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Relationships between percentage defoliation, dry weight, percentage reflectance, leaf-to-stem ratio, and green leaf area index in the alfalfa leaf spot pathosystem. (Forage & Grazing Lands).

AFALFA is one of the most important forage crops grown in the United States, as well as throughout the world (Hanson et al., 1988; Stuteville and Erwin, 1990). Alfalfa plays an important role in soil conservation and also in improving soil N. It is estimated that 9.62 million ha of alfalfa are currently in production in the United States (National Agricultural Statistics Service, 2001). Alfalfa, however, is susceptible to a number of foliar pathogens and each of these pathogens has the potential to cause disease injury that limits alfalfa production (Hampton et al., 1978; Stuteville and Erwin, 1990). Fungal diseases are by far the largest group of pathogens that cause alfalfa foliar diseases, and these diseases continue to be responsible for significant reductions in alfalfa yield and quality (Hanson et al., 1988; Nutter et al., 2002; Stuteville and Erwin, 1990). Even though alfalfa varieties and germplasms resistant to some alfalfa diseases are available, alfalfa varieties resistant to multiple foliar diseases are not currently available (Acharya and Huang, 2000; Sorensen et al., 1993, 1994; Woodward et al., 1993). The most prevalent and damaging foliar diseases of alfalfa in Iowa are spring black stem and leaf spot (caused by Phoma medicaginis Malbr. & Roum. var. medicaginis Boerema), summer black stem and leaf spot {caused by Cercospora medicaginis Ellis & Everh.), common leaf spot [caused by Pseudopeziza medicaginis (Lib.) Sacc.], and Leptosphaerulina leaf spot [caused by Leptosphaerulina trifolii (Rostr.) Petr. [syn. L. briosiana (Pollaci) J.H. Graham & Luttrell]} (Rizvi and Nutter, 1993).

Green leaf area index, the amount of green leaf area per unit ground area, is commonly used as a measure of vegetative growth and development (Best and Harlan, 1985). Alfalfa foliar diseases can cause severe reductions in GLAI due to leaf spotting, followed by premature defoliation. Reductions in forage yield and quality are directly related to reductions in GLAI and changes in leaf-to-stem ratios, which is a measure of alfalfa quality (Broscious et al., 1987; Broscious and Kirby, 1988; Campbell and Duthie, 1990; Gray, 1983; Stuteville and Erwin, 1990). These types of measurements, however, are extremely labor intensive and usually require destructive sampling to achieve accurate estimates (Best and Harlan, 1985). Green leaf area index is one of the most important agronomic characters of crops, and this agronomic variable is frequently used as an input in crop growth models (Wiegand et al., 1979).

Although visual assessment methods are commonly employed by plant pathologists, visual assessment methods often possess several negative attributes: such methods (i) usually require destructive sampling, (ii) are often highly subjective, and (iii) are typically both labor intensive and time consuming to perform (Nilsson, 1995; Nutter and Gaunt, 1996). Examples of different types of visual disease intensity assessment methods include disease incidence (the number of diseased leaves/the total number of leaves assessed x 100), disease severity (the diseased leaf area/the total leaf area x 100), and percentage defoliation (the number of defoliated primary leaves on an alfalfa stem/the total number of primary leaves that should be present x 100) (Nutter et al., 1991).

Remote sensing may provide an alternative method to quantify the effects of disease. Remote sensing is defined as the acquisition of information from a sampling unit (e.g., leaf, plant, plant canopy, plant population) without direct physical contact between the measuring device and the sampling unit (Nilsson, 1995; Nutter, 1990). Plants may respond to foliar pathogens in a number of ways, including leaf spots (chlorosis or necrosis of photosynthetic plant parts), leaf blights, defoliation, leaf curling, stunting, and wilting. Although many of these responses are difficult to quantify visually with acceptable levels of accuracy, precision, and speed, these same plant responses should also affect the amount and quality of electromagnetic radiation reflected from plant canopies (Nilsson, 1995; Nutter, 1990; Nutter and Gaunt, 1996). Thus, remote sensing instruments that measure and record changes in electromagnetic radiation may provide a better means to objectively quantify the effects of foliar diseases on host populations than the more traditional visual assessment methods (Nutter, 1989; Nutter and Littrell, 1996). Another potential advantage of remote sensing is that plant canopies (sampling units) can be repeatedly assessed across time, both noninvasively and nondestructively (Mitchell et al., 1990; Nilsson, 1995; Nutter and Gaunt, 1996).

Growth and duration of GLAI of a crop determines the percentage of the incident solar radiation that will be intercepted by the crop canopy across time, thereby influencing canopy photosynthesis, photosynthate translocation, and final yield (Dale et al., 1980). Even though respiration, absorption of nutrients from soil, and photosynthesis of organs other than leaves can also affect crop biomass and yield, it is not surprising that the yields of many crops are often closely related to GLAI and GLAI duration (Engel et al., 1987; Waggoner and Berger, 1987; Waston, 1947). Because remote sensing assessments often have a better relationship with yield than visual disease assessments and GLAI is often closely related to yield, remote sensing assessments also should have a better relationship with GLAI than do percentage defoliation assessments. A number of research studies have been conducted to quantify the relationship between remote sensing assessments and GLAI for several crops (Ajai et al., 1983; Almihanna, 1990; Best and Harlan, 1985; Haverkort et al., 1991), and these studies have demonstrated that reflectance measurements in the near-infrared region (720-850 nm) often have a significant linear relationship with GLAI.

Single-point models, multiple-point models, and AUC models are three types of empirical models that have been used to quantify the relationships between disease intensity assessments and yield, but these models also can be used to quantify the relationships between reflectance from crop canopies, biomass (dry weight), and GLAI (Campbell and Madden, 1990; Nutter and Gaunt, 1996; Teng, 1985). Single-point regression models could be developed that relate disease intensity or percentage reflectance assessments performed at one specific time during the growing season (or at a specific growth stage of the host) to dependent variables such as crop biomass (dry weight) and GLAI. Multiple-point GLAI models could be used to relate disease intensity and remote sensing assessments performed at two or more times during the course of a growing season to dry weight and GLAI. Because it is difficult to find rational and biological explanations, multiple-point models are not widely used, and therefore, multiple-point models were not used in the present study (Campbell and Madden, 1990; Nutter and Gaunt, 1996). Area under the curve models relate the areas under the disease or percentage reflectance curves to areas under the dry weight and GLAI curves, but the latter two methods usually require destructive sampling and are tedious and time-consuming to perform.

On the basis of the preceding points, it is our hypothesis that the percentage reflectance of sunlight from alfalfa canopies will have a stronger linear relationship with dry weight and GLAI than percentage defoliation assessments. If this hypothesis is true, then models based on remote sensing (percentage reflectance) measurements should predict dry weight and GLAI with greater precision (higher coefficients of determination and smaller standard errors of the estimate for y) compared with regression models using visual disease assessments (percentage defoliation). The objective of this study was to quantify and compare single-point and AUC models to quantify the relationships between percentage defoliation and percentage reflectance measured from alfalfa canopies with dry weight and alfalfa GLAI.

MATERIALS AND METHODS

Field Experimental Design

Alfalfa stands were established using variety ICI 630 at the Iowa State University Agronomy Research Farm at Ames, IA, in a Nicollet loam soil (fine-loamy, mixed, superactive, mesic Aquic Hapludolls) in 1995, and the Iowa State University Northeast Research Farm at Nashua, IA, in a Readlyn loam (fine-loamy, mixed, superactive, mesic Aquic Hapludolls) in 1996. Foliar disease epidemics were allowed to develop naturally at both locations. In order to generate a broad range of disease intensity levels that would result in a broad range of GLAI levels, fungicides that varied in efficacy (azoxystrobin, chlorothalonil, cupric hydroxide, mancozeb, and propiconazole) and various application frequencies were employed to differentially control foliar diseases of alfalfa (Table 1). Each plot was 12.2 m by 1.8 m with a 1.2-m nonfungicide-treated border surrounding each plot. Fungicide applications were initiated when alfalfa was [approximately equal to] 15 cm high. For treatments involving two or more fungicide applications per growth cycle, subsequent applications were applied 10 d after the previous application. All fungicides were applied in 718 L water [ha.sup.-1] equivalent using a CO2-pressurized sprayer operated at 276 kPa. A randomized complete block design with four replications was used for both experiments. There were six treatments in 1998 at Ames, IA. To increase the range of disease levels in 1999, two additional treatments, azoxystrobin (applied twice) and chlorothalonil (applied three times) per growth cycle were added to the 1999 experiment conducted at Nashua, IA.

Visual Disease Assessments

Visual percentage defoliation and disease severity assessments were performed on each sampling date. Three 0.48-m diameter circles were randomly selected from each plot using a random number generator. Three alfalfa stems were sampled from each circle by cutting the stems at the ground level (i.e., nine alfalfa stems per plot). These nine alfalfa stems were bulked and stored in an ice cooler for later processing in the laboratory. The nine alfalfa stems were visually assessed for percentage defoliation (number of primary leaves missing on each primary node divided by the total number of primary nodes per stem x 100) and disease severity (diseased leaf area/the total leaf area x 100) for both primary and secondary leaves. One person (rater) was responsible for all visual disease assessments and this rater was trained to assess disease severity using the disease assessment software training program Alfalfa. Pro (Nutter and Litwiller, 1993).

Dry Weight and GLAI Measurements

Leaf area measurements from each plot were obtained weekly by destructive sampling. After sampling the nine alfalfa stems (three from each circle), the remaining alfalfa stems from each circle were then removed by cutting all stems at the ground level. Alfalfa stems from each circle were placed in separate paper bags and dried at 60[degrees]C for 3 d in a forced-air oven, and the dry weight was then recorded. After completing visual disease assessments for the nine stem samples from each plot, the primary and secondary leaves were separated from petioles and stems. Subsamples (half of all the leaves) from the primary and secondary leaves of the nine stems were arbitrarily selected and used to determine leaf area using a Delta-T leaf area meter (Decagon Device, Inc., Pullman, WA). The dry weight of primary leaves, secondary leaves, and stems (including petioles) was obtained after drying in a forced-air oven for 24 h at 60[degrees]C. The leaf-to-stem ratio was calculated as the sum of the dry weight for primary and secondary leaves originating from the nine stems sampled from each plot divided by the dry weight of the nine stems (without leaves).

GLAI, leaf area index for primary leaves (PLAI) and leaf area index for secondary leaves (SLAI) were calculated using the following equations:

[1] PLAI = [(e + g/b + e + f + g + h x a) x c/g]/(3 x [pi] x [r.sup.2]

[2] SLAI= [f + h/b + e + f + g + h x a) x d/h]/(3 x [pi] x [r.sup.2]

To calculate PLAI and SLAI, a is the dry weight of all the alfalfa biomass (stems, leaves, and petioles) from one plot; b is the dry weight of the nine stems (without leaves) from each plot; c and d are the leaf areas of the primary and secondary leaves obtained from each nine-stem sample, respectively; e and f are the dry weights of the primary and secondary leaves for which leaf area was not measured, respectively; and g and h are the dry weights of the primary and secondary leaves for which leaf area was measured, respectively; and r is the radius of each of the three circles in one plot. GLAI (total) was calculated using the following equations:

[3] GLAI = PGLAI + SGLAI

[4] PGLAI = PLAI x (1 - [x.sub.1])

[5] SGLAI = SLAI x (1 - [x.sub.2]),

where PGLAI is GLAI for the primary leaves, SGLAI is GLAI for the secondary leaves, and [x.sub.1] and [x.sub.2] are the disease severities expressed in proportions for primary and secondary leaves, respectively.

Remote Sensing Assessments

Remote sensing assessments were conducted weekly for each growth cycle using a hand-held, multispectral radiometer (CROPSCAN, Inc., Rochester, MN). Two percentage reflectance assessments were obtained from each circle (six assessments from each plot). Both the incident and reflected radiation from plot canopies were measured simultaneously in eight narrow (50-nm) wavelength bands (460, 510, 560, 610, 660, 710, 760, and 810 nm). All percentage reflectance measurements were obtained between 1100 and 1500 h central standard time (CST) from a sensor height of [approximately equal to] 1 m above ground level during cloud-free periods (Guan and Nutter, 2001). The alfalfa canopy area for each remote sensing assessment was a 0.50-m diameter circle (approximately the same area as the area sampled for biomass). A bubble-spirit level mounted on the support pole of the radiometer was used to align the sensors to the appropriate (90[degrees]) angle (Nutter and Littrell, 1996).

AUDC and AURC Models

Area under the curve models were constructed using the area under the percentage defoliation curve (AUDC) or area under the percentage reflectance curve (AURC; 810 nm) (AURC) as separate independent variables, and area under the dry weight curve (AUDW), area under the PGLAI curve (AUCPGLAI), area under the secondary leaves curve (AUCSGLAI), area under the leaf-to-stem ratio curve (AUCLSR), or area under the GLAI curve (AUCGLAI), as separate dependent variables. Area under the percentage defoliation curve, AURC, AUDW, AUCPGLAI, AUCSGLAI, AUCLSR, and AUCGLAI were calculated using this following equation:

(6) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [a.sub.1] is percentage defoliation, percentage reflectance, dry weight, PGLAI, SGLAI, leaf-to-stem ratio, or GLAI obtained on the ith sampling date; [t.sub.1] is the ith disease assessment date from the first assessment date; and n is the total number of assessments conducted (Broscious et al., 1987; Campbell and Madden, 1990). Because the time duration of each growth cycle was different, all those variables were standardized by dividing the number of days in that growth cycle.

Data Analysis

Single-point models were developed for each sampling date to quantify relationships among the following variables: percentage reflectance, percentage defoliation, dry weight, PGLAI, SGLAI, GLAI, and leaf-to-stem ratio (Gomez and Gomez, 1984; Kleinbaum and Kupper, 1978; SAS Institute, 1996). Area under the curve models for each growth cycle also were developed by regressing the area under the curves for percentage reflectance and percentage defoliation on area under the dry weight, PGLAI, SGLAI, GLAI, and leaf-to-stem ratio curve. The goodness-of-fit of all models was based on: (i) the F-statistic, (ii) the coefficient of determination ([R.sup.2]), (iii) the standard error of the estimate for y (SEEy), and (iv) the coefficient of variation (CV).

RESULTS AND DISCUSSION

A range of foliar disease stress levels was generated by utilizing different fungicides and application frequencies. Fungicide treatments had significant effects on percentage defoliation, dry weight, percentage reflectance (810 nm), PGLAI, SGLAI, leaf-to-stem ratio, and GLAI of alfalfa. Progress curves of those variables for the nonfungicide control treatment (highest disease levels) and weekly chlorothalonil treatment (the lowest disease levels) were shown in Fig. 1 and 2. Usually, the biggest differences among treatments were achieved when measurements were taken on the date of harvest or 1 to 2 wk prior to harvest.

[FIGURES 1-2 OMITTED]

Linear regressions were conducted between percentage defoliation, percentage reflectance (810 nm), and GLAI. The coefficients of determination for single-point GLAI models based on percentage defoliation assessments or the percentage of sunlight reflected from alfalfa canopies (810 nm) were averaged across all sampling dates (Fig. 3). It was found that coefficients of determination relating either percentage defoliation or percentage reflectance (x, at 810 nm) to GLAI (y) were highest when percentage defoliation and percentage reflectance assessments were obtained on the date that alfalfa plots were harvested (last sampling date for each alfalfa growth cycle). Thus, models based on only harvest date assessments were developed and compared.

[FIGURE 3 OMITTED]

Coefficients of determination for the relationships between percentage reflectance at the eight wavelength bands and alfalfa yield for the six harvest dates of the two locations were averaged and shown in Fig. 4. Percentage reflectance in 760 and 810 nm were essentially the same, and both had better relationships with alfalfa yield than reflectance in other wavelength bands. Since the 810-nm wavelength band has been shown to have a better relationship with yield than 760 nm with yield in other pathosystems (Nutter, 1990; Nutter et al., 1990, 1993), percentage reflectance in this wavelength band was selected to develop single-point and AUC models.

[FIGURE 4 OMITTED]

Relationships between Percentage Defoliation and Dry Weight, PGLAI, SGLAI, GLAI, and Leaf-to-Stem Ratio Using Single-Point

Regression Models

There were significant linear relationships between percentage defoliation and dry weight, SGLAI, GLAI, and leaf-to-stem ratio for three of the six assessment dates (harvests) in 1998 and 1999 (Table 2). Dry weights decreased from 3.20 to 12.69 g [m.sup.-2] for each 1% increase in percentage defoliation, while SGLAI decreased 0.014 to 0.11 units for each 1% increase in percentage defoliation. In the three significant models, GLAI decreased 0.041 to 0.21 units for each 1% increase in defoliation, and leaf-to-stem ratio decreased 0.0069 to 0.0074 units for each 1% increase in percentage defoliation (Table 2). There were significant linear relationships between percentage defoliation and PGLAI for four of the six assessment dates, and PGLAI decreased 0.024 to 0.10 units for each 1% increase in percentage defoliation. Percentage defoliation explained up to 74, 88, 57, 76, and 89% of the variation in alfalfa dry weight, PGLAI, SGLAI, GLAI, and leaf-to-stem ratio, respectively (Table 2). Standard errors of the estimate for y and coefficients of variation for all regression models are provided in Table 2.

Relationships between Percentage Reflectance (810 nm) and Dry Weight, PGLAI, SGLAI, GLAI, and Leaf. to-Stem Ratio Using Single-Point Regression Models

There were significant linear relationships between percentage reflectance and dry weight, PGLAI, and GLAI for four of six assessment (harvest) dates in 1998 and 1999 (Table 2). Dry weights increased 7.83 to 15.85 g [m.sup.-2] for each 1% increase in percentage reflectance, while PGLAI and GLAI increased 0.041 to 0.086 units and 0.10 to 0.20 units for each 1% increase in percentage reflectance, respectively (Table 2). There were significant linear relationships between percentage reflectance and SGLAI, and leaf-to-stem ratios for three of the six assessment dates in 1998 and 1999 (Table 2). Leaf-to-stem ratios increased 0.0093 to 0.023 units for each 1% increase in percentage reflectance, while SGLAI increased 0.051 to 0.079 units for each 1% increase in percentage reflectance. Percentage reflectance explained up to 88, 91, 69, 94, and 60% of the variation in alfalfa dry weight, PGLAI, SGLAI, GLAI, and leaf-to-stem ratio, respectively (Table 2). Standard errors of the estimate for y and coefficients of variation for all regression models are provided in Table 2.

Nondestructive percentage reflectance measurements had a strong linear relationship with GLAI. The GLAI method of assessing plant health was much more labor intensive; it took more time and this method required destructive sampling, whereas the remote sensing provided a faster and more precise method that could nondestructively and noninvasively estimate GLAI. On average, percentage reflectance measurements explained 15, 3, 18, and 4% more of the variation in dry weight, PGLAI, SGLAI, and GLAI, respectively, than percentage defoliation assessments. These results demonstrate that the percentage reflectance of sunlight obtained from alfalfa canopies was a better predictor of dry weight, PGLAI, SGLAI, and GLAI than percentage defoliation assessments. Percentage reflectance, however, explained 10% less of the variation in leaf-to-stem ratio than percentage defoliation when data were averaged across the significant models (Table 2).

Stronger linear relationships between percentage defoliation, dry weight, percentage reflectance, PGLAI, SGLAI, and GLAI were found for the experiment conducted in 1999 at Nashua, IA, than for the experiment in 1998 at Ames, IA. This difference may be because there were two more fungicide treatments in 1999 than in the experiment conducted in 1998 at Ames, IA. Also, the ranges of percentage defoliation, percentage reflectance, dry weight, and GLAI for the experiment conducted in 1999 at Nashua, IA, were wider than those achieved in the 1998 experiment at Ames, IA (Fig. 2). Thus, more significant relationships between those variables were found for the experiment conducted in 1999 at Nashua, IA, than for the experiment in 1998 at Ames, IA (Cornell and Berger, 1987).

Relationships between Percentage Defoliation and Dry Weight, PGLAI, SGLAI, GLAI, and Leaf-to-Stem Ratio Using AUC Models

There were significant linear relationships between area under the defoliation curves and area under the PGLAI, SGLAI, and GLAI curves for two of the six alfalfa growth cycles in 1998 and 1999 (Table 3). For these models, areas under the PGLAI, SGLAI, and GLAI curves decreased 0.024 to 0.076, 0.024 to 0.042, and 0.048 to 0.075 units for each unit increase in AUDC, respectively (Table 3). There were significant linear relationships between area under the dry weight and leaf-to-stem ratio curves for only one of the six alfalfa growth cycles (Table 3). For these models, AUDW decreased 3.04 g [m.sup.-2] and AUCLSR decreased 0.0090 units for each unit increase in the AUDC (Table 3). Area under the defoliation curve models explained up to 42, 52, 44, 50, and 46% of the variation in area under the alfalfa dry weight, PGLAI, SGLAI, GLAI, and leaf-to-stem ratio curves, respectively (Table 3). Standard errors of the estimate for y and coefficients of variation for all regression models are provided in Table 3.

Relationships between Percentage Reflectance (810 nm) and Dry Weight, PGLAI, SGLAI, GLAI, and Leaf-to-Stem Ratio Using AUC Models

There were significant linear relationships between area under the percentage reflectance curves (AURCs) and area under the PGLAI, SGLAI, and GLAI curves for all six alfalfa harvest cycles in 1998 and 1999 (Table 3). For these models, area under the PGLAI, SGLAI, and GLAI curves increased from 0.028 to 0.35, 0.042 to 0.10, and 0.073 to 0.49 units for each unit increase in AURC. There were significant linear relationships between area under the reflectance curve and area under the dry weight curves for four of the six alfalfa harvest cycles, with alfalfa dry weight increasing between 6.08 to 10.87 g [m.sup.-2] for each unit increase in AURC (Table 3). Area under the percentage reflectance curve explained up to 74, 84, 83, and 90% of the variation in area under the dry weight, PGLAI, SGLAI, and GLAI, respectively (Table 3). There were no significant linear relationships between the areas under the percentage reflectance curves and areas under the leaf-to-stem curves for any of the six harvest cycles in 1998 and 1999 (Table 3). Standard errors of the estimate for y and coefficients of variation for all regression models are provided in Table 3.

Our results demonstrate that AURC can explain more of the variation in area under the dry weight, PGLAI, SGLAI, and GLAI curves than using the AUDC. Averaged across all of the significant models, AURC explained 28, 19, 23, and 32% more of the variation in area under the dry weight, PGLAI, SGLAI, and GLAI curves than AUDC, respectively. However, the AUDC model had a significant relationship with leaf-to-stem ratio for one of the six harvest cycles, whereas no significant linear relationships between AURC and AUCLSR were found (Table 3).

Comparison of Single-Point Models and Area under the Curve Models

Single-point models based on percentage defoliation explained more of the variations in dry weight, PGLAI, SGLAI, GLAI, and leaf-to-stem ratio than AUDC models. Averaged across the significant models, single-point models based on percentage defoliation explained 66, 75, 47, 72, and 62% of the variations in those variables, respectively; whereas AUC models based on percentage defoliation explained 42, 49, 43, 48, and 46% of the variation in those variables, respectively. There were significant relationships between percentage reflectance and dry weight for four out the six harvest cycles using single-point models or AUC models. Averaged across the significant models, single-point models based on percentage reflectance explained 11% more of the variation in dry weight than the AUC models. Area under the curve models based on percentage reflectance, however, had better relationships with PGLAI, SGLAI, and GLAI than single-point models. There were significant relationships between AURCs and area under the PGLAI, SGLAI, and GLAI curves for all six harvest cycles, whereas using single-point models signifies relationships between percentage reflectance and these same variables were detected only for four, three, and four harvests, respectively. There were no significant linear relationships between percentage reflectance and leaf-to-stem ratio using AUC models, whereas there were significant linear relationships between percentage reflectance and leaf-to-stem ratio for three of the six harvest dates using single-point models.

These results supported the hypothesis that percentage reflectance measurements would have a better relationship with GLAI (sum of PGLAI and SGLAI) than visual estimates of percentage defoliation. Because most alfalfa foliar pathogens are polycyclic in nature, the logistic model dy/dt = ry(1 - y) often best describes disease progress (Campbell and Madden, 1990). In this equation, y is the proportion of diseased host tissue, 1 - y is the proportion of the healthy (nondiseased) tissue, dy/dt is the absolute rate of change in disease with time, and r is the estimated rate of disease progress. Visual disease assessment methods provide proportional estimates of y since disease intensity is expressed as a proportion of the total amount of plant tissue, whereas remote sensing assessments in our study provided estimates of 1 - y expressed in absolute units of yield and/or biomass (i.e., kg dry weight, kg [ha.sup.-1], GLAI). Thus, remote sensing assessments should have a better relationship with biomass and yield than percentage defoliation assessments (Nutter and Littrell, 1996; Nutter et al., 1990).

The defoliation assessment method used in our study measured only the presence or absence of the primary leaves on the main nodes of each alfalfa stem sampled, and therefore, this method may not have assessed the true impact of foliar diseases on the growth and development of secondary lateral branches and leaves. Remote sensing measurements provided more precise information about the impact of foliar diseases on the entire alfalfa canopy as a whole, including disease effects on lateral branches. Thus, percentage reflectance measurements provided quantitative information concerning the overall health (GLAI) of alfalfa canopies that the defoliation assessment method could not.

Percentage defoliation explained more of the variation in dry weight, PGLAI, SGLAI, and GLAI when single-point models were used as opposed to AUC models. Area under the curve models might be expected to have a better relationship with these variables than single-point models. However, our results conclusively demonstrated that AUC models based on percentage defoliation were not as good as single-point models. This result may be related to the fact that the greatest injury (defoliation) to alfalfa caused by foliar diseases usually occurred late in the alfalfa growth cycle (Broscious et al., 1987; Campbell and Duthie, 1990). We found that disease assessments performed at or near the end of a growth cycle often had the best relationship with dry weight, PGLAI, SGLAI, and GLAI. Percentage defoliation assessments obtained near the end of the growth cycle also had a better relationship with dry weight, PGLAI, SGLAI, and GLAI because it was during this phase of the alfalfa growth cycle that rates of percentage defoliation with respect to time were at their highest. Cornell and Berger (1987) have shown that the higher the level of disease intensity (in this case, percentage defoliation), the higher the coefficient of determination will be in regression models.

Contrary to percentage defoliation, the relationships between percentage reflectance and PGLAI, SGLAI, and GLAI using AUC models explained more of the variations in those variables than did single-point models. This may be because PGLAI, SGLAI, and GLAI measurements were all obtained at single points in time (harvest date) for the single-point models, whereas PGLAI, SGLAI, and GLAI in the AUC models represented the integral accumulation of multiple assessments across a growth cycle (Waggoner and Berger, 1987). Accumulated PGLAI, SGLAI, and GLAI values resulted in a broader range of values than single-point values of PGLAI, SGLAI, and GLAI obtained at the time of harvest, and the range of response values also has been shown to have a significant effect on the coefficient of determination for regression models (Cornell and Berger, 1987).

Both PGLAI and SGLAI increased with respect to time faster at the beginning of an alfalfa growth cycle than towards the later part of a growth cycle (Fig. 2). PGLAI usually peaked one to several weeks prior to harvest, and then decreased until harvest due to the effects of foliar diseases and natural senescence occurring at a rate that was faster than the rate of production of new leaves. Thus, overall GLAI increased across time from the beginning of a growth cycle and peaked 1 to 3 wk prior to harvest (Fig. 1). A decrease in GLAI often occurred 1 to 3 wk before harvest because it was during this stage of the alfalfa growth cycle that the rate of defoliation for primary and secondary leaves exceeded the rate of growth of new leaves as alfalfa approached flowering.

CONCLUSIONS

This research conclusively demonstrated that remote sensing was superior to the visual assessment of percentage defoliation or the destructive measurement of dry weight, PGLAI, SGLAI, and GLAI. Percentage of sunlight reflected from alfalfa canopies explained more of the variation in these variables than percentage defoliation assessments obtained visually. In addition, remote sensing assessments required less labor and took less time to estimate GLAI than the best visual disease assessment method. Remote sensing also estimated GLAI nondestructively and repetitively, whereas GLAI and visual disease assessment methods required destructive sampling. A potential disadvantage of remote sensing is that reflectance measurements obtained from crop canopies may potentially be affected by factors other than disease stress (Guan and Nutter, 2001). These nondisease factors include the amount of incident radiation, sun angle, leaf wetness, and sensor height. To minimize the effects of all of the above nondisease factors on the measurement of percentage of sunlight reflected from crop canopies, percentage reflectance measurements should be obtained between 1100 and 1500 h CST when plant canopies are dry, with a constant sensor height (sampling unit area) and within a small range of incident radiation values for all measurements.
Table 1. Fungicides and application frequencies per alfalfa
growth cycle for the experiment conducted in 1998 at Ames
and in 1999 at Nashua, IA.
                                                        Active
                                         Application   ingredient
Treatment         Fungicide               frequency    [ha.sup.-1]

1                 Mancozeb ([dagger])      Once          1.97 kg
2                 Propiconazole
                    ([double dagger])      Once          0.25 L
3                 Cupric Hydroxide         Once          1.97 kg
4                 Chlorothalonil
                    ([section])            Twice         2.04 L
5                 Chlorothalonil           Weekly        2.04 L
6 ([paragraph])   Chlorothalonil           Three         2.04 L
7 ([paragraph])   Azoxystrobin (#)         Twice         0.57 L
8                 Nonfungicide Control

([dagger]) {[1,2-ethanediylbis (carbamodithioato)](2-)}manganese
mixture with {[1,2-ethandiylbis (carbamodithioate)](2-)}zinc.

([double dagger]) 1-[2-(2',4'-Dichlorophenyl)-4-propyl-1,3-dioxolan-
2-yl-methyl]-1H-1,2,4-triazole.

([section]) 2,4,5,6-tetrachloro-1,3-dicyanobenzene.

([paragraph]) Fungicide treatments used only in 1999 at Nashua, IA.

(#) methyl (E)-2-{[6-(2-cyanophenoxy)-4-pyrimidinyl]oxy}-
alpha-(methoxymethylene)benzeneacetate.
Table 2. F-statistics, intercepts, slopes, coefficients of
determination, standard errors of the estimate for y (SEEy),
and coefficients of variation for relationships between
percentage defoliation, dry weight, percentage reflectance,
green leaf area index for alfalfa primary leaves (PGLAI),
green leaf area index for alfalfa secondary leaves (SGLAI),
leaf-to-stem ratio, and green leaf area index for harvest
dates in 1998 at Ames and in 1999 at Nashua, IA.

                         Location
                           Year       Harvest   F-statistic   Intercept

Percentage             Ames 1998         1          4.89        889.03
  defoliation vs.                        2          0.12        508.03
  dry weight                             3          1.40        482.61

                       Nashua 1999       1         12.74       1033.07
                                         2         17.07        470.76
                                         3          1.87        307.98

Percentage             Ames 1998         1          0.01          2.49
  defoliation                            2         13.20          1.83
  vs. PGLAI                              3         10.30          2.03

                       Nashua 1999       1          9.70          5.50
                                         2         43.15          1.82
                                         3          3.16          1.36

Percentage             Ames 1998         1          0.00          2.63
  defoliation                            2          0.65          2.80
  vs. SGLAI                              3          1.31          3.40

                       Nashua 1999       1          8.00          6.19
                                         2          4.23          1.87
                                         3          4.39          2.58

Percentage             Ames 1998         1          0.01          5.12
  defoliation                            2          1.84          4.63
  vs. GLAI                               3          3.63          5.42

                       Nashua 1999       1         13.57         11.70
                                         2         18.84          3.70
                                         3          6.15          3.94

Percentage             Ames 1998         1          0.10          0.38
  defoliation vs.                        2          1.78          0.74
  leaf-to-stem ratio                     3          0.64          0.62

                       Nashua 1999       1          4.14          0.71
                                         2          7.27          0.92
                                         3         48.66          1.02

Percentage             Ames 1998         1          6.03       1466.82
  reflectance vs.                        2          1.40        -81.61
  dry weight                             3          8.35       -395.09

                       Nashua 1999       1         40.66        148.35
                                         2         23.00         50.30
                                         3         42.81        -60.49

Percentage             Ames 1998         1          0.72         -0.69
  reflectance                            2          9.17         -1.77
  vs. PGLAI                              3         15.87         -3.10

                       Nashua 1999       1          1.86         -0.25
                                         2         62.37         -1.81
                                         3         14.10         -0.71

Percentage             Ames 1998         1          0.76          0.32
  reflectance                            2          1.26         -2.00
  vs. SGLAI                              3          1.90         -3.13

                       Nashua 1999       1         12.08         -1.24
                                         2         13.30         -0.39
                                         3          8.81         -0.63

Percentage             Ames 1998         1          1.16         -0.37
  reflectance                            2          2.73         -3.76
  vs. GLAI                               3          5.46         -6.23

                       Nashua 1999       1          6.21         -1.49
                                         2         89.44         -2.20
                                         3         27.48         -1.34

Percentage             Ames 1998         1          6.11         -0.69
  reflectance vs.                        2          0.27          0.09
  leaf-to-stem ratio                     3          0.27          0.35

                       Nashua 1999       1          0.21          0.36
                                         2          7.93          0.03
                                         3          3.94          0.41

                                                                   CV
                              Slope          [R.sup.2]   SEEy      (%)

Percentage              -7.26 ([dagger])       0.55      16.34     2.63
  defoliation vs.       -1.54                  0.03      42.11     9.59
  dry weight            -3.36                  0.26      31.44     8.63

                       -12.69 ([dagger])       0.68      16.20     3.07
                        -3.20 ([dagger])       0.74      15.47     4.07
                        -2.10                  0.24      24.48    10.49

Percentage              -0.0041                0.00       0.17     7.40
  defoliation           -0.024 ([dagger])      0.77       0.06     7.96
  vs. PGLAI             -0.028 ([dagger])      0.72       0.10     9.32

                        -0.10 ([dagger])       0.62       0.15    10.48
                        -0.028 ([dagger])      0.88       0.09     8.27
                         0.015                 0.35       0.13    15.93

Percentage               0.00                  0.00       0.13     4.93
  defoliation           -0.025                 0.14       0.29    17.32
  vs. SGLAI             -0.034                 0.25       0.32    14.56

                        -0.11 ([dagger])       0.57       0.17     9.37
                        -0.014 ([dagger])      0.41       0.14     9.57
                        -0.026 ([dagger])      0.42       0.20    12.18

Percentage              -0.003                 0.00       0.25     5.07
  defoliation           -0.049                 0.31       0.34    13.78
  vs. GLAI              -0.062                 0.48       0.36    10.97

                        -0.21 ([dagger])       0.69       0.26     7.93
                        -0.042 ([dagger])      0.76       0.20     7.83
                        -0.041 ([dagger])      0.51       0.26    10.67

Percentage               0.0021                0.02       0.033    7.20
  defoliation vs.       -0.0068                0.31       0.048   11.04
  leaf-to-stem ratio    -0.0021                0.14       0.029    5.26

                        -0.0074 ([dagger])     0.41       0.017    4.01
                        -0.0069 ([dagger])     0.55       0.051    7.04
                        -0.0072 ([dagger])     0.89       0.016    2.15

Percentage             -17.27                  0.60      15.33     2.47
  reflectance vs.       13.01                  0.26      36.78     8.38
  dry weight            15.85 ([dagger])       0.68      20.78     5.70

                         9.63 ([dagger])       0.87      10.27     1.94
                         9.14 ([dagger])       0.79      13.80     3.63
                         7.83 ([dagger])       0.88       9.83     4.21

Percentage               0.062                 0.15       0.16     6.82
  reflectance            0.063 ([dagger])      0.70       0.070    9.10
  vs. PGLAI              0.086 ([dagger])      0.80       0.082    7.90

                         0.042                 0.24       0.21    14.81
                         0.079 ([dagger])      0.91       0.072    7.01
                         0.041 ([dagger])      0.70       0.090   10.76

Percentage               0.047                 0.16       0.12     4.52
  reflectance            0.09                  0.24       0.27    16.29
  vs. SGLAI              0.11                  0.32       0.31    13.81

                         0.079 ([dagger])      0.67       0.15     8.25
                         0.051 ([dagger])      0.69       0.10     6.97
                         0.061 ([dagger])      0.59       0.17    10.20

Percentage               0.11                  0.23       0.22     4.46
  reflectance            0.16                  0.41       0.31    12.83
  vs. GLAI               0.20 ([dagger])       0.58       0.32     9.85

                         0.12 ([dagger])       0.51       0.33    10.04
                         0.13 ([dagger])       0.94       0.10     4.00
                         0.10 ([dagger])       0.82       0.16     6.42

Percentage               0.023 ([dagger])      0.60       0.021    4.58
  reflectance vs.        0.0086                0.06       0.056   12.84
  leaf-to-stem ratio     0.0041                0.06       0.030    5.49

                         0.0014                0.03       0.021    5.13
                         0.019 ([dagger])      0.57       0.050    6.88
                         0.0093 ([dagger])     0.40       0.038    5.04

([dagger]) Significant at the 0.10 probability level.
Table 3. F-statistics, intercepts, slopes, coefficients of
determination, standard errors of the estimate for y (SEEy),
and coefficients of variation for relationships between area
under the percentage defoliation, dry weight, percentage
reflectance, green leaf area index for alfalfa primary leaves
(PGLAI), green leaf area index for alfalfa secondary leaves
(SGLAI), leaf-to-stem ratio, and green leaf area index curves
for each growth cycle in 1998 at Ames and in 1999 at Nashua, IA.

                         Location Year   Harvest   F-statistic

Percentage defoliation   Ames 1998          1          3.61
  vs. dry weight                            2          1.11
                                            3          1.83

                         Nashua 1999        1          0.41
                                            2          1.60
                                            3          4.32

Percentage defoliation   Ames 1998          1         10.60
  vs. PGLAI                                 2          4.34
                                            3          2.96

                         Nashua 1999        1          0.04
                                            2          2.59
                                            3          4.89

Percentage defoliation   Ames 1998          1          0.27
  vs. SGLAI                                 2          1.14
                                            3          0.50

                         Nashua 1999        1          0.14
                                            2          4.70
                                            3          4.31

Percentage defoliation   Ames 1998          1          4.24
  vs. GLAI                                  2          2.39
                                            3          1.29

                         Nashua 1999        1          0.10
                                            2          5.19
                                            3          6.03

Percentage defoliation   Ames 1998          1          2.12
  vs. leaf-to-stem                          2          0.11
  ratio                                     3          0.37

                         Nashua 1999        1          0.27
                                            2          0.39
                                            3          5.21

Percentage reflectance   Ames 1998          1          1.36
  vs. dry weight                            2          4.28
                                            3         11.43

                         Nashua 1999        1         14.44
                                            2         10.84
                                            3         13.88

Percentage reflectance   Ames 1998          1          5.10
  vs. PGLAI                                 2          4.81
                                            3         20.59

                         Nashua 1999        1          8.33
                                            2         17.45
                                            3         25.40

Percentage reflectance   Ames 1998          1          6.08
  vs. SGLAI                                 2         19.60
                                            3          4.62

                         Nashua 1999        1         16.68
                                            2         13.32
                                            3          7.72

Percentage reflectance   Ames 1998          1         18.94
  vs. GLAI                                  2         11.06
                                            3         13.37

                         Nashua 1999        1         23.82
                                            2         53.56
                                            3         21.61

Percentage reflectance   Ames 1998          1          0.06
  vs. leaf-to-stem                          2          0.11
  ratio                                     3          0.13

                         Nashua 1999        1          0.93
                                            2          0.00
                                            3          0.41

                         Location Year   Harvest    Intercept

Percentage defoliation   Ames 1998          1        208.64
  vs. dry weight                            2        581.43
                                            3        481.43

                         Nashua 1999        1        480.37
                                            2        286.72
                                            3        202.52

Percentage defoliation   Ames 1998          1          0.47
  vs. PGLAI                                 2          3.02
                                            3          2.66

                         Nashua 1999        1          2.35
                                            2          1.68
                                            3          1.41

Percentage defoliation   Ames 1998          1          1.33
  vs. SGLAI                                 2          3.12
                                            3          2.51

                         Nashua 1999        1          1.76
                                            2          1.45
                                            3          1.17

Percentage defoliation   Ames 1998          1          1.90
  vs. GLAI                                  2          6.09
                                            3          5.05

                         Nashua 1999        1          4.12
                                            2          3.13
                                            3          2.56

Percentage defoliation   Ames 1998          1          0.68
  vs. leaf-to-stem                          2          0.69
  ratio                                     3          0.83

                         Nashua 1999        1          0.71
                                            2          0.95
                                            3          1.11

Percentage reflectance   Ames 1998          1      -1073.18
  vs. dry weight                            2         43.75
                                            3       -198.74

                         Nashua 1999        1         10.74
                                            2        -24.32
                                            3        -47.52

Percentage reflectance   Ames 1998          1        -15.22
  vs. PGLAI                                 2          0.0056
                                            3         -1.93

                         Nashua 1999        1         -0.28
                                            2         -0.84
                                            3         -0.63

Percentage reflectance   Ames 1998          1         -3.71
  vs. SGLAI                                 2         -0.57
                                            3         -1.91

                         Nashua 1999        1         -2.33
                                            2         -1.20
                                            3         -0.63

Percentage reflectance   Ames 1998          1        -21.06
  vs. GLAI                                  2         -0.38
                                            3         -3.68

                         Nashua 1999        1         -2.61
                                            2         -2.05
                                            3         -1.26

Percentage reflectance   Ames 1998          1          0.25
  vs. leaf-to-stem                          2          0.59
  ratio                                     3          0.60

                         Nashua 1999        1          0.73
                                            2          1.02
                                            3          0.80

                         Location Year   Harvest        Slope

Percentage defoliation   Ames 1998          1        7.66
  vs. dry weight                            2      -11.58
                                            3       -6.63

                         Nashua 1999        1       -4.91
                                            2       -3.74
                                            3       -3.04 ([dagger])

Percentage defoliation   Ames 1998          1        0.076
  vs. PGLAI                                 2       -0.076 ([dagger])
                                            3       -0.048

                         Nashua 1999        1       -0.012
                                            2       -0.033
                                            3       -0.024 ([dagger])

Percentage defoliation   Ames 1998          1        0.035
  vs. SGLAI                                 2       -0.072
                                            3       -0.034

                         Nashua 1999        1       -0.029
                                            2       -0.042 ([dagger])
                                            3       -0.024 ([dagger])

Percentage defoliation   Ames 1998          1        0.08
  vs. GLAI                                  2       -0.15
                                            3       -0.08

                         Nashua 1999        1       -0.041
                                            2       -0.075 ([dagger])
                                            3       -0.048 ([dagger])

Percentage defoliation   Ames 1998          1       -0.0057
  vs. leaf-to-stem                          2       -0.0028
  ratio                                     3       -0.0038

                         Nashua 1999        1       -0.0046
                                            2        0.0048
                                            3       -0.0090 ([dagger])

Percentage reflectance   Ames 1998          1       29.01
  vs. dry weight                            2        6.37
                                            3       10.87 ([dagger])

                         Nashua 1999        1        8.33 ([dagger])
                                            2        7.69 ([dagger])
                                            3        6.08 ([dagger])

Percentage reflectance   Ames 1998          1        0.35 ([dagger])
  vs. PGLAI                                 2        0.028 ([dagger])
                                            3        0.071 ([dagger])

                         Nashua 1999        1        0.058 ([dagger])
                                            2        0.061 ([dagger])
                                            3        0.050 ([dagger])

Percentage reflectance   Ames 1998          1        0.10 ([dagger])
  vs. SGLAI                                 2        0.049 ([dagger])
                                            3        0.078 ([dagger])

                         Nashua 1999        1        0.083 ([dagger])
                                            2        0.062 ([dagger])
                                            3        0.042 ([dagger])

Percentage reflectance   Ames 1998          1        0.49 ([dagger])
  vs. GLAI                                  2        0.073 ([dagger])
                                            3        0.15 ([dagger])

                         Nashua 1999        1        0.14 ([dagger])
                                            2        0.12 ([dagger])
                                            3        0.092 ([dagger])

Percentage reflectance   Ames 1998          1        0.0061
  vs. leaf-to-stem                          2        0.0010
  ratio                                     3        0.0024

                         Nashua 1999        1       -0.041
                                            2        0.00
                                            3        0.0052

                         Location Year   Harvest   [R.sup.2]    SEEy

Percentage defoliation   Ames 1998          1        0.47      13.71
  vs. dry weight                            2        0.22      16.57
                                            3        0.31      18.54

                         Nashua 1999        1        0.06      12.38
                                            2        0.21      11.27
                                            3        0.42      11.92

Percentage defoliation   Ames 1998          1        0.73       0.080
  vs. PGLAI                                 2        0.52       0.055
                                            3        0.43       0.10

                         Nashua 1999        1        0.01       0.099
                                            2        0.30       0.078
                                            3        0.45       0.089

Percentage defoliation   Ames 1998          1        0.06       0.043
  vs. SGLAI                                 2        0.22       0.10
                                            3        0.11       0.18

                         Nashua 1999        1        0.02       0.12
                                            2        0.44       0.074
                                            3        0.42       0.093

Percentage defoliation   Ames 1998          1        0.51       0.12
  vs. GLAI                                  2        0.37       0.14
                                            3        0.24       0.26

                         Nashua 1999        1        0.02       0.20
                                            2        0.46       0.13
                                            3        0.50       0.16

Percentage defoliation   Ames 1998          1        0.35       0.013
  vs. leaf-to-stem                          2        0.03       0.012
  ratio                                     3        0.09       0.023

                         Nashua 1999        1        0.04       0.014
                                            2        0.06       0.029
                                            3        0.46       0.032

Percentage reflectance   Ames 1998          1        0.25      16.34
  vs. dry weight                            2        0.52      13.01
                                            3        0.74      11.40

                         Nashua 1999        1        0.71       6.93
                                            2        0.64       7.58
                                            3        0.70       8.59

Percentage reflectance   Ames 1998          1        0.56       0.10
  vs. PGLAI                                 2        0.55       0.054
                                            3        0.84       0.056

                         Nashua 1999        1        0.58       0.034
                                            2        0.74       0.047
                                            3        0.81       0.052

Percentage reflectance   Ames 1998          1        0.60       0.028
  vs. SGLAI                                 2        0.83       0.047
                                            3        0.54       0.13

                         Nashua 1999        1        0.74       0.065
                                            2        0.69       0.055
                                            3        0.56       0.080

Percentage reflectance   Ames 1998          1        0.83       0.074
  vs. GLAI                                  2        0.73       0.092
                                            3        0.77       0.14

                         Nashua 1999        1        0.80       0.092
                                            2        0.90       0.055
                                            3        0.78       0.10

Percentage reflectance   Ames 1998          1        0.01       0.016
  vs. leaf-to-stem                          2        0.03       0.012
  ratio                                     3        0.03       0.024

                         Nashua 1999        1        0.13       0.013
                                            2        0.00       0.030
                                            3        0.06       0.042

                         Location Year   Harvest   CV (%)

Percentage defoliation   Ames 1998          1        3.65
  vs. dry weight                            2        5.89
                                            3        7.27

                         Nashua 1999        1        3.68
                                            2        4.73
                                            3        7.66

Percentage defoliation   Ames 1998          1        3.74
  vs. PGLAI                                 2        5.28
                                            3       10.09

                         Nashua 1999        1        4.93
                                            2        6.26
                                            3        8.52

Percentage defoliation   Ames 1998          1        2.88
  vs. SGLAI                                 2        7.97
                                            3       13.09

                         Nashua 1999        1       13.36
                                            2        8.15
                                            3       11.81

Percentage defoliation   Ames 1998          1        3.51
  vs. GLAI                                  2        6.10
                                            3       10.63

                         Nashua 1999        1        6.93
                                            2        5.54
                                            3        8.67

Percentage defoliation   Ames 1998          1        2.38
  vs. leaf-to-stem                          2        2.00
  ratio                                     3        3.36

                         Nashua 1999        1        2.49
                                            2        2.90
                                            3        3.30

Percentage reflectance   Ames 1998          1        4.35
  vs. dry weight                            2        4.63
                                            3        4.47

                         Nashua 1999        1        2.06
                                            2        3.18
                                            3        5.52

Percentage reflectance   Ames 1998          1        4.74
  vs. PGLAI                                 2        5.14
                                            3        5.37

                         Nashua 1999        1        3.20
                                            2        3.79
                                            3        5.02

Percentage reflectance   Ames 1998          1        1.88
  vs. SGLAI                                 2        3.72
                                            3        9.46

                         Nashua 1999        1        6.95
                                            2        6.07
                                            3       10.24

Percentage reflectance   Ames 1998          1        2.10
  vs. GLAI                                  2        3.97
                                            3        5.86

                         Nashua 1999        1        3.13
                                            2        2.53
                                            3        5.72

Percentage reflectance   Ames 1998          1        2.92
  vs. leaf-to-stem                          2        1.99
  ratio                                     3        3.46

                         Nashua 1999        1        2.36
                                            2        3.00
                                            3        4.36

([dagger]) Significant at the 0.10 probability level.


Abbreviations: AUC, area under the curve; AUCLSR, area under the leaf-to-stem ratio curve; AUCPGLAI, area under the PGLAI curve; AUCSGLAI, area under the secondary leaves curve; AUDC, area under the percentage defoliation curve; AUDW, area under the dry weight curve; AURC, area under the percentage reflectance curve; CST, central standard time; GLAI, green leaf area index; PGLAI, green leaf area index for primary leaves; PLAI, leaf area index for primary leaves; SGLAI, green leaf area index for secondary leaves; SLAI, leaf area index for secondary leaves.

ACKNOWLEDGMENTS

This project was supported, in part, by Hatch Act and State of Iowa Funds and a grant from the North Central Pesticide Impact Assessment Program.

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Jie Guan and Forrest W. Nutter, Jr. *

351 Bessey Hall, Dep. of Plant Pathology, Iowa State Univ., Ames, IA 50011. Journal Paper No. J-19114 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA, Project No. 3394. Received 20 Apr. 2001. * Corresponding author (fwn@iastate.edu).
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Author:Guan, Jie; Nutter, Forrest W., Jr.
Publication:Crop Science
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Jul 1, 2002
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