Epidemilogical chrachterization of Okra Yellow Vein Mosaic Virus and its vector.
Okra (Abelmoschusesculentus L. Moench) belongs to "Malvaceae" family .It is a perfect villager's vegetable commonly known as Bhindi or lady finger. Being an important source of vitamins (vitamin A, vitamin C,), proteins (15-26 %) edible oil (14%) and minerals. It is an extremely popular tropical vegetable in the developing countries . In 2013, Okra was grown on an area of 1104 thousand hectares with the annual production of 8689 thousand tons globally . India, Nigeria, Sudan, Iraq and Ivory Cost are top leading okra producing countries in the world. Pakistan at 6th in world's okra production, produces 108 thousand tons on the area of 35 thousands acre annually (Fish, 2013).
Yellow Vein Mosaic Virus disease is the most serious disease in okra crop caused by a begomovirus "Okra Yellow Vein Mosaic Virus"  vectored by an important sucking pest Bemisiatabaci, having a wide host range . B. tabaci transfers the virus persistently with one hour of acquisition feeding period and 30 minutes of transmission feeding period . Okra yellow vein mosaic is a scourging okra disease, lowers the yield up to30 % annually but in epidemicconditions losses may reach up to 90% . The symptoms of OYVMV are homogenous interwoven network of yellow veins enclosing islands of green tissues within its leaf. In case of severe infection, the infected leaves become yellowish or creamy in color. Infected fruit shows stunting or reduction in size and deformed. The infected fruits are covered with yellow or creamy color, fibrous, small, and tough .
Disease is a dynamic process changes with time and space, significantly depends on classical disease triangle components (Susceptible host, virulent pathogen and the environment). Phytopathometery reflects the actual disease losses focusing to measure, predict and adopt the disease management strategies . Environmentis characterized into two components (biotic and a-biotic) for disease progression. A-biotic environment (temperature, humidity, rainfall and wind velocity) significantly impacts on plantpathosystem. Change in weather events significantly impacts on disease progression. Optimum environmental conditions are needed for successful infection and vector infestation.
The present study was aimed to assess the impact of different a-biotic environmental factors (maximum air temperature, minimum temperature, relative humidity, rainfall and wind velocity) directly (disease incidence) or indirectly (vector population) for disease forecasting.
MATERIALS AND METHODS
Certified seeds of 12 Okra varieties/Lines (Long finger, Lush green, Green gold, China Red, SabzPari, JKOH-635, OK-407, PusaSawani, Punjab Selection, N-220, Najuka, ArkaAnamika) were taken from Ayub Agricultural Research Institute (AARI), Faisalabad. To avoid from soil borne infection, seeds were treated by Dynesty @ 2ml [Kg.sup.-1] for 10 minutes and air dried. Field trial was conducted at experimental area of Department of Plant Pathology, University of Agriculture, Faisalabad. The seeds were sown in augmented design with two repeats by keeping the row to row and plant to plant distance at 75 and 30 cm respectively. Infection was relied upon natural inouclum. All agronomic practices were adopted. Percent disease incidence was recorded by selecting diseased plants showing clear symptoms (yellow veins enclosing islands of green tissues) among total plants and was calculated by using the formula :
Disease incidence % = infected plants/Total plants x 100
Data was recorded on weekly basis. B. tabaci population was recorded atback side of lower, middle and upper leaves of randomly selected okra plants from each variety/lineat 10:00 am daily.
Environmental data (maximum air temperature, minimum temperature, relative humidity, rain fall and wind velocity) was recorded from weather station of Department of Crop Physiology, University of Agriculture, Faisalabad installed two hundred meters away from the experimental site. Recorded OYMV disease incidence and B. tabaci population on twelve varieties was correlated with environmental data using Pearson's correlation. Varieties exhibiting significant correlations were selected for regression analysis. The recorded data was analyzed by using "Statistical Analysis System (SAS 9.3)" computer simulated software  and graphical approaches were made through "Microsoft Office 2013" .
RESULTS AND DISCUSSION
Significant correlation between disease incidence of OYVMV and maximum air temperature, minimum temperature, relative humidity and weak negative correlation was seen with rainfall and wind velocity in all varieties (Table 1).Three varieties (Long Finger, Lush Green and Green Gold) were used for regression analysis. B. tabaci population had shown significant correlation with maximum air temperature, minimum temperature and relative humidity while rainfall and wind velocity had non-significant correlation with B. tabaci population (Table 2).
Maximum air temperature played a significant role in the development of OYVMV. Relationship between disease incidence and maximum air temperature was explained by simple linear regression model. Significant positive correlation was seen among all varieties.With the increase in maximum air temperature, disease incidence increased significantly. Maximum disease incidence was recorded at 36[degrees]C to 39[degrees]Cmaximum air temperature. (Fig 1). Relationship between B. tabaci population and maximum air temperature was explained by simple linear regression model. Significant positive correlation was seen between B. tabaci population and maximum air temperature on okra varieties (Long Finger, Lush Green and Green Gold). B. tabaci population increased with the increase of maximum air temperature. Maximum population was recorded at 38[degrees]C to 39[degrees]C (Fig 2)
Strong positive correlation was observed between disease incidence and minimum temperature. With the increase in minimum temperature the disease development was also increased. Maximum disease incidence was noted at minimum temperature 28.6[degrees]C to 28.8[degrees]C. Significant correlation between disease incidence and minimum temperature was noted by okra varieties (Long Finger, Lush Green and Green Gold)(Fig 3). Strong positive correlation was observed between B. tabaci population and minimum temperature. As minimum temperature increased, B. tabaci population increased. Maximum B. tabaci population was noted at minimum temperature 28.8[degrees]C (Fig 4).
A-biotic environmental factors (Temperature, humidity, rainfall, wind velocity) significantly impacts directly and indirectly on disease progression. Temperature plays a critical role in disease susceptibility and in resistance . Vertical resistance is controlled by a single or two genes greatly depends on genetic expression of R genes . The same happens in horizontal resistance where the mechanism of resistance is controlled by many genes . Wheather the resistance is horizontal or vertical, temperature is an important factor. R genes requires dire need of optimum temperature for the proper function of many enzymes which involve in transcriptional and translational processes . Failing to express R genes of a resistant variety, it may become susceptible against a pathogen. The same is true in case of the pathogen, it may lose its pathogenicity by changing climatic events. In complex plant-pathogen interaction, the environmental factors are critical for disease development especially in vector transferred disease. The Okra Yellow Vein Mosaic Virus disease is vectored by B. tabaci . Both the virus and the insect needs optimum environmental conditions for their survival. From the above study; considering the maximum air temperature, disease incidence was found maximum at 36[degrees]C to 36[degrees]C and the maximum population was at 38 to 39[degrees]C. Bemisiatabaciis a sucking insect, acquires the virus from the infected plant through persistent manner. Several biological reactions in B. tabacibody cells needs optimum temperature for their continuity. From this study, maximum air temperature from 38[degrees]C to 39[degrees]C favored B. tabaci population which reflects the optimum condition for the reproduction process. More the whiteflies, more the chances of virus acquisition and transmission and ultimately more the disease incidence will. The same is true in case of minimum temperature where disease incidence was found maximum at 28.6[degrees]C to 28.8[degrees]C at the same temperature (28.8[degrees]C) maximum B. tabaci population was noted.  determined the influence of weather factors on the incidence of OYVMV. They found that relative humidity and wind velocity had a highly significant and negative correlation with disease incidence. Maximum air temperature and total rainfall had a highly non-significant, positive and negative correlation with disease incidence respectively.  reported that density of B.tabaci was the lowest in kharifand winter crops. B. tabaci population on rabicrop started from first week of December with 5.12 whiteflies/3 leaves and there was a gradual increase in B. tabaci population with the increase in temperature
Linear trend was seen between disease incidence and relative humidity. Negative correlation was observed between disease incidence and relative humidity. As relative humidity increased, decrease in disease incidence was seen. Maximum disease incidence was noted at 42 to 45 % relative humidity. (Fig 5). Linear trend was seen between B. tabaci population and relative humidity. Strong negative correlation was observed between B. tabaci population and relative humidity. As relative humidity was increased, B. tabaci population was decreased. Maximum B. tabaci population was noted at 42 % to 46 % relative humidity (Fig 6).
Negative trend between disease incidence and rainfall was assessed. Increase in rainfall resulted into decrease of disease incidence. Maximum disease incidence was noticed at 0.1 to 0.5 mm of rainfall. Non-significant correlation between disease incidence and rainfall was observed by the three okra varieties (Fig 7). Between B. tabaci population and rainfall, negative trend was assessed. Increase in rainfall resulted into decrease of B. tabaci population. Maximum B. tabaci population was noticed at 0.1 to 0.5 mm of rainfall. Weak correlation between disease incidence and rainfall was observed by okra varieties (Fig 8).
Humidity plays a significant role in disease progression and in B. tabaci population. Rainfall and relative humidity directly link to each other, more the rainfall more will be the humidity. After few hours of the rain, the atmospheric contains maximum water vapors which gradually decreases with the passage of time. A single female of B. tabaci lays approximately more than 119 eggs at the lower side of the leaf surface . The rain splashes wash off the eggs, nymphs and adults from the leaf surface, hence, this is the main reason of population decline. With the passage of time, the B. tabaci population again builds up while on the other hand, the humidity in air decreases gradually. The same happened in our experiment where disease incidence and vector population decreased significantly by the rainfall and increase in humidity
Linear trend was seen between disease incidence and wind velocity. Negative correlation was observed between disease incidence and wind velocity. As wind velocity increased, disease incidence was decreased. Maximum disease incidence was noted at 5.5 to 6 km/h wind velocity. No significant correlation between disease incidence and wind velocity was observed in three okra varieties (Fig 9). Negative linear trend was seen between B. tabaci population and wind velocity. Strong negative correlation was observed between B. tabaci population and wind velocity. As wind velocity was increased, B. tabaci population decreased. Maximum B. tabaci population was noted at 5.25 to 6 km/h wind velocity. Weak correlation between B. tabaci population and wind velocity were observed in three okra varieties (Fig 10).
The wind velocity plays a vital role in viral acquisition and transmission efficacy of B. tabaci. The insect (B. tabaci) needs continuously more than an hour to acquire the virus from an infected leave and the same time is required for its transmission to the health plant. The strong wind recurrent greatly disturbs insect's attachment to the plant which results into failure of efficient Okra yellow vin mosaic virus acquisition and transmission causing to deline the disease incidence.
Pan et al.  explained that among the environmental factors temperature, rainfall, sunlight, humidity expressed significant correlation with disease incidence and B. tabaci population dynamics. Activity of B. tabacion summer started during first week of April 06 and reached a peak during last week of April 06 (14.91 whiteflies/3 leaves) and there was a decrease in B. tabaci abundance with the onset of monsoon.  reported that both pest (B. tabaci) and natural enemy populations decreased due to rainfall. Threhan  reported that high temperature and low rainfall were found to favor rapid multiplication of the pest. Same results were reported by Ozgur et al. . Ali et al.  determined the correlation of environmental factors (maximum and minimum temperature, relative humidity and rainfall) with percent plant infection of okra yellow vein mosaic virus (OYVMV). There was a significant correlation between environment and disease severity. Ali et al.  determined the correlation of environmental conditions (maximum and minimum air temperature, relative humidity, rainfall, clouds and wind velocity) with okra yellow vein mosaic virus (OYVMV) disease severity and B. tabaci population on commercially grown varieties of okra i.e. Pahuja, Safal, SubzPari and SurkhBhindi. Minimum temperature and relative humidity had significant correlation with OYVMV disease severity and B. tabaci population. The disease incidence increased with the rise in minimum temperature and B. tabaci population decreased with increase in the relative humidity.
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Faisal Siddique, Safder Ali, Muhammad Ehetisham-ul-Haq, Muhammad Atiq and Abdul Rashid
Department of Plant Pathology, University of Agriculture, Faisalabad, Pakistan.
Address For Correspondence:
Muhammad Ehetisham-ul-Haq, Department of Plant Pathology, University of Agriculture, Faisalabad, Pakistan. E-mail: email@example.com
Received 12 February 2016; Accepted 28 April 2016; Available online 24 May 2016
Table 1: Correlation of environmental conditions with okra yellow vein mosaic virus disease on different okra verities/lines Varieties/ Maximum air Minimum Relative Rain Wind LINES temperature Temperature Humidity Fall Velocity Long Finger .83 * 0.91 ** -.93 ** -0.77 -0.617 0.03 0.02 0.01 0.32 0.06 Green Gold .857 * 0.91 ** -.97 * -0.79 -0.732 0.03 0.02 0.01 0.26 0.1 China Red .865 * .805 * -.920 ** 0.39 -0.62 0.03 0.05 0.01 0.26 0.13 Lush Green .834 * 0.95 ** -.98 ** -0.72 -0.70 0.01 0.01 0.01 0.17 0.12 SabzPari .809 * .900 * -.936 ** 0.47 -0.58 0.05 0.02 0.01 0.21 0.15 JKOH-635 .810 * 0.889 * -.993 ** 0.31 -0.78 0.05 0.02 0 0.3 0.06 OK-407 .838 * 0.77 -.877 * 0.4 -0.55 0.04 0.06 0.03 0.25 0.17 PusaSawani 0.7 .872 * -.958 ** 0.23 -.837 * 0.1 0.03 0.01 0.35 0.04 Punjab 0.55 0.72 -.885 * -0.03 -.890 * Selection 0.17 0.09 0.02 0.48 0.02 N-220 0.59 0.77 -.851 * 0.14 -.865 * 0.15 0.06 0.03 0.41 0.03 Najuka .826 * .833 * -.992 ** 0.23 -.846 * 0.04 0.04 0 0.36 0.04 Arka-Anamika .816 * 0.77 -0.62 0.49 -0.78 0.04 0.04 0 0.62 0.1 Table 2: Correlation of environmental conditions with B. tabacipopulation on different okra varieties/lines Varieties Maximum Minimum Relative Rain Wind air Temperature Humidity fall velocity temperature Long Finger .919 * 0.78 * -.952 ** -0.76 0.808 0.031 0.03 0.002 0.284 0.08 Green Gold .858 * 0.74 * -.979 ** -0.71 -0.732 0.031 0.03 0.002 0.284 0.08 Lush Green .906 * 0.89 * -.950 ** -0.88 -0.719 0.017 0.052 0.007 0.286 0.085 China Red 0.777 0.724 -.847 * 0.324 -0.527 0.061 0.083 0.035 0.297 0.181 SabzPari .866 * 0.658 -.874 * 0.215 -0.69 0.029 0.114 0.026 0.364 0.099 JKOH-635 0.776 0.742 -.888 * 0.264 -0.612 0.062 0.076 0.022 0.334 0.136 OK-407 .889 * .930 * -.949 ** 0.557 -0.606 0.022 0.011 0.007 0.165 0.139 PusaSawani 0.704 0.747 -.935 ** 0.099 -0.774 0.092 0.073 0.01 0.437 0.062 Punjab .952 ** 0.705 -.905 * 0.294 -0.762 Selection 0.006 0.092 0.017 0.315 0.067 N-220 0.792 .819 * -.864 * 0.469 -0.476 0.055 0.045 0.03 0.213 0.209 Najuka 0.711 .842 * -.888 * 0.387 -0.525 0.089 0.037 0.022 0.26 0.182 Arka-Anamika 0.73 .852 * -.891 * 0.587 -0.543 0.089 0.037 0.022 0.26 0.182
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|Author:||Siddique, Faisal; Ali, Safder; Ehetisham-ul-Haq, Muhammad; Atiq, Muhammad; Rashid, Abdul|
|Publication:||Advances in Environmental Biology|
|Date:||Apr 1, 2016|
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