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Resistance to Common Bacterial Blight Among Phaseolus Species and Common Bean Improvement.

Common bean is a legume crop of worldwide significance (Singh, 1992). In the tropics and subtropics, it is frequently and severely attacked by CBB, a systemic (Burkholder, 1921), seed-transmitted (Aggour et al., 1989b) disease caused by Xanthomonas campestris pv. phaseoli (Smith) Dye (Xcp) (Saettler, 1989; Schuster and Coyne, 1981).

CBB is widespread in Latin America, particularly in northwestern Argentina, south central Brazil, Venezuela, Central America and Cuba, and coastal Mexico. These regions typically grow small-seeded, susceptible cultivars of the common bean race Mesoamerica (Singh et al., 1991).

The pathogen attacks all aerial plant parts, including leaf petioles, pods, and seeds, but the characteristic symptoms of chlorotic borders around the necrotic lesions are more severe and conspicuous on leaves of Susceptible cultivars. Similarly, the movement of bacterial populations through vascular tissue may depend on the level of CBB resistance (Goodwin et al., 1995). Susceptible cultivars accumulate larger bacterial populations, these moving faster through vascular tissues, than do CBB-resistant genotypes. CBB-infected seeds with visible symptoms can lose their color or are stained and their value is thus lowered. However, planting infected seeds does not necessarily result in systemic transmission of the bacteria from the vascular tissues of the grown plants to the new seeds (Aggour et al., 1989b).

Bacteria can survive for months on plant debris left on the soil and in seeds (Gilbertson et al., 1990). Severity of yield losses varies according to cultivar, levels of infection, environment, and stage of crop growth. Heavy and early infection, high humidity, temperatures fluctuating between [is less than] 20 and [is greater than] 25 [degrees] C, and alternately dry and wet weather can cause more than 40% yield loss in susceptible cultivars (Serracin et al., 1991). Other factors influencing disease severity are photoperiod (Arnaud-Santana et al., 1993a), inoculation method and bacterial concentration (Aggour et al., 1989a), and stage of crop maturity at infection (Coyne and Schuster, 1974). A Phaseolus genotype may also show resistance in leaves, but susceptibility in pods, or vice versa; it may also be resistant to some strains of the bacterium but susceptible to others (Aggour et al., 1989a).

Disease incidence can be reduced by intercropping, for example, with maize (Zea mays L.) (Fininsa, 1996) or by chemicals such as copper hydroxide and potassium methyldithiocarbamate, particularly when applied early. However, chemical control does not significantly reduce pod infection nor is seed yield increased (Weller and Saettler, 1976). Cultivar resistance is thus the most effective strategy, and is pivotal to all Other CBB control measures, including integrated disease-and-crop management practices.

Moderate resistance to CBB has been found in common bean; comparatively higher levels in some scarlet runner bean (P. coccineus) accessions and the highest levels in tepary bean (P. acutifolius) (Coyne and Schuster, 1973; Mohan, 1982; Yoshii et al., 1978). Although more than 80% of P. acutifolius cultivars are highly resistant to CBB, fewer than 25% of wild P. acutifolius accessions possess similar resistance (CIAT, 1996).

In common bean, a single recessive gene controls CBB resistance in trifoliolate leaves of an induced mutant, Bulgarian snap bean line A-8-40 (Adams et al., 1988). CBB resistance is also inherited quantitatively, with predominance of additive gene action and low to high heritability (Arnaud-Santana et al., 1994; Silva et al., 1989). Eskridge and Coyne (1996), using inbredbackcross data, estimated that between one and five genes control CBB resistance. Studies involving DNA-based (RAPD, RFLP) markers support the existence of two to six quantitative trait loci (QTLs) responsible for CBB resistance in common bean (Jung et al., 1996; Nodari et al., 1993).

In P. acutifolius, three linked dominant genes (one for a different isolate of Xcp) were identified as controlling CBB resistance (Dursun et al., 1996; Freytag, 1989). But Drijfhout and Blok (1987), after crossing resistant (PI 319443) and susceptible (Oaxaca 88 and PI 313488) P. acutifolius, reported a single dominant gene to be responsible for CBB resistance in leaves and pods.

McElroy (1985) concluded that one major and a few minor genes controlled resistance in lines XAN 159, XAN 160, and XAN 161, which were developed at CIAT from a P. vulgaris x P. acutifolius (PI 319443) population received from the University of California-Riverside. Kolkman and Michaels (1994) reported that both tepary bean accessions PI 319443 and PI 440795 carried identical genes for CBB resistance. But in F2 populations among crosses of OAC 88-1 (CBB resistance introgressed from PI 440795 at the University of Guelph, ON, Scott and Michaels, 1992), XAN 159, and XAN 161, segregation for susceptibility was observed, suggesting that a different gene for resistance was probably transferred from the P. acutifolius in each of these genotypes. Nonetheless, CBB resistance in OAC 88-1 and XAN 159 is linked to the same RAPD marker (P.N. Miklas, 1997, personal communication).

All breeding and genetic studies on CBB resistance in Latin America have been carried out within the last 25 yr. The sources of CBB resistance often used were those initially introgressed from tepary bean at the University of Nebraska, Lincoln, for example, GNN #1 Sel 27, Tara, and Jules (Coyne and Schuster, 1969, 1970). But because photoperiod and temperature are known to affect common bean's reaction to CBB (Arnaud-Santana et al., 1993a) and because these Nebraska sources of CBB resistance are poorly adapted to the tropics (Webster et al., 1983), better tropical adaptation was sought (Beebe and Pastor-Corrales, 1991; Mohan and Mohan, 1983). A major breakthrough in breeding for CBB resistance was achieved when lines XAN 159, XAN 160, XAN 161, and OAC 88-1 were developed from new P. vulgaris x P. acutifolius crosses.

CBB-resistant lines have also been developed from P. vulgaris x P. coccineus crosses in Puerto Rico (Freytag at al., 1982; Miklas et al., 1994) and Canada (Park and Dhanvantari, 1987). Recently, CBB resistance from GNN #1 Sel 27, PI 207262, and XAN 159 have been combined with other resistance genes at CIAT and in Brazil, Puerto Rico, and USA. These sources of CBB resistance are also being transferred into common bean cultivars of different market classes.

Hybridization between P. vulgaris and P. acutifolius, by embryo rescue, was initiated at CIAT, Palmira, Colombia, in 1989 (Mejia-Jimenez et al., 1994). More than 10 000 advanced-generation progenies were obtained from recurrent and congruity backcrosses (i.e., backcrossing alternately to either species) of the interspecific [F.sub.1] hybrids and gene pyramiding (i.e., combining different sources of CBB resistance genes). For the last 5 yr, these progenies have been systematically screened under field conditions at CIAT's Quilichao Experiment Station, Colombia, giving rise to six CBB-resistant lines.

This paper compares the levels of CBB resistance available in some promising accessions of four Phaseolus species, the levels of CBB resistance introgressed from the primary, secondary, and tertiary gene pools into common bean lines and cultivars, and the levels of CBB resistance of lines developed from gene pyramiding. It also discusses difficulties faced so far and suggests alternative breeding strategies and methods for future needs.

MATERIALS AND METHODS

All germplasm screening and breeding studies for CBB resistance were carried out in the field at Quilichao. Between 1994 and 1998, 162 promising germplasm accessions of common (38 accessions), lima (35), scarlet runner (55), and tepary (34) beans were systematically screened for CBB reaction. Also screened were 70 advanced breeding lines from different programs around the world and reported CBB-resistant lines derived from interspecific crosses (16 lines) and gene pyramiding (33 lines). The outstanding genotypes from these groups were also compared with six CBB-resistant lines: VAX 1, VAX 2, VAX 3, VAX 4, VAX 5, and VAX 6 (Table 1), recently developed at CIAT from interspecific hybridization of P. vulgaris and P. acutifolius and gene pyramiding.

Table 1. Parents used to develop common bacterial blight-resistant common bean breeding lines VAX 1 to VAX 6 from P. vulgaris x P. acutifolius interspecific hybridization and gene pyramiding at CIAT, Colombia, 1989-1997.
                                Parent lines

                                 PI 207262
                    Jules        and Jules
                 [arrow down]   [arrow down]
                    XAN 87        XAN 112
                 [arrow down]   [arrow down]
Breeding line       A 769         A 775        ICA Pijao

VAX 1                 X             X             X
VAX 2                 X             X             X
VAX 3                 X             X             X
VAX 4                 X             X             X
VAX 5                 X             X             X
VAX 6                 X             X             X

                                     G 40020([dagger])
                                        [arrow down]
                                           XAN 159
                                        [arrow down]
Breeding line    G 40001([dagger])         XAN 263

VAX 1                  X
VAX 2                  X
VAX 3                  X
VAX 4                  X                    X
VAX 5                  X                    X
VAX 6                  X

                             PI 207262, Tara
                              and G 40020
                              [arrow down]
                                XAN 90 and
                                XAN 263
                              [arrow down]
Breeding line                    XAN 309
VAX 1
VAX 2
VAX 3                               X
VAX 4
VAX 5
VAX 6                               X


([dagger]) Phaseolus acutifolius; all others are P. vulgaris. Common bean cultivars Jules and Tara derive their CBB resistance from P. acutifolius (Coyne and Schuster, 1969, 1970).

To develop the six VAX lines, the common bean cultivar ICA Pijao was crossed with tepary bean accession G 40001 in 1989 by embryo rescue (Mejia-Jimenez et al., 1994). The [F.sub.1] of the interspecific hybrid was then crossed, first with the advanced common bean line A 775 and then with line A 769. This was followed by three generations of inbreeding before conducting intensive screening and selection for CBB resistance. By 1995, this resulted in common bean lines VAX 1 and VAX 2 (Table 1). An advanced CBB-resistant sister line (PVPA 9576-1) of these two was crossed with previously developed CBB-resistant CIAT lines XAN 263 and XAN 309, resulting in lines VAX 4 and VAX 5, and VAX 3 and VAX 6, respectively (Table 1).

An unreplicated, 2-m-long, single-row plot was first used. The spacing between rows was 60 cm with about 10 cm between plants within a row. Susceptible and resistant checks of known CBB reaction were planted throughout the nurseries. The nurseries were inoculated by spraying the canopy with back-pack solo sprayers, two to four times at 7- to 10-d intervals, beginning about 3 wk after sowing. A bacterial concentration of [10.sup.7] to [10.sup.8] cfu/mL was used.

All common bean genotypes were evaluated on a 1-to-9 scale, as described by Schoonhoven and Pastor-Corrales (1987). Genotypes receiving scores of 1 (no visible symptoms) to 3 (about 2% of leaf surface covered by small CBB lesions) were considered resistant; those receiving scores of 4 ([is less than] 5% of the leaf area covered by small CBB lesions) to 6 ([is less than] 10% leaf area covered by medium-sized and large CBB lesions) were classified as intermediate; and those with scores of 7 (about 10% leaf area affected) to 9 ([is greater than] 25% leaf area affected by CBB) were susceptible.

Thus, all common bean genotypes that received scores of 1 to 6 were evaluated in replicated trials by the same procedure in subsequent plantings. All susceptible genotypes were eliminated. The three leaflets of the first or second trifoliolate leaf of all those genotypes still resistant or intermediate were then inoculated with twin surgical blades (Pastor-Corrales et al., 1981) about 3 wk after sowing. Subsequently, all entries were also inoculated three to five times (at weekly intervals) by spraying the canopy to verify the CBB reaction.

Promising entries thus identified and reported in this article were compared with each other in a final trial with two replicates between 1996 and 1998. Plot size, distance between and within rows, and inoculation and evaluation methods were similar to those of previous years. However, in addition to inoculating the canopy and trifoliolate leaf, developing pods were also inoculated with a hypodermic needle, multiple needles (florist's frog), and/or surgical blades.

Trifoliolate leaves inoculated with surgical blades were evaluated 7 to 15 d after inoculation. Spray-inoculated canopies were evaluated periodically from when the first symptoms appeared on the susceptible check--common bean cultivar ICA Pijao--until leaves senesced or the crop matured. The pods were evaluated about a week after inoculation. Also, growth habit (Singh, 1982) and seed size and color were recorded for all entries. All data were analyzed by SAS (SAS Institute Inc., 1985).

RESULTS

CBB Resistance in Phaseolus Species

Table 2 indicates the number of promising accessions evaluated, and the ranges and means for CBB reaction in leaves inoculated in two ways for four Phaseolus species. While susceptible accessions were found in all Phaseolus species, the lowest levels of resistance were recorded in P. coccineus, P. lunatus, and P. vulgaris. Table 3 shows that the highest CBB resistance (scores of 1.2-2.0), as determined by all three inoculation methods, were recorded for P. acutifolius, accessions G 40029 and G 40156. But not all P. acutifolius accessions possessed high levels of resistance; some were intermediate (e.g., G 40022) and others highly susceptible (e.g., G 40110). Some P. lunatus accessions exhibited scores of 4.2 to 6.2; P. coccineus had scores of 4.8 to 5.5; and the three most promising P. vulgaris landraces had scores of 4.5 to 6.4. The susceptible cultivar ICA Pijao had scores of 8.0 to 8.9.

Table 2. The ranges and means for reaction to common bacterial blight (CBB) in leaves of promising accessions of different Phaseolus species and of advanced breeding common bean lines derived from interspecific hybridization and gene pyramiding. Two inoculation methods were used: surgical blade and aspersion. CIAT-Quilichao, Colombia, 1994-1998.
                                      CBB score([dagger])

                                         Surgical blade

                                Number
Germplasm identification      evaluated    Range     Mean

P. acutifolius                   34       1.0-9.0    3.0
P. coccineus                     55       5.0-9.0    7.0
P. lunatus                       35       6.2-9.0    6.3
P. vulgaris landraces            38       5.9-9.0    6.8
Lines or cultivars               70       4.1-9.0    6.5
Lines from P. vulgaris x
  P. acutifolius                  5       3.8-7.0    5.8
Lines from P. vulgaris x
  P. coccineus                   11       5.2-7.0    6.7
Lines from gene pyramiding       33       1.5-7.0    4.5
  LSD (0.05)                                1.3      0.3

                                CBB score([dagger])

                                     Aspersion

Germplasm identification            Range    Mean

P. acutifolius                     1.0-9.0    2.9
P. coccineus                       4.8-9.0    6.2
P. lunatus                         4.2-9.0    5.6
P. vulgaris landraces              6.0-9.0    6.8
Lines or cultivars                 3.4-9.0    5.8
Lines from P. vulgaris x
  P. acutifolius                   4.3-5.0    4.6
Lines from P. vulgaris x
  P. coccineus                     4.2-7.0    5.8
Lines from gene pyramiding         2.0-7.0    3.9
LSD (0.05)                           1.6      0.4


([dagger]) Scores: 1 = immune with no visible symptoms; 3 = about 2% leaf surface covered by small CBB lesions; 5 = about 5% leaf surface covered by small and medium-sized CBB lesions; 7 = about 10% leaf area affected; 9 = >25% leaf area diseased (Schoonhoven and Pastor-Corrales, 1987).

Table 3. Origin, growth habit, seed color and size, and common bacterial blight reaction of promising Phaseolus acutifolius, P. coccineus, P. lunatus, and P. vulgaris accessions. Three inoculation methods were used; CIAT-Quilichao, Colombia, 1994-1998.
                                                     Growth
Identification             Origin([dagger])   habit([double dagger])

P. acutifolius
   G 40001                        MEX                   III
   G 40022                        USA                   lII
   G 40029                        USA                   III
   G 40034                        MEX                   III
   G 40035                        MEX                   III
   G 40038                        MEX                   III
   G 40110([paragraph])           MEX                   III
   G 40155                        MEX                   III
   G 40156                        MEX                   III
P. coccineus
   G 35006                        GTA                   III
   G 35007                        GTA                   III
   G 35016                        MEX                   III
   G 35066                        MEX                   III
   G 35105                        MEX                   III
   G 35113                        MEX                   III
   G 35116                        MEX                   III
   G 35121                        MEX                   III
   G 35148                        MEX                   III
   G 35157                        MEX                   III
P. lunatus
   G 25254                        GTA                   I
   G 25835                        PER                   III
   G 25890                        CLB                   III
   G 25947                        PER                   III
   G 26007                        USA                   I
P. vulgaris
   G 6772                         MEX                   III
   G 1320                         MEX                   III
   G 4399                         MEX                   III
   ICA Pijao([paragraph])         CLB                   II
   LSD (0.05)

                                            Seed
Identification                   Color                  Size

P. acutifolius
   G 40001                  White                      Small
   G 40022                  Beige                      Small
   G 40029                  Cream speckled             Small
   G 40034                  White                      Small
   G 40035                  Black                      Small
   G 40038                  Cream                      Small
   G 40110([paragraph])     Black                      Small
   G 40155                  White                      Small
   G 40156                  White                      Small
P. coccineus
   G 35006                  Variable                   Large
   G 35007                  Variable                   Large
   G 35016                  Variable                   Large
   G 35066                  Gray speckled              Large
   G 35105                  Variable                   Large
   G 35113                  Variable                   Large
   G 35116                  Variable                   Large
   G 35121                  Variable                   Large
   G 35148                  Variable                   Large
   G 35157                  Variable                   Large
P. lunatus
   G 25254                  Pink                       Small
   G 25835                  Cream                      Large
   G 25890                  Pink mottled               Small
   G 25947                  Gray speckled              Large
   G 26007                  White                      Medium
P. vulgaris
   G 6772                   Beige                      Small
   G 1320                   Beige                      Small
   G 4399                   Beige                      Small
   ICA Pijao([paragraph])   Black                      Small
   LSD (0.05)

                            Mean bacterial blight score([sections])

Identification              ASP            SRB            MNP

P. acutifolius
   G 40001                  1.9            3.0            4.0
   G 40022                  4.1            2.5            3.0
   G 40029                  1.2            1.4            2.0
   G 40034                  1.4            1.2            3.5
   G 40035                  3.0            1.8            5.0
   G 40038                  1.7            1.8            3.0
   G 40110([paragraph])     8.0            7.3            7.0
   G 40155                  2.0            2.3            2.0
   G 40156                  1.3            1.7            2.0
P. coccineus
   G 35006                  6.3            6.2             --
   G 35007                  6.1            6.2             --
   G 35016                  6.7            5.3             --
   G 35066                  5.5            4.9             --
   G 35105                  5.0            6.2             --
   G 35113                  6.2            5.3             --
   G 35116                  5.8            5.3             --
   G 35121                  6.4            5.6             --
   G 35148                  5.5            4.8             --
   G 35157                  5.4            5.4             --
P. lunatus
   G 25254                  6.8            6.1             --
   G 25835                  6.8            6.9             --
   G 25890                  6.2            4.2             --
   G 25947                  6.6            6.0             --
   G 26007                  6.8            6.8             --
P. vulgaris
   G 6772                   6.4            6.1            4.5
   G 1320                   6.8            6.8            5.3
   G 4399                   6.4            6.4            7.5
   ICA Pijao([paragraph])   8.7            8.9            8.0
   LSD (0.05)               1.6            1.3            1.6


([dagger]) CLB = Colombia; GTA = Guatemala; MEX = Mexico; PER = Peru; and USA = United States of America.

([double dagger]) I = determinate upright; II = indeterminate upright; and III = indeterminate, prostrate, semiclimbing (Singh, 1982).

([sections]) Mean of three crop seasons, evaluated on a scale of 1 = immune with no visible symptoms; 3 = about 2% leaf surface covered by small CBB lesions; 5 = about 5% leaf surface covered by small and medium-sized CBB lesions; 7 = about 10% leaf area affected; 9 = [is greater than] 25% leaf area diseased (Schoonhoven and Pastor-Corrales, 1987). Inoculation methods: ASP = aspersion, SRB = surgical blade (for leaves only), MNP = multiple needles (for pods only).

([paragraph]) Susceptible check.

Introgression of CBB Resistance from Wild Populations and Landraces of the Primary, Secondary, and Tertiary Gene Pools of P. vulgaris

Examples of use of CBB resistance genes from only the wild or cultivated germplasm forming the primary gene pool of P. vulgaris are rare indeed. As far as we know, the only line that derives such resistance (from a landrace accession G 4399, also known as Tamaulipas 9-B, from Mexico) is XAN 91, developed at CIAT more than a decade ago (Beebe and Pastor-Corrales, 1991). Its resistance is intermediate (scores of 5.2-6.3).

Table 4 includes the CBB reaction of the most promising common bean lines that derive their resistance from P. acutifolius and P. coccineus. Both groups of lines, in general, exhibited intermediate levels of CBB reaction.

Table 4. Origin, growth habit, seed color and size, and common bacterial blight (CBB) reaction of some common bean cultivars and breeding lines developed by introgression of CBB resistance from P. autifolius and P. coccineus into P. vulgaris. Three inoculation methods were used; CIAT-Quilichao, Colombia, 1994-1998.
                                                      Growth
Identification             Origin([dagger])   habit([double dagger])

P. vulgaris x
 P. acutifolius
   GNN #1 Sel 27                  UNE                   III
   Jules                          UNE                   III
   Tara                           UNE                   III
   OAC 88-1                       UGC                   II
   XAN 159                        CIAT                  I
P. vulgaris x
 P. coccineus
   ICB 3                          UPR                   II
   ICB 6                          UPR                   II
   ICB 11                         UPR                   II
   ICB 37                         UPR                   II
   I 9365-1-9                     UPR                   II
   I 9365-5-DR                    UPR                   II
   TARS VCI-4B                    TARS                  III
   XR 235-1-1                     MITA                  III
   ICA Pijao([paragraph])         ICA                   II

   LSD (0.05)

                                            Seed
Identification                   Color                  Size

P. vulgaris x
 P. acutifolius
   GNN #1 Sel 27            White                      Medium
   Jules                    White                      Medium
   Tara                     White                      Medium
   OAC 88-1                 White                      Small
   XAN 159                  Gray speckled              Large
P. vulgaris x
 P. coccineus
   ICB 3                    Black                      Small
   ICB 6                    Red                        Small
   ICB 11                   Black                      Small
   ICB 37                   White                      Small
   I 9365-1-9               Beige                      Small
   I 9365-5-DR              Red                        Small
   TARS VCI-4B              Pinto                      Medium
   XR 235-1-1               White                      Small
   ICA Pijao([paragraph])   Black                      Small

   LSD (0.05)

                            Mean bacterial blight score([sections])
Identification                ASP           SRB               MNP

P. vulgaris x
 P. acutifolius
   GNN #1 Sel 27              6.1           5.0               3.0
   Jules                      5.6           5.0               2.8
   Tara                       6.4           4.9               1.8
   OAC 88-1                   6.2           4.3               4.7
   XAN 159                    3.8           4.0               5.3
P. vulgaris x
 P. coccineus
   ICB 3                      5.2           4.2               3.3
   ICB 6                      6.6           5.6               4.5
   ICB 11                     7.0           6.7               4.5
   ICB 37                     7.0           6.4               6.0
   I 9365-1-9                 6.7           5.9               6.0
   I 9365-5-DR                6.9           6.6               6.5
   TARS VCI-4B                4.5           3.8               3.0
   XR 235-1-1                 6.8           5.8               3.5
   ICA Pijao([paragraph])     8.7           8.9               8.0

   LSD (0.05)                 1.6           1.3               1.6


([dagger]) CIAT = Centro Internacional de Agricultura Tropical; ICA = Instituto Colombiano Agropecuario; MITA = Mayaguez Institute of Tropical Agriculture; TARS = Tropical Agricultural Research Service; UGC = University of Guelph, Canada; UNE = University of Nebraska; UPR = University of Puerto Rico.

([double dagger]) I = determinate upright; II = indeterminate upright; and III = indeterminate, prostrate, semiclimbing (Singh, 1982).

([sections]) Mean of three crop seasons, evaluated on a scale of 1 = immune with no visible symptoms; 3 = about 2% leaf surface covered by small CBB lesions; 5 = about 5% leaf surface covered by small and medium-sized CBB lesions; 7 = about 10% leaf area affected; 9 = >25% leaf area diseased (Schoonhoven and Pastor-Corrales, 1987). Inoculation methods: ASP = aspersion, SRB = surgical blade (for leaves only), MNP = multiple needles (for pods only).

([paragraph]) Susceptible common bean cultivar.

The line XAN 159 had higher levels of CBB resistance in leaves but lower resistance in pods, compared with the initial Nebraska lines GNN #1 Sel. 27, Jules, and Tara. Table 4, in conjunction with Tables 2 and 3, also shows that, while the highest levels of CBB resistance available in P. vulgaris and P. coccineus have been introgressed into common bean, those for P. acutifolius still remain to be achieved (e.g., P. acutifolius accessions G 40029 and G 40156, Table 3).

Pyramiding CBB Resistance Genes from Across Phaseolus Species

Only limited efforts have been made to systematically recombine and accumulate CBB resistance from different Phaseolus species into a common bean genotype. For example, GNN #1 Sel 27, Jules, and/or Tara, which derive their resistance from P. acutifolius, were crossed with common bean accessions such as PI 207262 (Beebe and Pastor-Corrales, 1991; Mohan and Mohan, 1983). This resulted in lines BAT 93, XAN 112, IAPAR BAC 20, IAPAR BAC 31 (Table 5), and IAPAR BAC 44, among others.

Table 5. Origin, growth habit, seed color and size, and common bacterial blight (CBB) reaction of some common bean breeding lines obtained by pyramiding resistance genes from different sources. Three inoculation methods were used; CIAT-Quilichao, Colombia, 1994-1998.
                                                    Growth
Identification           Origin([dagger])   habit([double dagger])

BAC 31                        IAPAR                  III
WBB-20-1                      UPR                    I
G 17341                       CU                     III
NY 79-3776-1                  CU                     III
Wilk. 2                       CU                     I
XAN 263                       CIAT                   II
XAN 309                       CIAT                   II
XAN 328                       CIAT                   II
XAN 330                       CIAT                   II
XAN 332                       CIAT                   II
VAX 1                         CIAT                   III
VAX 2                         CIAT                   III
VAX 3                         CIAT                   II
VAX 4                         CIAT                   II
VAX 5                         CIAT                   II
VAX 6                         CIAT                   II
ICA Pijao([paragraph])        ICA                    II
  LSD (0.05)

                                        Seed
Identification                Color                  Size

BAC 31                   Pinto                      Small
WBB-20-1                 White                      Small
G 17341                  Pinto                      Small
NY 79-3776-1             Pinto                      Small
Wilk. 2                  White                      Small
XAN 263                  Red                        Small
XAN 309                  Red                        Small
XAN 328                  Red                        Small
XAN 330                  Red                        Small
XAN 332                  Red                        Small
VAX 1                    Cream striped              Small
VAX 2                    Cream                      Small
VAX 3                    Red                        Small
VAX 4                    Cream                      Small
VAX 5                    Black                      Small
VAX 6                    Red                        Small
ICA Pijao([paragraph])   Black                      Small
  LSD (0.05)

                         Mean bacterial blight score([sections])
Identification             ASP           SRB               MNP

BAC 31                     5.0           8.0               8.0
WBB-20-1                   6.2           4.4               2.5
G 17341                    3.6           3.2               2.3
NY 79-3776-1               3.7           3.4               2.5
Wilk. 2                    3.1           3.3               4.8
XAN 263                    2.8           2.8               4.0
XAN 309                    2.9           2.7               2.5
XAN 328                    4.4           4.2               4.0
XAN 330                    4.8           3.6               4.0
XAN 332                    4.4           3.6               2.5
VAX 1                      3.6           2.6               3.4
VAX 2                      3.0           3.0               3.3
VAX 3                      2.0           2.0               2.4
VAX 4                      1.5           2.4               2.4
VAX 5                      2.6           2.7               2.7
VAX 6                      1.8           2.0               2.4
ICA Pijao([paragraph])     8.7           8.9               8.0
  LSD (0.05)               1.6           1.3               1.6


([dagger]) CIAT = Centro Internacional de Agricultura Tropical; CU = Cornell University; IAPAR = Instituto Agronomico do Parana; ICA = Instituto Colombiano Agropecuario; UPR = University of Puerto Rico.

([double dagger]) I = determinate upright; II = indeterminate upright; and III - indeterminate, prostrate, semiclimbing (Singh, 1982).

([sections]) Mean of three crop seasons, evaluated on a scale of 1 = immune with no visible symptoms; 3 = about 2% leaf surface covered by small CBB lesions; 5 = about 5% leaf surface covered by small and medium-sized CBB lesions; 7 = about 10% leaf area affected; 9 = [is greater than] 25% leaf area diseased (Schoonhoven and Pastor-Corrales, 1987). Inoculation methods: ASP = aspersion, SRB = surgical blade (for leaves only), MNP = multiple needles (for pods only).

([paragraph]) Susceptible common bean cultivar.

Subsequently, concerted efforts to combine different CBB resistance genes were made by R.E. Wilkinson of Cornell University in the 1980s, and at CIAT. Although, the exact pedigree and germplasm used at Cornell University are not known, Wilkinson seems to have combined CBB resistance genes from all three species: P. vulgaris, P. coccineus, and P. acutifolius, including line XAN 159 or its sisters. The three most promising genotypes from this work, G 17341, NY 79-3776-1, and Wilkinson 2, are listed in Table 5.

Table 5 shows that, over the years, steady progress has been made in raising the levels of CBB resistance, and the common bean lines obtained from gene pyramiding at the Cornell University and CIAT possess, by far, the highest levels of CBB resistance. Moreover, VAX 3, VAX 4, and VAX 6 possess levels of CBB resistance that are as high as those found in P. acutifolius accessions. These lines also possess much better tropical adaptation, plant type, and seed color (Table 5).

Transfer of CBB Resistance into Cultivars

For some time, in USA (Coyne and Schuster, 1969, 1970), Brazil (Mohan and Mohan, 1983), and Colombia (Beebe and Pastor-Corrales, 1991), the initial source of CBB resistance, GNN #1 Sel 27, was used for cultivar development. Subsequently, line XAN 159 (or its sisters), and other sources of resistance have been used in breeding programs in Brazil (Rava et al., 1996), Canada (Park and Dhanvantari, 1994), Colombia (Beebe and Pastor-Corrales, 1991), and USA (Arnaud-Santana et al., 1993b). Table 6 summarizes the CBB reaction of some promising common bean lines and cultivars of different commercial classes that were available to us. This level of CBB resistance is comparable with, or slightly better than, those introgressed from P. coccineus and P. acutifolius (Table 4).

Table 6. Origin, growth habit, seed color and size, and common bacterial blight (CBB) reaction of some common bean breeding lines. Three inoculation methods were used; CIAT-Quilichao, Colombia, 1994-1998.
                                                     Growth
Identification            Origin([dagger])   habit([double dagger])

Race Mesoamerica
   A 716                        CIAT                  II
   AND 915                      CIAT                  II
   APN 137                      CIAT                  III
   BAT 93                       CIAT                  II
   G 18484                      GTA                   II
   MUS 105                      CIAT                  II
   XAN 91                       CIAT                  II
   XAN 200                      CIAT                  II
   BAC 14                       IAPAR                 II
   ICA
    Pijao([paragraph])          ICA                   II
Race Durango
   Chase                        USA                   III
   MAM 48                       CIAT                  III
   Pinto UI
    114([paragraph])            USA                   III
Race Nueva Granada
   AFR 603                      CIAT                  II
   AND 1071                     CIAT                  I
   BLM 85                       CIAT                  I
   CAL 123                      CIAT                  I
   DRK 120                      CIAT                  II
   G 4079                       USA                   I
   G 4081                       USA                   I
   G 18168                      HTI                   I
   G 18221                      KYA                   I
   G 20539                      KYA                   II
   SUG 131                      CIAT                  II
   SUG 135                      CIAT                  II
   ZAA 91                       CIAT                  I
   ZAA 93                       CIAT                  I
   Montcalm                     USA                   I
   Diacol
    Calima([paragraph])         CLB                   I
     LSD (0.05)

                                          Seed
Identification                 Color                  Size

Race Mesoamerica
   A 716                  Black                      Small
   AND 915                Cream                      Small
   APN 137                Brown                      Small
   BAT 93                 Beige                      Small
   G 18484                Black                      Small
   MUS 105                Black                      Small
   XAN 91                 Gray                       Small
   XAN 200                Black                      Small
   BAC 14                 Cream striped              Small
   ICA
    Pijao([paragraph])    Black                      Small
Race Durango
   Chase                  Pinto                      Medium
   MAM 48                 Pinto                      Medium
   Pinto UI
    114([paragraph])      Pinto                      Medium
Race Nueva Granada
   AFR 603                Red mottled                Large
   AND 1071               Cream mottled              Large
   BLM 85                 White                      Medium
   CAL 123                Red mottled                Large
   DRK 120                Red                        Large
   G 4079                 Pink                       Large
   G 4081                 Red                        Large
   G 18168                Red                        Large
   G 18221                Red mottled                Large
   G 20539                Cream mottled              Large
   SUG 131                Cream mottled              Large
   SUG 135                Cream mottled              Large
   ZAA 91                 Purple mottled             Large
   ZAA 93                 Purple mottled             Large
   Montcalm               Red                        Large
   Diacol
    Calima([paragraph])   Red mottled                Large
     LSD (0.05)

                          Mean bacterial blight score([sections])
Identification              ASP           SRB               MNP

Race Mesoamerica
   A 716                    5.2           4.0               4.0
   AND 915                  5.6           4.2               7.0
   APN 137                  6.5           4.7               3.5
   BAT 93                   6.8           5.2               4.5
   G 18484                  5.4           3.8               2.5
   MUS 105                  4.8           4.3               2.3
   XAN 91                   6.3           5.2               6.0
   XAN 200                  5.5           3.4               2.5
   BAC 14                   5.7           4.1               2.0
   ICA
    Pijao([paragraph])      8.7           8.9               8.0
Race Durango
   Chase                    6.0           7.8               4.5
   MAM 48                   4.9           6.7               5.8
   Pinto UI
    114([paragraph])        8.0           7.7               6.8
Race Nueva Granada
   AFR 603                  5.4           5.4               2.5
   AND 1071                 6.6           6.2               2.0
   BLM 85                   6.4           5.3               4.6
   CAL 123                  6.1           5.6               8.0
   DRK 120                  5.9           5.3               2.3
   G 4079                   6.2           6.9               8.0
   G 4081                   5.9           6.8               5.0
   G 18168                  6.1           6.8               5.0
   G 18221                  6.5           6.0               7.5
   G 20539                  6.7           6.5               7.5
   SUG 131                  6.3           5.1               7.5
   SUG 135                  5.4           5.4               7.5
   ZAA 91                   4.1           4.0               7.0
   ZAA 93                   6.0           5.9               2.0
   Montcalm                 6.0           8.0               3.0
   Diacol
    Calima([paragraph])     8.0           9.0               8.0
     LSD (0.05)             1.6           1.3               1.6


([dagger]) CIAT = Centro Internacional de Agricultura Tropical; CLB = Colombia; GTA = Guatemala; HTI = Haiti; IAPAR = Instituto Agronomico do Parana; ICA = Instituto Colombiano Agropecuario; KYA = Kenya; and USA = United States of America.

([double dagger]) I = determinate upright; II = indeterminate upright; and III = indeterminate, prostrate, semiclimbing (Singh, 1982).

([sections]) Mean of three crop seasons, evaluated on a scale of 1 = immune with no visible symptoms; 3 = about 2% leaf surface covered by small CBB lesions; 5 = about 5% leaf surface covered by small and medium-sized CBB lesions; 7 = about 10% leaf area affected; 9 = >25% leaf area diseased (Schoonhoven and Pastor-Corrales, 1987). Inoculation methods: ASP = aspersion, SRB = surgical blade (for leaves only), MNP = multiple needles (for pods only).

([paragraph]) Susceptible common bean cultivar.

DISCUSSION

Our comparative study of the four species of Phaseolus in tropical Colombia demonstrated the occurrence of the highest levels of CBB resistance in some P. acutifolius accessions. The levels of CBB resistance in P. coccineus, P. lunatus, and P. vulgaris were intermediate. These findings agree with levels of resistance in these species reported earlier (Coyne and Schuster, 1973; Mohan, 1982). We evaluated only a very small number of accessions of the four Phaseolus species and some P. acutifolius had accessions with much higher levels of CBB resistance than those so far reported and used in common bean breeding programs. A strong justification therefore exists for more systematic and exhaustive screening of all available accessions from wild and cultivated populations of the primary, secondary, and tertiary gene pools of P. vulgaris.

Because Xcp races have been identified from some bean-growing environments (Rava, 1984; Schuster et al., 1973), an international CBB nursery must be established to evaluate pure lined, resistant accessions of different Phaseolus species (and breeding lines and cultivars of common bean) across sites endemic to CBB and in the greenhouse for their reaction in canopy, trifoliolate leaf, and pods, and for their overall usefulness before being used in breeding and genetic studies.

In P. acutifolius, most cultivars are resistant to CBB, whereas most of the wild populations available at CIAT are highly susceptible (CIAT, 1996). This may indicate a founder effect, where most cultigens originate from a few wild populations. As far as we know, only the P. acutifolius landraces PI 319443 and PI 440795 have been tested for allelism; both were found to possess the same CBB-resistant gene (Kolkman and Michaels, 1994). This was further confirmed by the presence of the same RAPD marker in common bean lines XAN 159 and OAC 88-1, derived, respectively, from the two P. acutifolius accessions (P.N. Miklas 1997, personal communication). Thus, a test of allelism against specific Xcp isolates needs to be performed between different CBB-resistant P. acutifolius accessions before using them in interspecific hybridization and gene pyramiding for common bean improvement.

Tables 3, 4, and 6 show that, although the levels of CBB resistance available in landraces of P. vulgaris and P. coccineus have been successfully transferred into cultivated common bean, some P. acutifolius accessions still possess much higher levels of resistance. Thus, in the common bean lines that derived from interspecific hybridization with P. acutifolius, most likely not all the CBB-resistant genes or QTLs were introgressed. Lack of information about the specificity between Xcp isolates and different QTLs conferring resistance makes the task of recovering all QTLs from tepary bean and transferring them to common bean relatively difficult. Thus, the identification and use of tightly linked molecular markers for each QTL should facilitate gene transfer and pyramiding. Moreover, P. acutifolius accessions possessing the highest levels of CBB resistance (e.g., G 40029 and G 40156, Table 3) must be used in interspecific hybridization.

Although interspecific hybridization and gene pyramiding have often been carried out without knowing the underlying genetics of CBB resistance, Table 5 shows that substantial progress has been made for CBB resistance breeding through gene pyramiding. These results, especially the performance of lines VAX 3, VAX 4, and VAX 6 developed at CIAT, suggest that the soundest strategy for breeding for CBB resistance is to pyramid resistance genes from several different Phaseolus species.

Although the initial transfer of partial CBB resistance from tepary to common bean was achieved with lines of commercial or near-commercial seed types (e.g., GNN #1 Sel 27), lines developed from wide crosses and gene pyramiding were often not of commercial seed type (e.g., BAT 93, XAN 91, XAN 159, VAX 1, VAX 4, and VAX 6). Neither did the lines and cultivars presented in Table 6 have the levels of CBB resistance found in pyramided resistant lines (Table 5). This high, pyramided resistance must therefore be transferred to, and combined with, other desirable traits in cultivars of different market classes.

To summarize, breeding for CBB resistance in the tropics can now be expedited by maximizing the use of the best pyramided sources of resistance (Table 5), DNA-based markers (Jung et al., 1996; Nodari et al., 1993), and different inoculation methods.

Problems for CBB Resistance Breeding

By far the most serious problem in CBB resistance breeding has been the instability of resistance (Table 7). Often, after more than a dozen generations of selfing, CBB-resistant lines continue to segregate. As far as we know, no line carrying high levels of CBB resistance is found to be true breeding, despite many generations of selfing and selection under severe disease pressure. Thus, to maintain high levels of CBB resistance, single-plant selections must be made under high disease pressure in each generation. The cause of instability is not known: the CBB-resistant gene(s) may be unstable; or, because three or more genes or QTLs are involved in controlling CBB resistance, the population size and number of single-plant selections made for pure lining are not large enough to provide a line that is homozygous for and contains all CBB resistance genes.

Table 7. Variation observed for common bacterial blight reaction within advanced breeding lines of common bean, CIAT-Quilichao, Colombia, 1994-1998.
Identification   Common bacterial blight score([paragraph])

XAN 309          2, 3, 4, 5, 7
VAX 1            2, 3, 4, 5, 6, 7
VAX 2            2, 3, 4, 5, 7
VAX 3            1, 2, 5, 7
VAX 4            1, 2, 4
VAX 5            1, 2, 3, 5, 7
VAX 6            1, 2, 3, 5, 6


([dagger]) Recorded on single plants: 1 = immune with no visible symptoms; 3 = about 2% leaf surface covered by small CBB lesions; 5 = about 5% leaf surface covered by small and medium-sized CBB lesions; 7 = about 10% leaf area affected; 9 = > 25% area of trifoliolate leaves diseased (Schoonhoven and Pastor-Corrales, 1987). The surgical blade method of inoculation was used.

Another serious problem in breeding for CBB resistance in common bean is the differential CBB reaction of different plant organs (e.g., leaves versus pods), especially in large-seeded germplasm of Andean origin (Table 6; Beebe and Pastor-Corrales, 1991), Differences in leaf and pod reactions to CBB have also been observed in P. acutifolius (Table 3; Zaiter et al., 1989), the species with the highest CBB resistance. Other problems are low correlations of leaf, pod, and seed reactions (Arnaud-Santana et al., 1994); and apparent association of resistance with stages of plant development, indeterminate growth habit, and/or delayed maturity (Coyne and Schuster, 1974).

Breeding Strategies and Methods

Any common bean improvement program for CBB resistance should take into account that (i) different levels of CBB resistance are found in the common bean's primary, `secondary, and tertiary gene pools; (ii) different inoculation methods and bacterial concentrations produce different CBB reactions; (iii) different strains of Xcp and strain-specific resistance are found, at least in temperate environments; (iv) differential CBB reactions occur according to plant organ; (v) CBB resistance may be associated with the stage of plant development, growth habit, and/or maturity; (vi) strain-specific CBB resistance may be controlled by a single recessive gene, single dominant gene, or inherited quantitatively with low to high heritability and involving one to six genes or QTLs; (vii) DNA-based linked markers are available for some QTLs responsible for CBB resistance; and (viii) CBB reaction is also affected by photoperiod and temperature.

CBB resistance is often screened by artificial wounding methods and higher Xcp populations than probably occur naturally in bean-growing environments. Also difficult to do is directly transferring CBB resistance from exotic sources into cultivars of different market classes and combining it with other desirable agronomic traits. Thus, all available sources of CBB resistance would need to be evaluated against the prevalent strains of Xcp by appropriate inoculation methods and bacterial concentrations. For integrated genetic improvement (Singh, 1997), parental recombination and selection activities may need to be divided into (i) introgression of CBB-resistance genes from common bean's primary (distantly related landraces and wild populations), secondary, and tertiary gene pools; (ii) parental development and breeding for CBB resistance per se (i.e., broadening the genetic base, gene pyramiding, and character improvement); and (iii) cultivar development, that is, simultaneous selection for multiple qualitative and quantitative traits for specific market classes and bean production regions from elite x elite populations (Kelly et al., 1998).

Introgression of CBB Resistance

Often separate hybridization and selection programs would be required to transfer CBB resistance from each wild population or distantly related landrace of common bean, and from the secondary and tertiary gene pool accessions. Crosses with the tertiary gene pool require embryo rescue. Recurrent and congruity backcrossings (Mejia-Jimenez et al., 1994), followed by at least one or more generations of inbreeding, are needed before evaluation and selection for CBB resistance can begin.

Parental Development and Breeding for CBB Resistance Per Se

Different sources of CBB resistance available (e.g., G 1320, G 4399, G 6772) or introgressed (e.g., XAN 91) from the primary, secondary (e.g., XR-235-1-1, ICB 3, TARS VCI-4B), and tertiary (GNN #1 Sel 27, OAC 88-1, XAN 159) gene pools to common bean need to be systematically combined to pyramid different genes for CBB resistance. The genetic base would thus be broadened, and the levels and durability of resistance increased. Different CBB resistance genes within each species may need to be combined first, followed by that between P. vulgaris and P. coccineus genes because they possess comparatively lower levels of CBB resistance. Finally, this pyramided resistance would need to be combined with that of P. acutifolius sources. Access to the widest range of pathogenic variation in Xcp, availability of tightly linked molecular markers to CBB resistance genes from different sources, and recurrent selection would greatly aid this task.

Cultivar Improvement

The highest levels of CBB resistance available in pyramided lines would need to be combined with other major desirable traits such as seed size and color, canning and cooking quality, growth habit, maturity, and resistance to other biotic and abiotic stresses for specific groups of cultivars for each major bean-growing environment. The levels of CBB resistance in cultivars that would suffice under field conditions should be determined a priori. Multiple-parent elite x elite crosses within a gene pool and races would need to be made for simultaneous selection and improvement of all desirable traits. Marker-assisted gamete (Singh, 1994, 1998), [F.sub.2]-derived family, and/or single-seed descent (Urrea and Singh, 1994) breeding methods could be used for this purpose. The choice of breeding method would depend largely on the resources and facilities available, and the resistance level required. While making multiple-parent crosses, breeders should ensure a sufficient genetic contribution of the CBB-resistant parents so that a comparatively larger frequency of desirable recombinants with CBB resistance are available in segregating populations and families. Thus, in a multiple-parent cross (three or more parents), preferably two CBB-resistant parents from different sources are used (Table 8). Moreover, donor parents for CBB resistance and all other desirable traits should be similar to the commercial cultivar under improvement in seed color and size, growth habit, maturity, and adaptation (Kelly et al., 1998). Recovering lines with high CBB resistance is often difficult when and if the genetic contribution of its donor parents is [is less than or equal to] 25% in a multiple-parent cross (Table 8).

Table 8. Number of common bean [F.sub.1]-derived [F.sub.2] families that were resistant, intermediately resistant, or susceptible to common bacterial blight (CBB). The families were derived from populations obtained when using 0, 1, 2 or 5 CBB-resistant parents. The range and mean values of these families reaction to CBB are also given; CIAT-Quilichao, Colombia, 1994-1998.
                                          No. of [F.sub.1]-derived
                                             [F.sub.2] families
Population identification      No. of
                                CBB-
                               resistant
                               parents     Resistant   Intermediate

Catrachita/J 117//
  A 429/EMP 473                    0            0           0
Catrachita/G 2883//
  G 3017/Othello                   0            0           0
Catrachita/RM 35//
  G 17341/De Celaya                1            0           6
XAN 309/A 193//MAR 3/G 5653        1            0           9
MAM 38/G 17341//J 117/
  XAN 159                          2            0          13
VAX 2///A 429/J 117//
  G 17341/G 3017                   2            1          43
VAX 1///J 117///PVPA
  9576-14/XAN 159//
  PVPA 9576-21/G 17340             5           11          27
VAX 1////SEA 7///XAN 330/
  XAN 265//PVPA 9576-21/
  G 17341                          5           56         125

                                No. of
                            [F.sub.1]-derived           CBB
                            [F.sub.2] families     scores([dagger])

Population identification   Susceptible   Total    Range    Mean

Catrachita/J 117//
  A 429/EMP 473                 29         29        7-9     8.0
Catrachita/G 2883//
  G 3017/Othello                88         88        7-9     8.3
Catrachita/RM 35//
  G 17341/De Celaya             21         27        5-9     7.2
XAN 309/A 193//MAR 3/G 5653     12         21        4-8     6.7
MAM 38/G 17341//J 117/
  XAN 159                        1         14        3-7     5.5
VAX 2///A 429/J 117//
  G 17341/G 3017                18         62        3-9     5.9
VAX 1////J 117///PVPA
  9576-14/XAN 159//
  PVPA 9576-21/G 17340           9         47        2-8     4.8
VAX 1////SEA 7///XAN 330/
  XAN 265//PVPA 9576-21/
  G 17341                        7        188        3-8     4.3


([dagger]) Scored on a 1 = immune with no visible symptoms; 3 = about 2% leaf surface covered by small CBB lesions; 5 = about 5% leaf surface covered by small and medium-sized CBB lesions; 7 = about 10% leaf area affected; 9 = > 25% diseased leaf canopy (Schoonhoven and Pastor-Corrales, 1987).

The aspersion method of inoculation was used.

Because of the relatively larger costs and resources required for inoculating trifoliolate leaves with razors or surgical blades, or multiple or hypodermic needles, sequential screening methods, like those currently in use at CIAT, can be adopted: all early generation, large-scale, field-grown, segregating populations and families should be initially inoculated by aspersion for canopy and pod evaluations. The trifoliolate leaves of advanced generation families and lines uniform for CBB resistance in canopy and pods should then be inoculated with either razors, surgical blades, or multiple needles. Those still resistant should then be screened for pod reaction to CBB against specific Xcp strains in both field and greenhouse, using any of the inoculation methods used for trifoliolate leaf screening, but preferably multiple needles. Greenhouse screening would be required when and if the field screening is not reliable and chances for escapes are real, and when screening against Xcp strains originating from outside the bean production region. Under the warm tropical environments of Colombia, greenhouse screening is not required.

For simultaneous selection for other desirable characters, including resistance to other diseases, insects, and abiotic stresses, a separate series of complementary nurseries may need to be grown, either at the same location or at different locations, and in the same or different cropping seasons (Singh et al., 1998). For example, for leafhopper (Empoasca kraemeri Ross & Moore) screening, warm dry environments are required, whereas the development and spread of CBB in breeding nurseries require warm temperatures, high relative humidity, intermittently dry and wet weather, and high winds.

ACKNOWLEDGMENTS

We thank the Instituto Colombiano para el Desarrollo de la Ciencia y Tecnologia "Francisco Jose de Caldas" (COLCIENCIAS) and the Inter-American Development Bank (IDB) for the scholarship awarded to Carlos German Munoz. We also thank Marcial A. Pastor-Corrales and Carlos Jara for bacterial inocula; Isabel Cristina Giraldo and Patricia Montoya for typing the manuscript, and Elizabeth de Paez for editorial assistance.

Abbreviations: CBB, common bacterial blight; cfu, colony forming units; GNN #1 Sel 27, Great Northern Nebraska #1 Selection 27; QTLs, quantitative trait loci; Xcp, Xanthomonas campestris pv. phaseoli.

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Shree P. Singh(*) and Carlos G. Munoz

S. Singh, Univ. of Idaho, 3793 North 3600 East; Kimberly, ID 83341-5076; C.G. Munoz, International Center for Tropical Agriculture (CIAT), A.A. 6713, Cali, Colombia. Received 23 Feb. 1998. (*) Corresponding author (singh@kimberly.uidaho.edu).
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Date:Jan 1, 1999
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