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Crop Science 40:1565-1572 (2000)
© 2000 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Selection for Bean Golden Mosaic Resistance in Intra- and Interracial Bean Populations

Shree P. Singha, Francisco J. Moralesb, Phillip N. Miklasc and Henry Teránb

a Univ. of Idaho, 3793 North 3600 East, Kimberly, ID 83341-5076, USA
b International Center for Tropical Agriculture (CIAT), A.A. 6713, Cali, Colombia
c USDA-ARS-IAREC, 24106. Bunn Rd., Prosser, WA 99350, USA

singh{at}kimberly.uidaho.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Bean golden mosaic (BGM) resistance within any one bean (Phaseolus vulgaris L.) race is inadequate. Pyramiding genes from different races would increase resistance and reduce pesticide use. Our objectives were to: (i) pyramid and compare BGM resistance of genotypes from intraracial versus interracial populations, (ii) verify pyramided resistance by means of molecular markers, and (iii) combine BGM resistance with resistance to other diseases. One intraracial (GV 10625) and four interracial (DC 10622, GV 10624, GV 10626, and GV 10627) populations were developed at CIAT. Plants within each F1-derived F2 (F1:2) were advanced in bulk. The F3 was grown in the field under common bacterial blight [CBB, caused by Xanthomonas campestris pv. phaseoli (Smith) Dye] and angular leaf spot [ALS, caused by Phaeoisariopsis griseola (Sacc.) Ferraris] pressure. Single plant selections were made in the F3 and F4. Plants within F4:5 were harvested in bulk to screen for ALS, bean common mosaic (BCM), BGM, and CBB. The 39 selected genotypes, 12 parents, and six checks were again evaluated for the four diseases. Seventeen genotypes, parents, and checks also were screened for the RAPD marker OR2530 linked with bgm-1 gene and the SCAR marker SW12700 linked with a QTL controlling BGM resistance. Selected genotypes differed in their BGM reaction. None of the genotypes from the intraracial population were resistant to BGM. Five genotypes with the highest resistance were obtained from GV 10626 and GV 10627. DC 10622 and GV 10624 produced genotypes with intermediate resistance. Three genotypes from DC 10622 and four from GV 10627 possessed OR2530 marker linked with resistance derived from `Garrapato' and SW12700 marker linked with resistance derived from `Porrillo Sintetico'. All genotypes possessed the I gene for BCM and were intermediate to ALS. Five also were intermediate for CBB. Combined use of interracial populations, disease screening, and molecular markers should be emphasized for resistance breeding.

Abbreviations: ALS, angular leaf spot • BCM, bean common mosaic • BGM, bean golden mosaic • CBB, common bacterial blight • QTL, quantitative trait loci • RAPD, random amplified polymorphic DNA • SCAR, sequence characterized amplified region


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
BEAN GOLDEN MOSAIC (caused by a geminivirus) (Goodman and Bird, 1978) was reported as a new threat to bean (Phaseolus vulgaris L.) production in Brazil (Costa, 1965), the Caribbean (Bird et al., 1973), and Central America (Gálvez and Cárdenas, 1980; Gámez, 1971) in the 1960s and 1970s. By the early 1980s, the disease had not only spread northward along the Atlantic and Pacific coasts (Chiapas, Veracruz, Sinaloa, and Sonora) of Mexico (López-Salinas and Becerra, 1994) and to northwestern Argentina but had begun to cause severe (up to 100%) yield losses (Costa and Cupertino, 1976; Gálvez and Morales, 1989). Hundreds of thousands of hectares either were abandoned or could not be cultivated without the heavy use of systemic insecticides such as carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate) and phorate (O,O-diethyl S-ethylthiomethyl phosphorodithioate). This situation was most serious in traditional bean production regions of Brazil (e.g., Goias, Minas Gerais, Parana, and São Paulo) and Argentina (e.g., Jujuy, Salta, Santiago de Estero, and Tucumán) (Gálvez and Morales, 1989; Morales, 1994b; Vizgarra et al., 1992). Production costs were elevated substantially and it imposed serious health hazards to growers and field laborers throughout the region. In the 1990s, the disease also was reported in Florida, USA (Blair et al., 1995) and Santa Cruz de la Sierra, Bolivia (Morales, 1994b).

The occurrence and severity of BGM in Latin America is believed to be associated with the introduction and expansion of soybean [Glycine max (L.) Merr.] cultivation in the region, intensified and continuous bean cultivation, and cultivation of crops such as cotton (Gossypium hirsutum L.), tobacco (Nicotiana tabacum L.), and tomato (Lycopersicon esculentum Mill.) in nearby fields (Morales, 1994a). Prolonged dry and relatively warm (24 to 28°C mean growing temperature) weather favors the multiplication of the insect vectors, the sweet potato whitefly (Bemisia tabaci Genn.) and the silver-leaf whitefly (Bemisia argentifolii Bellows & Perring) (Goodman and Bird, 1978). For some unknown reasons, however, BGM incidence is minimal in southern Brazil (above 25°S latitude) and it has not bean reported in Chile, Peru, Paraguay, Uruguay, Ecuador, Panama, and Canada.

Plant age at the time of infection and bean cultivar grown also are among the primary determinants of disease incidence and severity. Morales and Niessen (1988) found that infection late in the season resulted in lower disease incidence and severity. They also noted that disease symptoms include plant dwarfing, plant malformation, leaf chlorosis (yellowing or mosaic), flower and pod abortion, pod malformation, and reduction in seed quality and yield. Howarth et al. (1985) and Gilbertson et al. (1993) sequenced the geminivirus causing bean golden mosaic. Gilbertson et al. (1991, 1993) and Maxwell et al. (1994) reported genetic diversity among bean infecting whitefly transmitted geminivirus from Mexico, the Caribbean, and Central and South America. Gilbertson et al. (1991, 1993) also observed 60 to 95% nucleic acid sequence homology between the geminivirus isolates from these regions. Nonetheless, a differential reaction of the virus isolates on bean genotypes remains to be demonstrated.

The search for sources of resistance began simultaneously with the appearance of the disease in Brazil, the Caribbean, Central America, and Mexico. Genotypes such as Porrillo Sintetico and Turrialba 1 were found to be resistant to infection, plant dwarfing, and delayed expression of leaf chlorosis as plants grew older (Beebe and Pastor Corrales, 1991; Gálvez and Cárdenas, 1980). These genotypes are characterized by small-seeds, indeterminate, upright type II growth habit, I gene resistance to BCM, and are derived from the race Mesoamerica (Singh et al., 1991). This resistance was bred into other small-seeded susceptible genotypes of black, cream, cream striped, pink, and red colored bean (Beebe, 1994; Beebe and Pastor Corrales, 1991; Bianchini, 1999). Among the most popular genotypes possessing this resistance are `DOR 41' in Argentina, `Dorado' (or DOR 364) in Honduras, `ICTA Ostua' and `ICTA Quetzal' in Guatemala, and `Negro Huasteco' in Mexico (Beebe, 1994; Beebe and Pastor Corrales, 1991; López-Salinas and Becerra, 1994). Subsequently, it was realized that under severe disease pressure, this resistance was not adequate, plants showed leaf chlorosis and yields were drastically reduced (CIAT, 1986). However, P. coccineus L. accessions, such as N-15 (or G 35171) and N-16 (or G 35172), were found to be immune under similar BGM pressure in fields in Brazil (S.T. Mohan, personal communication, 1982, 1999) and Guatemala (Beebe and Pastor Corrales, 1991; CIAT, 1986). Subsequently, F.J. Morales at CIAT in Colombia verified the BGM resistance of G 35171 and G 35172 in the greenhouse inoculations (F.J. Morales, 1994, unpublished data).

Discovery, albeit inadvertently, of bean genotype A 429 and its sister selections (e.g., A 430 and A 431), possessing extremely high level of resistance to BGM in the early 1980s, was a major milestone (CIAT, 1986). By systematic screening of parents of A 429, Morales and Niessen (1988) identified the original source of A 429 resistance to leaf chlorosis (i.e., medium-seeded race Durango landrace cultivar Garrapato, also known as G 2402). They also reported new sources of BGM resistance for each of its components (infection, plant dwarfing, leaf chlorosis, flower abortion, pod deformation, and seed yield) in other bean races of the Andean and Middle American gene pools (Singh et al., 1991).

Inheritance (using a quantitative genetic model) of components of BGM resistance and combining ability among selected bean genotypes belonging to races Durango, Mesoamerica, and Nueva Granada was reported (Morales and Singh, 1991; Vizgarra et al., 1992). However, subsequent use of controlled environments and more systematic genetic studies lead to the unequivocal identification of two independent recessive genes, bgm or bgm-1 (Blair and Beaver, 1993; Urrea et al., 1996), and bgm-2 (Velez et al., 1998), controlling resistance to leaf chlorosis. Gene bgm-1 was initially identified in A 429 and traced back to Garrapato and bgm-2 was identified in DOR 303. One recessive gene dwf for resistance to plant dwarfing was found in XAN 176 (Velez et al., 1998). Molina Castañeda and Beaver (1998) reported a dominant gene Bgp for resistance to pod deformation that seemed to require bgm-1 for expression. In addition, two major and one minor quantitative trait loci (QTL) conditioning reduced and delayed onset of leaf chlorosis were reported in DOR 364 (Miklas et al., 1996). The major QTL in DOR 364 traced back to Porrillo Sintetico, and the minor QTL appeared to be derived from Honduras 46. A RAPD marker tightly linked with bgm-1 (Urrea et al., 1996) has facilitated indirect selection for BGM resistant germplasm in the absence of virus pressure and has minimized the need for maintaining and using whitefly colonies (McMillan et al., 1998; Stavely et al., 1996). The utilization of RAPD markers linked with QTL from DOR 364 for indirect selection of BGM resistance is still being explored (S. Beebe and P. Miklas, 1999, personal communication).

The importance of pyramiding complementary genes from bean races Durango and Mesoamerica for higher resistance to infection, leaf chlorosis, and pod deformation caused by the BGM virus was unequivocally demonstrated (Morales and Singh, 1993). Moreover, the highly resistant genotypes, such as `Don Silvio' (DOR 482), `Tio Canela 75', and `Turbo III' developed for production in Central America to date combine resistance from A 429 and DOR 364. This also provided further support for developing genotypes with even higher BGM resistance than found in A 429 and DOR 364 from interracial and inter-gene pool crosses. Thus, this study was initiated at CIAT, Cali, Colombia, in 1991-1992. We had the following three objectives. First, systematically pyramid and compare BGM resistance from the common bean races of Andes (race Nueva Granada) and Middle America (races Durango and Mesoamerica). Second, verify pyramided resistance with linked RAPD and SCAR markers. And third, combine BGM resistance with resistance to ALS, BCM, and CBB.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Two single (GV 10625 and GV 10626), one double (GV 10624), and two multiple-parent (DC 10622 and GV 10627) crosses were made by means of hand emasculation and pollination at CIAT in 1991 to 1992. The two parents involved in intraracial cross GV 10625 = Porrillo Sintetico/XAN 263 (Table 1) belonged to the small-seeded bean race Mesoamerica (Singh et al., 1991). Porrillo Sintetico was reported to be tolerant to BGM (Gálvez and Cárdenas, 1980) and XAN 263 resistant to CBB (Singh and Muñoz, 1999) and both possessed I gene resistance to BCM. Of the two parents involved in inter-gene pool cross GV 10626 = `Royal Red'/Garrapato, the large-seeded Royal Red belongs to race Nueva Granada and medium-seeded Garrapato to race Durango. The single cross between races Mesoamerica and Durango was not made because we had already pyramided BGM resistance from such a cross (Morales and Singh, 1993). The double cross GV 10624 = A 429/XAN 263//Royal Red/Garrapato and the two multiple-parent crosses, DC 10622 = `Sierra'/3/OAC 88-1/`Jatu Rong'//OAC 88-1/Garrapato and GV 10627 = Turbo III/3/XAN 132/Garrapato//DOR 303/RXAH 18274C, were both interracial as well as inter-gene pool crosses. Each of these possessed genotypes with characteristics of all three races, namely, Durango, Mesoamerica, and Nueva Granada (Table 1). Also, all crosses except GV 10626 involved one or more parents tolerant to CBB.


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  		Table 1 Characteristics of 12 parents used in five bean populations for selection for resistance to bean golden mosaic, bean common mosaic, common bacterial blight, and angular leaf spot

 
Approximately 75 F1 seeds were produced for each cross. Seed from each F1 plant was harvested, threshed, cleaned, and kept in a separate envelope. The F1:2 families were grown in a single row plot, each 3 m long, spaced 60 cm apart in the field at CIAT-Palmira (soil was a fine silty, mixed isohypothermic, Aquic Hapludoll, with a pH of 7.5; 24°C mean growing temperature). The nursery was periodically sprayed with pesticides to protect from diseases and insects. Plants within a row were harvested in bulk. The F3 was grown at CIAT-Quilichao (soil was a very fine kaolinitic, isohypothermic, plinthic Kandiudox, with a pH of 4.5; 24°C mean growing temperature). The nursery was inoculated with the bacterial isolate Xcp 123 causing CBB with a backpack sprayer three times, beginning 3 wk after sowing, at an interval of 8 to 10 d. A bacterial concentration of 107 to 108 colony forming units mL-1 was used. Similarly, it was also inoculated three times with a mixture of the pathogen isolates Pg 3 of Andean and Pg 15, Pg 32, Pg 61, and Pg 65 of Middle American origins (2 x 104 conidia per mL) causing ALS, collected locally in the previous cropping season. All families (except in GV 10626) uniformly susceptible (scores of 7 or higher, van Schoonhoven and Pastor-Corrales, 1987) to ALS and CBB were discarded. Plants (an average of 80 plants per population) intermediate (scores of 4–6) or resistant (score of 3 or lower) to either disease alone or both diseases were harvested individually. No selection for growth habit, maturity, seed characteristics or any other trait was practiced. The F4 plant-to-progeny rows were grown at CIAT-Palmira. From each family, a single plant was harvested and its seed increased in F5 at the same location.

The 12 parents involved in all five populations, six checks, and F4:6 genotypes (an average of 70 genotypes/population) were grown in the greenhouse at CIAT-Palmira in 1995. Ten seeds, each in a separate 76-mm polystyrene cup, were grown for each entry. A randomized complete block design with two replications was used. Each plant was mechanically inoculated with the Guatemalan strain of BGM virus at the primary leaf stage, as described by Morales and Niessen (1988). Data were recorded for infection, leaf chlorosis, and pod deformation. For the latter two traits, an evaluation scale of 1 to 9, where 1= symptomless; 2 and 3 = low symptom expression; 4, 5 and 6 = intermediate symptom expression; and 7, 8 and 9 = high symptom expression, was used for data recording. Selected genotypes from each population (a total of 39 including all 17 reported in this article), 12 parents, and six checks were again evaluated for BGM in the greenhouse at CIAT-Palmira in 1998. Experimental design, inoculation, and evaluation methods were the same as in the previous experiment. Moreover, they were also evaluated for BCM with the NL3 necrosis inducing strain (Drijfhout, 1978) in the greenhouse at CIAT-Palmira and for ALS and CBB in the field at CIAT-Quilichao. For BCM, entries were rated as resistant (i.e., possessing I gene), susceptible or variable. For ALS and CBB an evaluation scale of 1 to 9, as described above for BGM, was used (van Schoonhoven and Pastor-Corrales, 1987).

The extraction of DNA from bean genotypes followed the mini-prep procedure of Afanador et al. (1993). Amplification and visualization of the codominant RAPD marker OR2530 linked with bgm-1 followed PCR and electrophoresis protocols of Urrea et al. (1996). The OR2530 is linked in coupling with the resistant gene bgm-1. A SCAR was developed for the W12700 RAPD marker linked with a QTL for delayed leaf chlorosis identified in DOR 364 (Miklas et al., 1996) because the RAPD was not readily amplified across different laboratories. The RAPD was converted to a SCAR marker by the TA cloning kit (Invitrogen, Carlsbad, CA). The renamed SCAR SW12700 was amplified by a pair of 24-bp primers (5' TGG GCA GAA GTT CTA GCA TGT GGC 3' - forward; 5' TGG GCA GAA GCA CAG TAT GAT TTG 3' - reverse). A profile of 1 cycle of 60 s at 94°C; 30 cycles of 30 s at 94°C, 30 s at 72°C, and 60 s at 72°C; followed by 1 cycle of 5 min at 72°C was used. All genotypes in this study were assayed for the OR2530 RAPD and SW12700 SCAR markers. A SAS statistical package (SAS Institute Inc., 1985) was used for data analysis.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
The range and mean for infection, leaf chlorosis, and pod deformation caused by BGM for 12 parents and one intraracial and four interracial populations are given in Table 2 . From these values and from the reaction of BGM resistance donor parents representing each of the three bean races namely, Durango (Garrapato), Mesoamerica (Porrillo Sintetico), and Nueva Granada (Jatu Rong and Royal Red) (Table 3 ; Morales and Niessen, 1988), it is evident that use of only one source of resistance in intraracial populations would not be adequate for developing BGM resistant genotypes. Genetic variability within bean races for most characters, including seed yield, is limited (Singh et al., 1989). Thus, even the selected genotypes with the lowest scores from the small-seeded intraracial (within race Mesoamerica) population GV 10625 were still susceptible to one or more BGM reactions. Even when intensive and repeated cycles of selection were practiced under BGM pressure in fields in Central America genotypes derived from such intraracial populations at best were intermediate as evidenced from BGM scores of DOR 390, DOR 391, and DOR 500 (Table 3). This may also explain why the resistance of cultivars such as Dorado, ICTA Ostua, and Negro Huasteco, developed from similar intraracial populations, was inadequate under severe BGM pressure in Central America and elsewhere (Morales, 1994a; Morales and Niessen, 1988).


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  		Table 2 Range and mean for the 12 parents and F4-derived F8 genotypes from intraracial (GV 10625) and interracial (DC 10622, GV 10624, GV 10626, and GV 10627) bean populations for infection, leaf chlorosis, and pod deformation caused by bean golden mosaic virus evaluated in the greenhouse at CIAT, Cali, Colombia, in 1998

 

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  		Table 3 Reaction of parents, selected F4-derived F8 genotypes from intraracial (GV 10625) versus interracial (DC 10622, GV 10624, GV 10626, and GV 10627) bean populations, and checks for angular leaf spot, bean common mosaic, common bacterial blight, and infection, leaf chlorosis, and pod deformation caused by bean golden mosaic evaluated at CIAT, Cali, Colombia, in 1998

 
In contrast, genotypes with highest BGM resistance were developed from the interracial populations GV 10626 and GV 10627. GV 10626 is a single-cross population between race Durango of Middle America and race Nueva Granada of the Andean gene pool. Similarly, GV 10627 is an interracial and inter-gene pool population involving genotypes from races Durango, Mesoamerica, and Nueva Granada. Complementation of BGM resistance genes between races Durango (Garrapato) and Nueva Granada (Royal Red) may be seen from population GV 10626. Despite the fact that the two parents showed intermediate or susceptible reaction, two selected genotypes possessed the lowest values for infection, leaf chlorosis, and pod deformation caused by BGM (Table 3). Morales and Singh (1993) reported complementation for BGM resistance between races Durango (`UI 114') and Mesoamerica (`ICA-Pijao'). Thus, pyramiding resistance through interracial and inter-gene pool hybridization would be the most appropriate strategy for increasing the level of resistance and broadening the genetic base of BGM resistance in common bean.

Jatu Rong, OAC 88-1, and Royal Red lacked the molecular marker OR2530 linked to the recessive BGM resistance gene bgm-1 and SW12700 marker linked to a QTL for BGM resistance (Table 3). Similarly, both markers were absent in DOR 390, DOR 500, and XAN 309, and the BGM susceptible check Top Crop. The OR2530 marker was present in Garrapato (race Durango from Mexican highland) and its derived genotypes such as A 429, DOR 482, and Turbo III. It has also been used successfully to introgress resistance into snap bean (Stavely et al., 1996). The SCAR marker SW12700 was found in Porrillo Sintetico and DOR 303, Sierra, RXAH 18274C, XAN 132, and XAN 263. Pedigree analysis of these parents and the BGM resistant genotypes developed in this study support the suggestion by Miklas et al. (1996) that the origin of the SW12700 marker and linked QTL for resistance is the small-seeded race Mesoamerica (e.g., cv. Porrillo Sintetico). We also found the SW12700 marker present in Dorado (DOR 364) and in ICA Pijao that has Porrillo Sintetico in its pedigree.

Of 17 selected genotypes, seven possessed both molecular markers, two only possessed SW12700, four possessed OR2530, and one was segregating for OR2530. Considerable variation for infection, leaf chlorosis, and pod deformation was observed within and between each of these groups of genotypes. From the presence and absence of the two markers, and BGM reaction of 17 selected genotypes, parents, and checks it appears that the bgm-1 gene linked to the OR2530 marker was more effective in reducing infection, leaf chlorosis, and pod deformation caused by BGM than the QTL linked to the SW12700 marker. Moreover, the latter alone may not provide adequate protection under severe BGM pressure. The effectiveness of bgm-1 was increased in the presence of the QTL indicated by SW12700 in three lines selected from GV 10627 and check DOR 482. Use of race Durango resistance may be important in interracial and inter-gene pool populations. Indirect support for this could be obtained from examining BGM reaction of DOR 303 developed from a population between races Mesoamerica and Nueva Granada. DOR 303 was susceptible to pod deformation and was at best intermediate for infection and leaf chlorosis (Table 3). Because genotypes possessing the highest resistance were developed from crosses of race Durango with races Mesoamerica (Morales and Singh, 1993) and Nueva Granada (this study), the BGM resistance genes found in race Durango are complementary to those present in the two latter races. Moreover, this may be additional evidence of their different evolutionary origins (Singh, 1989; Singh et al., 1991; Gepts et al., 1986). Attempts need to be made to identify complementary genes or QTL and tightly linked molecular markers for BGM resistance present in Royal Red and other Andean germplasm.

The extent of diversity and complementation between genes involved in inheritance of each of the three components of BGM resistance reported here is unknown. Similarly, the additional importance of the population involving resistance genes from all three common bean races (e.g., cross GV 10627) could not be determined because only one isolate of the BGM virus was used in the greenhouse inoculations. Perhaps extensive testing of these genotypes in different bean production regions severely affected by BGM could aid in this process. Use of tightly linked markers (Miklas et al., 1996; Urrea et al., 1996) for target genes found in different bean races should facilitate both determination of their genetic divergence and value for pyramiding complementary genes. Moreover, identification of molecular markers tightly linked to each of the other components of BGM resistance traits (Molina Castañeda and Beaver, 1998; Morales and Niessen, 1988; Velez et al., 1998) should also receive priority.

One selected genotype from GV 10627 and three genotypes from DC 10622 despite possessing both molecular markers had only intermediate resistance to BGM. In contrast, two genotypes from GV 10626 despite possessing only OR2530 marker found in A 429 and Garrapato exhibited the highest BGM resistance. This would suggest that there are additional genes or QTL (probably not found in Prorrillo Sintetico), albeit with small effects, that are complementary to the major genes found in Garrapato. These unknown genes impart partial resistance to infection, leaf chlorosis, and pod deformation among other traits including plant dwarfing and flower and pod abortion. This is supported by the BGM reaction of Royal Red, DOR 390, DOR 500, and XAN 309, each of which despite not carrying either molecular marker exhibited intermediate reactions to one or more components of BGM resistance. These quantitative genes seem to be present in both Andean (e.g., Royal Red) and Middle American gene pools. These need to be combined with major genes and QTL such as those linked with markers OR2530 and SW12700, Bgp for pod deformation (Molina Castañeda and Beaver, 1998), dwf for plant dwarfing (Velez et al., 1998), and others. However, despite the report of these major independent genes controlling different BGM reactions, it is not clear if any of these genes have or do not have pleiotropic effects on one or more BGM reactions. Our data indicate that the major BGM resistance gene bgm-1 linked to OR2530 marker imparts resistance to infection, leaf chlorosis, and pod deformation.

The utility of the SW12700 SCAR marker for indirect selection of the resistance QTL (identified in DOR 364 and present in Porrillo Sintetico) across different genetic backgrounds has not been extensively studied. However, a combination of direct disease screening and use of markers such as OR2530 and SW12700 should facilitate and expedite gene pyramiding and broadening BGM resistance. As noted previously (Urrea et al., 1996), the OR2530 RAPD marker was only observed in BGM resistant genotypes which possessed bgm-1 source germplasm as part of the pedigree, as no susceptible genotypes with the resistance-linked fragment have been observed yet outside the original mapping population.

Pyramiding high levels of BGM resistance from different sources and its combination with resistance genes for other diseases (e.g., ALS, BCM, and CBB) can be problematic if care is not taken in selecting and assuring adequate genetic contribution of resistant donor parents. For example, in population DC 10622 all complementary BGM resistant genes from the two donor parents (Garrapato and Jatu Rong) were not recovered and the three selected genotypes carried only intermediate resistance to each component trait. Similarly, from population GV 10624 all complementary BGM resistance genes from A 429, Garrapato, and Royal Red could not be recovered, despite all three selected genotypes from DC 10622 possessing both markers. This probably occurred because no selection for BGM resistance was practiced between the F1 and F5. The OR2530 and SW12700 markers were not available when this study was initiated and it was not practically feasible to screen large populations in segregating generations in the greenhouse using mechanical inoculations. Moreover, the joint contribution of the two BGM resistance donor genotypes in the multiple-parent interracial population DC 10622 was only 25% and the susceptible genotypes Sierra (the last female parent of the cross) and OAC 88-1 contributed 75% of the genes. In contrast, in population GV 10627, although the contribution of two potential BGM resistance donor genotypes (Garrapato and DOR 303) was similar, but because the last female parent Turbo III contributing 50% of the genes was intermediate and carried the two major genes for BGM resistance, three genotypes with extremely high level of resistance were identified. Turbo III was derived from a single-cross population between XAN 112 (race Mesoamerica) and A 429 (race Durango). Thus, it may be advisable to first pyramid BGM resistance from races and gene pools into specific market classes of bean (i.e., parental germplasm development). Because epistatic interactions among the different resistance sources exist, it becomes difficult with direct disease screening to know exactly which sources of resistance are combined in improved genotypes. Combining and retaining resistance genes will become easier once tightly linked molecular markers become available for the different resistance sources, in addition to OR2530 for bgm-1 and SW12700 for the QTL. For example, genotypes are directly selected for resistance to chlorosis and pod deformation in disease nurseries in the greenhouse or field while simultaneously maintaining heterozygosity for Bgm-1/bgm-1 by indirect selection of both alleles of the codominant RAPD marker OR2570/530. This is necessary (or effective) because other resistances can be difficult to detect in a homozygous bgm-1 background. Upon selfing these genotypes and then selecting individual plants for the OR2530 marker, bgm-1 can be effectively pyramided with other resistance genes and QTL (Urrea et al., 1996).

Extremely high levels of resistance to BGM were reported in the secondary gene pool of common bean (P. constaricensis, P. coccineus, and P. polyanthus) (CIAT, 1986). Bean genotypes MD 820 and MD 829 with high levels of resistance to leaf chlorosis and plant malformation and stunting were recently developed from initial P. vulgaris x P. coccineus cross (Bianchini, 1999). BGM resistance from these genotypes and resistance from other accessions of the secondary gene pool should be introgressed and pyramided with high levels of resistance already accumulated in the five selected common bean genotypes developed in this study. Subsequently, these highly resistant and well-adapted genotypes should be crossed with similar genotypes developed for other traits (i.e., elite x elite crosses) within the same market class for cultivar development (Kelly et al., 1998).

Although an average of 75 F1-derived families were developed for each population, and not all genotypes were evaluated for BGM reaction, all BGM resistant genotypes or genotypes with the lowest scores originated from a single F1-derived family in populations DC 10622, GV 10624, and GV 10625. Furthermore, only three F1-derived families produced all highly resistant genotypes in populations GV 10626 and GV 10627. Had selection been practiced in the F1 using OR2530 for bgm-1 and SW12700 for the QTL or in F2 for markers and/or BGM resistant phenotypes these F1-derived families could have been identified early, and all susceptible families could have been eliminated. This emphasizes the value of gamete selection in the multiple-parent F1 and their derived families in early generations for dominant traits and dominant and codominant molecular markers linked to the desired dominant or recessive genes such as bgm-1 (Singh, 1994; Singh et al., 1998).

Selected genotypes highly resistant to BGM derived from different interracial and inter-gene pool populations possessed different flower color, growth habit, maturity, seed color, and seed size. Moreover dry bean genotypes of cream striped (Singh et al., 1998), red (DOR 482, Tio Canela), and red mottled (J. Beaver, 1998, personal communication) seed types and snap bean (McMillan et al., 1998; Stavely et al., 1996) possessing high levels of BGM resistance have been developed. This may be indicative of the absence of any linkage between BGM resistance genes and undesirable seed, growth habit, and maturity traits.

Since all populations involved at least one parent carrying the dominant I gene for resistance to BCM (Drijfhout, 1978) and one or more susceptible parents, screening of advanced generation (e.g., F6 and beyond) selected BGM resistant genotypes for BCM was justified. This has been done at CIAT routinely for most advanced generation selected genotypes irrespective of their origins. Despite the fact that no selection for BCM resistance was practiced between F1 and F6, all 17 selected genotypes from the five populations were resistant to BCM. This may be expected for a trait inherited by a single dominant gene from populations involving one or more resistant parents and making >50% genetic contributions. Moreover, growing environments at CIAT farms at Palmira and Quilichao are prone to natural BCM infection, especially when maize (Zea mays L.) or other crops harboring high populations of the BCM virus insect vector are grown in surrounding fields. This would have facilitated selection against BCM susceptible plants in early segregating generations.

Although selection for ALS and CBB was only practiced in the F3, all 17 selected genotypes from five populations possessed either intermediate or resistant reactions to ALS. Similarly, five of 17 genotypes also possessed intermediate CBB resistance. Because the gamete selection maximizes phenotypic differences within and between families in each population (Singh, 1994; Singh et al., 1998), it might have facilitated selection for a higher frequency of resistant F1:3 families for qualitatively and quantitatively inherited CBB (Nodari et al., 1993; Silva et al., 1989) and qualitatively inherited ALS (Cardona, 1962; Santos-Filho et al., 1976). Nonetheless, Miklas et al. (1996) observed a negative phenotypic correlation (-0.23 P < 0.05) between BGM resistance (derived from DOR 364) and CBB resistance (derived from GN#1 Sel. 27 through `Jules') in a recombinant inbred line mapping population (DOR 364/XAN 176). The negative correlation was caused by repulsion linkages between QTL conditioning resistance to the diseases in two separate regions of the genome. The selected genotypes from the GV 10624, GV 10625, and GV 10627 populations have resistance to BGM and CBB, in part, from the same sources used by Miklas et al. (1996). The genotypes A 429, DOR 303, and Turbo III acquire BGM resistance in part from Porrillo Sintectico. Similarly, XAN 132, XAN 263, and RXAH 18274C derive a portion of their CBB resistance from GN#1 Sel. 27 (Rodríguez et al., 1995; Singh and Muñoz, 1999); whereas OAC 88-1 does not (Miklas et al., 2000). Thus, a negative association between traits may, in part, explain why only a few genotypes carried CBB resistance and even they attained only intermediate and not high CBB resistance of donor genotypes. Two or more independent major genes with additional modifier genes usually impart a high level of CBB resistance (McElroy, 1985; Miklas et al., 2000; Silva et al., 1989). Thus, complex resistance could also have contributed to the lack of high CBB resistance among genotypes selected for BGM resistance in this study. Use of germplasm with higher levels of CBB (Singh and Muñoz, 1999) and ALS (Pastor-Corrales et al., 1998) resistance in future populations, and simultaneous selection by means of both direct disease screening and molecular markers linked to target genes should help increase the levels of ALS, CBB, and BGM resistance.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Selection for BGM resistance in the intraracial bean population (within the common bean race Mesoamerica) was not successful. In contrast, genotypes with extremely high levels of BGM resistance were developed from two out of four interracial (and intergene pool) populations. The interracial populations with <50% genetic contributions of the BGM resistant germplasm produced genotypes with only moderate resistance. All 17 selected genotypes possessed the I gene for BCM resistance and were intermediate or resistant to ALS. Five of these were also tolerant to CBB. Moreover, all BGM resistant genotypes in this study were selected from only very few F1-derived families. Thus, gamete selection for resistance to BGM and other diseases in interracial bean populations should be effective.

The RAPD marker OR2530 linked with the recessive BGM resistance gene bgm-1 was present in Garrapato and its derived genotypes including A 420, DOR 482, and 12 of 17 selected genotypes in this study. The SCAR marker SW12700 linked with a QTL for BGM resistance was present in Porrillo Sintetico, DOR 303, and 10 of 17 selected lines. Genotypes possessing the former marker had higher levels of resistance than those with the latter molecular marker alone. Since some genotypes possessing both markers had only intermediate BGM resistance, use of both direct disease screening and molecular marker-assisted selection from early segregating generations should offer the greatest potential for obtaining BGM resistance in interracial bean populations.SAS Institute 1985


   ACKNOWLEDGMENTS
 
We are grateful to Mauricio Castaño for assistance with BCM screening, to Consuelo Martínez for help with BGM screening, and to Patricia Zamorano de Montoya for typing the initial draft of the manuscript.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Published as Idaho Agricultural Experiment Station Journal Article No. 00706, Univ. of Idaho, College of Agriculture, Moscow, ID 83844.

Received for publication February 9, 2000.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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