Carlos Jara

Pathogenic variation in, sources of, and breeding for resistance to Phaeoisariopsis griseola causing angular leaf spot in common bean

If we are to breed common bean (Phaseolus vulgaris L.) for durable resistance to diseases, we must understand pathogenic variation and find sources of resistance. Our first objective was to determine the patterns of pathogenic variation found among isolates of Phaeoisariopsis griseola (PG), the fungus that causes angular leaf spot (ALS) in common bean. We characterized 433 PG isolates from 11 Latin American and 10 African countries, using differential cultivars, isozymes, and/or random amplified polymorphic DNA (RAPD) markers. We also systematically screened, for ALS resistance, common bean accessions from the world collection held at CIAT, and assessed the progress so far made in breeding for resistance to ALS. Despite their great diversity within and between countries on both continents, the PG isolates were classified into two major groups: Andean, and Middle American. Although each group had internal differences for virulence, and biochemical and molecular characteristics, the ‘Andean’ PG isolates were more virulent on common beans of Andean origin, than on those of Middle American origin, thus, suggesting a host-pathogen co-evolution. The ‘Middle American’ PG isolates, although more virulent on common beans from Middle America, also attacked Andean beans, thus, exhibiting a much broader virulence spectrum. To find sources of resistance, we tested 22,832 common bean accessions against naturally occurring PG isolates in the field at CIATs Experiment Station, Quilichao, Colombia, between 1985 and 1992. The resulting 123 intermediate (scores of 4 to 6) and resistant (scores of 1 to 3) accessions were then tested in the greenhouse against selected 14 PG isolates of diverse origins. Nineteen accessions were intermediate or resistant to at least 13 of 14 PG isolates. Similarly, of 13,219 bred lines tested in the field between 1978 and 1996, 89 were intermediate or resistant. Of these, 33 bred lines proved intermediate or resistant to at least eight of nine PG isolates to which they were challenged in the greenhouse. We suggest that, to breed for durable resistance to ALS, common bean populations should be developed from crosses between Andean and Middle American gene pools. The populations should then be systematically evaluated and selected against the broadest range of the most virulent PG isolates of diverse evolutionary origins.

Seed compositional and disease resistance differences among gene pools in cultivated common bean

It is widely accepted that two major gene pools exist in cultivatedcommon bean, one Middle American and one Andean. Recently another gene pool,designated as North Andean and a fourth group (not considered to be adistinct gene pool) have been reported by the senior author and hiscolleagues. Many of the agronomic and seed compositional attributes of the twomajor gene pools are well known, but the seed compositional value and diseaseresistance of the North Andean gene pool has not yet been characterized. Torectify this situation, the seed compositional characters, percentage of proteinconcentrations (phaseolin, lectin and α-amylaseinhibitor), the nutrient elements (calcium, phosphorus, iron andzinc) and the disease and pest attributes (angular leaf spot,anthracnose, common bacterial blight and empoasca damage) were considered.The Middle American gene pool gave higher lectin, calcium, phosphorus, sulfurand zinc than the Andean gene pool but lower phaseolin and iron. The NorthAndean gene pool is more like the Andean gene pool for phaseolin, resistance toangular leaf spot and anthracnose from Andean pathogen isolates, but more likethe Middle American gene pool for lectin, zinc, sulfur and resistance toanthracnose from Middle American pathogen isolates. On the other hand, it hadthe highest iron concentration and was more resistant to common bacterialblight. These results indicate the potential value of this gene pool in a commonbean breeding program.

Sources of resistance to angular leaf spot (Phaeoisariopsis griseola) in common bean core collection, wild Phaseolus vulgaris and secondary gene pool

Angular leaf spot (ALS) is one of the most devastating diseases of common bean (Phaseolus vulgaris L.) in tropical and subtropical countries. The causal fungus, Phaeoisariopsis griseola(Sacc.) Ferr. is highly variable and a diverse source of resistance genes is required to manage this disease. We evaluated a common bean core collection,primary and secondary gene pools and lines derived from inter-specific crosses of P. vulgaris and P. coccineus or P. polyanthus (secondary gene pool) for resistance to angular leaf spot. Of the 1441 accessiones in the core collection, only 2.2% were resistant to both Andean and Mesoamerican races of P. griseola, 28% were resistant only to Andean and 9% to Mesoamerican races. Of the 32 resistant accessions, 68%originated from Bolivia, Colombia,Guatemala and Mexico. More accessions from these countries should be examined for P. griseola reaction. Very few wild P. vulgaris accessions (4%), were resistant to ALS. In contrast, high levels of resistance (62%) were found in the secondary gene pool. Among the 1010 lines from inter-specific crosses, 109 lines were highly resistant. These genotypes from the primary and secondary common bean gene pools resistant to Andean and Mesoamerican races of P. griseola offer a potential for developing broad and durable ALS resistance.

Sources of resistance to Colletotrichum lindemuthianum in the secondary gene pool of Phaseolus vulgaris and in crosses of primary and secondary gene pools

Use of genetic resistance is the most practical and economic way to manage anthracnose of common bean. Colletotrichum lindemuthianum, the causal agent of bean anthracnose, is a highly variabile pathogen, and there are no host resistance genes that are effective against all known races of the pathogen. To diversify sources of resistance, we screened the core collection of the secondary gene pool of Phaseolus spp. and interspecific lines derived from simple and complex crosses of primary and secondary genotypes for their resistance to anthracnose. High levels of resistance were observed in the secondary gene pool. None of the 162 accessions tested was susceptible to C. lindemuthianum. Of the two species composing the secondary gene pool, P. polyanthus displayed higher levels of resistance than P. coccineus, and all accessions tested were resistant. The response of P. coccineus was more variable, with six genotypes showing an intermediate reaction. Among the 75 lines from interspecific crosses, 49 were resistant to the three races (races 6, 15, and 3481) used in this study, and higher levels of resistance were found in lines that had P. polyanthus as one of the parents in the crosses than in the lines derived from P. coccineus. These lines constitute a valuable source of resistance and may aid in the development of stable resistance to anthracnose.

Patterns of resistance to angular leaf spot, anthracnose and common bacterial blight in common bean germplasm

Diseases and insect pests are major causes of low yields of common bean (Phaseolus vulgaris L.) in Latin America and Africa. Anthracnose, angular leaf spot and common bacterial blight are widespread foliar diseases of common bean that also infect pods and seeds. One thousand and eighty-two accessions from a common bean core collection from the primary centres of origin were investigated for reaction to these three diseases. Angular leaf spot and common bacterial blight were evaluated in the field at Santander de Quilichao, Colombia, and anthracnose was evaluated in a screenhouse in Popayan, Colombia. By using the 15-group level from a hierarchical clustering procedure, it was found that 7 groups were formed with mainly Andean common bean accessions (Andean gene pool), 7 groups with mainly Middle American accessions (Middle American gene pool), while 1 group contained mixed accessions. Consistent with the theory of co-evolution, it was generally observed that accessions from the Andean gene pool were resistant to Middle American pathogen isolates causing anthracnoxe, while the Middle American accessions were resistant to pathogen isolates from the Andes. Different combinations of resistance patterns were found, and breeders can use this information to select a specific group of accessions on the basis of their need.