Characterization of Pseudocercospora griseola, identification of local sources of resistance and effectiveness of gene pyramiding in controlling angular leaf spot in common bean

Common bean (Phaseolus vulgaris L.) is one of the most widely grown legume crops for food and income by small holder farmers in Uganda. Despite its importance, bean production is still constraint by angular leaf spot (ALS), which still cause losses both in quantity and quality of beans. This study was conducted to determine the distribution and variability of Pseudocercopsora griseola, identify new sources of resistance and elucidate the effectiveness of pyramided genes in enhancing ALS resistance. To determine the distribution and variability of P. griseola, disease surveys, ALS disease differential bean cultivars and molecular markers were deployed. The results revealed the existence of ALS in all regions and districts surveyed and disease distribution varied depending on region, and altitude. Disease incidence and severity were highest in the north and lowest in south-western Uganda. In terms of districts, the highest disease incidence (68.5%) and severity (score of 7.35 on a 1-9 scale) were recorded in Dokolo while the lowest incidence (29%) and severity (score of 4.07 on a 1-9 scale) were observed in Kisoro. Both disease incidence (60%) and severity (4.95) were higher for bean fields located in low altitude areas than incidence (33.6%) and severity (score of 1.84 on a scale of 1-9) in fields found in high altitude areas. Among the Ugandan P. griseola isolates tested for variability, ALS disease differential bean cultivars defined 12 pathotypes and selected molecular markers defined 30 haplotypes which belonged to two major groups: middle-America and Andean group which were highly variable. Pathotypes 61:63 and 17:39 were the most virulent and prevalent respectively, while pathotype 5:55 was specific to high altitude areas. The research findings provided information needed to develop ALS control strategies in Uganda. On the other hand, eighty Uganda bean landraces were evaluated for ALS resistance using four P. griseola pathotypes (1:6, 17:39, 21:39 and 61:63) under screenhouse conditions at Kawanda. The severity of ALS on the bean lines ranged between 4.5 and 40.5 (based on area under disease progressive curve values ) being variable depending on the genotype and the pathotype Area under disease progressive curve (AUDPC) values were highest for plants inoculated with pathotype 61:63 compared to the rest of other pathotypes Results also showed that landrace U00297 was the only bean line resistant to all the four pathotypes with a disease severity values ranging between 7.6 and 13.5 on the AUDPC scale. Inheritance of ALS resistance in U00297 and its allelic relationship with other ALS known sources of resistance were also elucidated. Segregation ratios of F2 populations revealed that U00297 resistance to pathotype 17:39 was conferred by a dominant gene, while digenic epistatic gene interactions were responsible for U00297 resistance to 61:63, 21:39 and 1:6. The allelism test indicated that the dominant gene in U00297 was independent of resistance genes harboured by AND277 and G5686. Hence, U00297 has a potential of being used as an independent source of ALS resistance in breeding programs if its resistance is validated under field conditions. To study the effectiveness of pyramided genes in enhancing ALS resistance, genes from existing ALS resistance sources (Mexico 54, AND277 and G5686) were pyramided in a cascading scheme. The results showed that single crosses ( SC) between resistant parents (R x R) best fitted for 15:1 and 61:3 ratios while the triple cross (TC) best fitted for 249:7 and 247:9 ratios, suggesting that SC segregated for two and three genes while TC segregated for four genes. The combined resistance genes in SC and TC crosses exhibited additive effects within the cross and slightly increased the level of resistance to 61:63 when all the resistance sources were combined. The four parent cross (FPC) exhibited the highest level of resistance to 61:63 compared to the original ( Mexico 54, AND277 and G5686) resistance sources. The TC lines with combined resistance were more effective than the individual sources for transferring resistance to susceptible commercial cultivars of major importance in Uganda. Hence, pyramiding was successful in accumulating four ALS resistant genes into the susceptible parents indicating significant progress in breeding for ALS resistance.