Resistance spectrum to anthracnose disease and the genetic effect of pyramided genes on yield in common bean
Bean anthracnose, caused by Colletotrichum lindemuthianum (Sacc. et. Magn) Lams. Scrib., is one of the most widespread and economically important fungal diseases of the common bean. The pathogen possesses a high genetic and pathogenic variability, which causes it to overcome resistance in bean cultivars. Pyramiding of resistance genes in commercial varieties is a strategy that would ensure more effective resistance and thus a reduction in yield losses. This study was conducted with the overall objective to enhance broad-spectrum resistance to anthracnose disease among common bean varieties for increased productivity in Uganda. The specific objectives were; a) To determine the pathogenic variability of Colletotrichum lindemuthianum in Uganda; b) To assess effectiveness of single and pyramided resistance genes against bean anthracnose disease; and c) To determine genetic variability and relationship between pyramided genes and yield associated traits among advanced common bean populations To determine pathogenic variation of C. lindemuthianum, samples of common bean tissues with anthracnose symptoms were collected from Kabarole, Sironko, Mbale, Oyam, Lira, Kapchorwa, Maracha and Kisoro. 51 isolates were used to inoculate 12 standard differential cultivars under controlled conditions. Five plants per cultivar were inoculated with each isolate and evaluated for their reaction using a 1 – 5 severity scale (Inglis et al., 1988). Races were classified using the binary nomenclature system proposed by Pastor Corrales (1991). Sequence Characterized Amplified Region (SCAR) markers were used to facilitate the process of pyramiding and tracking three anthracnose resistance genes/ alleles (Co-42/ Co-43, Co-5 and Co-9) using a cascading pedigree pyramiding scheme. Detached leaf trifoliates of F4:6 plants were screened under controlled conditions with four C. lindemuthianum races and severity scored on a 1 – 9 scale (Balardin et al., 1997). 53 F4:5 and 69 F4:6 families were evaluated in the field for yield and agronomic performance; and correlation and path analysis done to establish relationship between pyramided resistance genes and yield. Disease severity and yield traits data were subjected to ANOVA to reveal extent of variability. Twenty seven (27) pathogenic races were identified. Sironko district had the highest number of races followed by Mbale and Kabarole. Races 2047 and 4095 were the most frequent. Race 4095 was the most aggressive and followed by races 2479, 2047 and 2045. Races, 4094 and 2479 caused a susceptible reaction on the differential cultivar G2333, known to possess a high degree of resistance. Differential cultivars G2333, Cornell 49-242, TU and AB136 were the most resistant. The five pyramid group means were significantly different from each other (P<0.01) of which, Co-42+Co-5+Co-9 and Co-42+Co-5 exhibited the lowest mean disease score to all the four races indicating a high degree and spectrum of resistance. The group Co-43+Co-9 had the highest mean disease severity. The single-genes were significantly different from each other (P<0.01). The Co-42 and Co-5 genes both conferred resistance to all the four races 352, 713, 767 and 2047, while the group with none of the o resistance gene inherited (5.9±0.21) was overcome by all the four races. Co-42 group had the least mean severity across races followed by Co-5, Co-43, Co-9 and No-gene. The single gene Co-42 was not significantly different from the best pyramids Co-42+Co-5+Co-9 and Co-42+Co-5 (P<0.01) and was better than the other three pyramids. Similarly the Co-5 single gene was not significantly different from Co-42+Co-5, Co-42+Co-9 and Co-5+Co-9 pyramid groups (P<0.01). The single gene Co-9 was found to be antagonistic in combination with other resistance genes and should be avoided in resistance gene pyramiding programs. The two single genes Co-42 and Co-5 are recommended for use in resistance gene pyramiding programs. ANOVA revealed significant variability (P<0.01 and P<0.05) for all yield traits among advanced bean lines, and further revealed that Phenotypic Coefficients of Variability (PCV) estimates were higher than the Genotypic Coefficients of Variability (GCV) estimates indicating importance of environmental effects in the expression and improvement of these traits. Broad sense heritability (hb2) and genetic advance among populations was low for number of pods per plant, number of seeds per plant and seed weight per plant implying that selection would not be effective in improving these traits but other breeding strategies such as heterosis breeding would be appropriate for improving the traits. Three F4:6 lines with pyramided genes namely D22.214.171.124.160.5.6, C126.96.36.199.142.4 and C188.8.131.52.136.2 were among the best 10 yielders. However, number of pyramided genes had a significant negative correlation with seed weight per plant (-0.17), number of pods per plant (-0.24, p<0.05) and number of seeds per plant (-0.19, p<0.1); and path coefficient analysis revealed a significant (P<0.05) negative indirect effect of number of pyramided genes on seed weight per plant via number of seeds per plant (-0.25). These findings suggest a yield penalty arising from pyramiding resistance genes against the C. lindemuthianum pathogen.