Abstract:

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
I
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 I - 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 F46 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 F46 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. HJ and AB 136 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±0.2l) 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 (hi,2) 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 F46 lines with
pyramided genes namely D16.3.3.1 1.160.5.6, C44.1.4.5.142.4 and C16.1.3.8.136.2 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
(/’<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.