Abstract:

Bean anthracnose (Colletotrichum lindemuthianum)isa destructive disease of susceptible
common bean in Uganda especially in the high altitude and low temperature areas.Marker
assisted pyramiding of race specific resistance (R) alleles has been proposed as one of the
approaches to develop resistant varieties. This requires knowledge of resistant alleles and
cordominant markers tightly linked to them. This research therefore sought to improve marker
assisted pyramiding of Co-2 anthracnose resistance gene in dry been cultivars; with specific
objectives of (i) identifying effective anthracnose resistant alleles and their source and (ii)
mapping the Co-2 anthracnose resistance gene using SNP markers. A set of 11 lines obtained
from Embrapa Brazil, 12 anthracnose differential varieties and two Ugandan susceptible checks
were evaluated in the field for two seasons in 1Cabale, South Western Uganda. The experiment
was laid in a randomised complete block design with bean lines as treatments in three replicates.
Three C. /indemuthianum races (336, 375, and 381) were used to screen the bean cultivars under
controlled conditions and thus evaluated for anthracnose severity. To map the Co-2 gene, an F2
mapping population was developed from crosses between Cornell 49-242 (Co-2 gene) used as
male and K132 (a susceptible check) used as female. 146 F2 families were developed and
advanced to 143 F2.3families and phenotyped with anthracnose race 336. Disease severity data
were subjected to a chi-square test to determine its conformity to F2 and F2:3 segregation ratios
of 3:1 and 1:2:1, respectively. DNA was extracted from the F2 plants and used for genotyping
and SNP marker development while the other portion was screened with SCAR markers for Co-2
anthracnose resistance gene (SCAReoli and SQ4) to validate the F2 phenotypic data. Five (SEL
1308(with severity of 1), 1C23 (1.25), BRS 9435 Cometa (1.333), BRS Ametista (2.0) and BRS
Horizonte (2.75) out of the 11 Brazillian lines screened were as highly resistant as the standard bean differentials PI 207262 (1.75), 02333 (1.833), TU (1.833),AB 136(1.917). Anthracnose
resistance alleles, Co-42, Co-5, Co-10 and Co-2carried in SEL 1308,. 1(23, TU, Ouro Negro and
Come] 49-242 respectively were highly effective against all field races of the pathogen, thus
making these cultivars potential sources of resistance for anthracnose resistance breeding
programs in Uganda. The alleles in BRS 9435 Cometa, BRS Ametista and BRS Horizonte are
still unknown but highly effective in Uganda. These should be characterised. Out of the 146 F2
plants screened with race 336 of C. lindemuthianum, 33 were susceptible, 113 were resistant.
Regarding the 143 F2:3 families screened, 25 showed complete resistance, 79 both resistance
and susceptibility (segregating) while 39 were completely susceptible. When the observed
frequencies were subjected to a chi-square test of goodness of fit, they conformed to the expected
segregation ratio of 3:1 at F2 generation and 1:2:1 for F2:3 generation.This implies that the gene
conferring resistance to race 336 of C. lindemuthianum in this study is inherited in a Mendelian
manner and thus is a single dominant (major) gene and likely to be the Co-2 gene in Comel 49
242. This type of resistance can easily be introgressed in susceptible bean cultivars and therefore
SNPs tightly linked to this gene could be useful co-dominant markers for pyramiding in bean
anthracnose resistance breeding.