Diversity of Colletotrichum lindemuthianum and reaction of common bean germplasm to anthracnose disease

The production of common bean (Phaseolus vulgaris L.) in Uganda is largely constrained by bean anthracnose (Colletotrichum lindemuthianum (Sacc.and Magn.), among other diseases. This pathogen is capable of causing total crop loss on susceptible varieties when planted in environments that are favourable to the pathogen. Previous results from Tanzania estimate that for each 1% increase in anthracnose incidence, seed yield decreases by 9kg/ha (Allen, et al., 1996). The most effective and appropriate control measure for this disease is the use of host plant resistance (Schwartz et al., 1982). Unfortunately, C. lindemuthianum is very variable, and this hinders the development and deployment of durable resistance (Scwartz et al., 1982). This particular work was aimed at characterizing Ugandan population of Colletotrichum lindemuthianum using bean differentials and molecular markers, and screening for anthracnose resistance among Ugandan local and released bean genotypes. The two methods of pathogen characterization were also compared for efficiency. Twenty one Random Amplified Polymorphic DNAs (RAPDs) markers, two rep-PCR primers and virulence on a set of 12 bean anthracnose differential cultivars were used to assess the genetic and physiological diversity of 74 C. lindemuthianum strains isolated from Kabale, Mbale, Apac, Mpigi and Wakiso districts. On the other hand, ninety six Ugandan local and improved bean germplasm were also screened for resistance against one highly virulent Ugandan bean anthracnose race (race 3). Analysis of molecular data resulted into 3 major clusters (Fig. 1 and 2). Comparison of results from the race differentials with those from the molecular techniques did not show a strong relationship. This could partly be due to apparent intra-race diversity revealed using genome wide spanning molecular markers. This implies that such markers are not suitable in assessing relationships between pathogenicity and molecular marker derived fingerprints. Pathotype characterization of 47 isolates with race differentials was carried out in the screen house at Kacwekano ZARDI, Kalengere substation in Kabale district and resulted into 21 races, of which 9 infected only Mesoamerican differential cultivars, where as 3 were virulent on Andean counterparts. Seven races infected cultivars from both gene pools, where as race 0 did not infect any of the differential cultivars. Races 1024, 1536, 1538, 1856, 1857, 1989, 3086 and 4033 were the most virulent as they incited symptoms on either or both of the highly resistant differential cultivars AB136 and G2333 with an average severity level of 4. Races 0 and 6 were the most wide spread; they were isolated from the districts of Kabale, Mbale and Apac (race 0) and Mpigi and Kabale (race 6). Lastly, inoculum from race 3 of Colletotrichumum lindemuthianum was used to infect 96 bean genotypes in two seasons. Of the 96 genotypes screened, 56% (54 genotypes) were susceptible while the rest were resistant. In this experiment, Andean genotypes were the most affected (accounting for 73% of total infected genotypes) than Mesoamerican genotypes (Table 14).