Abstract:

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 AB 136 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).