Studies on bean common mosaic virus on beans (phaseolus vulgaris.) in Kenya
Bean common mosaic virus (BCMV)occurs on bean, Phaseo1 us vulgaris L. , in Kenya. In order to establish the incidence of common bean mosaic in small scale farms, a survey was conducted during the long rainy season in 1981. Disease incidence of up to 63 percent was recorded in bean fields. Fourteen virus isolates from susceptible beans in Central, Eastern, Western, Nyanza and Rift Valley provinces and the Nairobi region were collected. These isolates were identified as isolates of BCMV on the basis of host range, symptomatologv, non persistent aphid transmission, particle morphology, size and serology. The isolates were subsequently differentiated into six strains on the basis of reactions they induced on bean differentials hosts. Some of the BCMV isolates differed from those strains of BCMV reported in literature, but others were identical to the NL3 strain and the US2 strain. The six strains were classified into four different pathogenicity groups of which four of the strains are related to NL3 and US2. The other two strains (NY and 11 A) were found to be new. The NL3 strain types consisted of two necrosis inducing strains and one non-necrosis inducing strain. The first necrosis inducing strain type comprised of isolates to, 510 and ICC; the second necrosis inducing strain type comprised of isolate 4 and AhT while the non-"ecrosis inducing strain type comprised of isolates E4, "I, K, T, 86 and iIC4. The US2 strain type was isolate Nch2. In order to investigate the presence of resistance among the available, bean accessions, 454 bean cultivars were screened by mechanical inoculation against individual or a complex of the isolates of BCMV. Resistance was obtained in 77 out of the 454 cul ti vars screened. The effect of BCMV on the yield of beans was investigated by testing three varieties that were predominant in the Kenyan market against three BCMV strain types represented by isolates Mch2, 5l0 and d. Tnese strain types caused significant reduction in the yield of the bean cultivars 'Rose cocc 'Canadian wonder' and 'Mwezi moja'. The yield of these cultivars was reduced by 55, 54 and 67 percent, respectively.