Occurrence and control of seedborne pathogenic fungi and bacteria of important grain legumes grown by small scale farmers in Kenya

Abstract

Surveys were conducted in several locations of Central and Eastern Provinces of Kenya during the months of October to December 1980. During these surveys, bean (Phaseolus vulgaris L.) cowpea (Vigna unguiculata (L) Walp) and pigeon pea (Cajanus cajan (L.) Millsp.) seeds were collected from small-scale farmers. Thee seeds were brought to the laboratory for a variety of studies leading to the identification and quantification of internally borne fungi and bacteria. An effort was also made to find out the best seed treatment pesticide against the pathogens. Benlate and captan were used against seedborne fungi while streptomycins was used against seedborne bacteria. Improper cleaning, handling and storage, seed pests especially Acanthoscelides sp. and Callosobruchus sp. and seedborne fungi and bacteria were found to contribute significantly to the poor quality and low viability of the seeds. Seventeen different genera of fungi were isolated from bean, 15 from cowpea and 9 from pigeon pea seeds. Among the fungi isolated, Fusarium spp. which occurred in all the seed types and nearly all the seed lots caused significant reductions in germination. Alternaria spp. was another common pathogen, but it was not of very significant pathogenic importance except in some bean seed lots. Penicillium spp. and Aspergillus spp. were found to cause serious damage to cowpea and pigeon pea seeds. Two bacterial pathogens, Xanthomonas phaseoli and Pseudomonas phaseolicola were isolated from bean seeds while only one, Xanthomonas phaseoli f.sp. vignicola was isolated from cowpeas. X. phaseoli occurred in more bean samples collected from Central Province than Eastern Province. Its incidence of occurrence ranged from 8% in Kiambu District to 32% in Embu District. P. phaseolicola occurred less frequently in Central Province than in Eastern Province. However, where P. phaseolicola was found, the incidences were higher (19-36%) than X. phaseoli X. phaseoli f.sp. vignicola was only isolated from Ishiara in Embu and Abogeta in Meru, Eastern Province, where incidences were 15% and 8% respectively. Kirinyaga in Central and Embu in Eastern Provinces were the most highly infested areas, while Machakos and Kitui both in Eastern Province were the least infested. On this basis, Machakos and Kitui Districts were considered potential areas of production of disease-free seed for planting. Among the fungicides tested, captan was found to be the best for seed treatment. It lowered the percent total fungi in seeds and increased both germination in vitro and emergence in the field. Benlate was equally useful but may not be economically viable for small-scale farmers. Coppery oxychloride was the least effective, and was therefore not recommended for seed treatment. Streptomycin was better than copper oxychloride against seedborne bacterial pathogens. Farmers are, however, advised to use disease-free seed for planting.