The Occurrence of Seed-borne Bacteria and Fungi on Common Bean (Phaseolus vulgaris L.) Under Different Farming Ecosystems of Mutasa District, Manicaland Province

Abstract

Seed quality is the basic important factor for bean crop production. Seed-borne diseases affect seed quality and viability. A study was conducted to assess the occurrence of seed-borne fungal and bacterial diseases on common bean landraces in three ecological regions of Mutasa district. Eight seed lots representing each landrace were obtained from farmers during the field survey for laboratory and field assessments. The blotter test method was used to analyse fungal pathogens. Bacterial pathogens were analysed using the growing-on test and the liquid assay method. Germination tests were conducted using ISTA standards. The detached leaf inoculations, spraying of live plants in a greenhouse and field plot assessments were used to evaluate Angular leaf spot disease tolerance in bean landraces. Data were analysed using Genstat 5, release 3.2. Of the thirty respondents in the farming systems survey, 53.3% of the farmers grew beans in summer, 66.7% during early winter and only 4% grew the crop in early summer. All the farmers used farm retained bean seed. The kidney class of sweet bean is the only bean class grown in Mutasa district. In all the eight seed samples, the pathogenic fungi recorded were Fusarium moniliforme, Botryodiplodia theobromae, and Collectotrichum lindemuthianum. The percentage of seed samples infected by fungi was Fusarium moniliforme (100%), Botryodiplodia theobormae (12.5%), and Collectotrichum lindemuthianum (12.5%). The infection of samples by storage fungi was Aspergillus flavus (75%), Cladosporium sp. (100%), Aspergillus niger (50%) and Rhizopus sp. (75%). There were significant differences (p<0.05) in normal germination rate between Red speckled, Black speckled, Honzo sweet (long) and Honzo sweet (small and round) seeds. No bean landraces were tolerant to Angular leaf spot from laboratory and field plot assessments. Xanthomonas axonopodis pv. phaseoli was isolated from samples of Honzo sweet (s&r) and Moza. No landraces were tolerant to bacterial blight in the field assessment. From the results, farmers should grow more than one class of beans to reduce disease incidence through a diversified gene pool of bean varieties. Retained seed should be treated with fungicides such as captan or copper oxy-chloride to suppress seed-borne fungi and bacteria. i