Identification and Characterization of Fungal Pathogens Associated With Cassava Brown Leaf Spot Disease in Western Kenya

Abstract

Cassava (Manihot esculenta Crantz) is an important food security crop due to its adaptability to marginal areas. However, brown leaf spot (BLS) is one of the fungal diseases considered to be a major threat in cassava production, causing more than 30% economic damage on susceptible cultivars. To breed cassava for durable resistance, identification and characterization of the fungal pathogens causing BLS is a pre-requisite. Therefore, the aim of this study was to identify and characterize fungal pathogens causing cassava BLS in Western Kenya. Cassava leaf samples with brown leaf spot symptoms were collected from three Counties (Kakamega, Bungoma and Busia) of Western Kenya. The fungal pathogens were isolated by culturing the leaf samples on potato dextrose agar (PDA) amended with the antibiotics. Pathogenicity of the fungal isolates was tested on leaves of susceptible cassava cultivar TMS 60444 using detached leaf assay. A total of 60 fungal isolates were obtained of which thirty five isolates were confirmed to be pathogenic based on detached leaf assay of the susceptible cassava cultivar TMS 60444. Out of the 35 pathogenic isolates, single spore cultures of 13 randomly selected representative isolates were characterized; and they exhibited varied morpho-cultural characteristics including colony colour, colony margin, colony texture, colony diameter and presence of pigments on PDA medium. Based on ITS sequences and phylogenetic analysis, the 13 representative fungal isolates were identified as Trichothecium roseum (four isolates), Fusarium equiseti (four isolates), Colletotrichum siamense (two isolates), Colletotrichum karstii, Diaporthe phaseolorum (one isolate) and Bipolaris setariae (one isolate). This is the first report to identify T. roseum, C. siamense, C. karsti and D. phaseloreum as causal agents of cassava BLS. This study reveals that BLS in Western Kenya is caused by a diverse group of fungal pathogens with different morphocultural and molecular characteristics. There is need to carry out further studies to investigate the severity of BLS when the cassava host is co-inoculated with the identified fungal pathogens.