Morphological Characterization of Ugandan Isolates of Sphaceloma sp. Causing Cowpea Scab Disease

Scab is an important fungal disease of cowpea, affecting both young and old tissues including stems, leaves and pods of susceptible cowpea genotypes, leading to significant yield losses of up to 100% under severe infections. Colony characteristics on agar media, symptomatology, phylogenetic affinity of hosts and host range have been used to justify taxonomic distinctions. The correct identification and description of a pathogen is paramount in understanding its control or developing genotypes resistant to it. This study involved the isolation and culture of the scab fungus (Sphaceloma sp.) from infected plant parts (leaves and pods) collected from farmers’ fields across major cowpea growing districts and agro-ecological zones in Uganda. The fungus was characterized using growth habit on potato dextrose agar (PDA) media, conidia features, variability in radial growth rate (mm/day) among the isolates and pathogenicity and virulence of some isolates on 20 selected cowpea genotypes with different levels of resistance. A total of 495 Sphaceloma sp. isolates comprising of 419 from infected leaves and 76 from infected pods were obtained following isolation and culture. There was a wide variation in the isolates based on the amount, nature, colour, depth and rate of mycelia growth, features of conidia and number of septations. Based on the mean incidence, severity, AUDPC and pathogenicity on the 20 genotypes, the isolates were put into three pathogenicity groups. Isolates were mostly slow growing (> 14 days to cover entire 90 mm petri dish). Genotypes NE 31 and NE 70 showed broad spectrum of resistance to the isolates and could therefore be recommended as parental lines in the cowpea breeding programme to develop cultivars with wide horizontal resistance to the scab disease.