Abstract:
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.
