Prevalence and severity of Cercospora leaf spot and Cowpea severe mosaic virus diseases of cowpea in Trinidad 143

Cercospora leaf spot {CLS) and Cowpea severe mosaic virus (CPSMV) diseases are considered serious constraints to cowpea production in Trinidad. The prevalence of the CLS pathogens at the University Field Station (UFS) was monitored in cowpea crops over nine seasons. Additionally, an island-wide epidemiological survey was conducted over the wet and dry seasons to identify the causal agent(s) of CLS, establish their distribution, and to record the incidence and severity of CLS and CPSMV diseases in Trinidad. Multiple regression analyses were carried out to determine the effect of a number of climatic, edaphic, and management factors on the incidence and severity of the two diseases. Pseudocercospora cruenta (Sacc.) was observed in epidemic proportions in every cowpea crop grown over the 9 seasons and was the only CLS pathogen observed in the island-wide study involving 54 cowpea fields. Cercospora apii s. lat. [pseudonym C. canescens (Ellis and Martin)] was found in cowpea crops at the UFS only in three out of the nine seasons studied. It always occurred in mixed infection with P. cruenta, and at low levels of incidence. Sixty-two per cent of the variation in the incidence of CLS was influenced by crop maturity, season, and site. Crop age and seasonal rainfall jointly accounted for 69% of the variation in CLS crop severity scores. Cercospora leaf spot appeared as a late-onset disease, prevalent in cowpea crops particularly under wet conditions. Cowpea severe mosaic virus was ubiquitous and found in every field surveyed. Seasonal rainfall and crop age jointly explained 38% of the variation in CPSMV incidence in farmers' fields, while seasonal rainfall alone explained 30% of the variation in severity scores. Despite the use of crop protection chemicals, both diseases were found in epiphytotic proportions in all the cowpea varieties cultivated in Trinidad. The implications of these findings to controlling these diseases are discussed.