Identification and survival of the causal organism of leaf smut disease of cowpea in Nigeria.

Protomycopsis phaseoli (Ramak and Subram) is the causal agent of the cowpea leaf smut disease in Nigeria and not Entyloma vignae as claimed by some authors. This pathogen formed dark ash-grey to sooty-black lesions of 3-10 mm in diameter, while young lesions had yellow haloes. P. phaseoli produced dark reddish-brown chlamydospores that are globose to oval measured 23.8 microm, thick-walled and rugose. The chlamydospores germinated and produced globose vesicles. The pathogen grew on potato dextrose agar only when the leaf tissue was dipped in acidified water (1% H2SO4). The organism was slowly growing at 24-28 degrees C with snow white colour. Chlamydospores of P. phaseoli in infected cowpea leaves survived longer when buried in the soil for five months than when they were left on the soil surface for the same period at temperatures (26-27 degrees C) and humidity (70-82%) prevailing in Ibadan. Destruction of leaf debris before crop emergence, long period of rotation and no tillage cropping are suggested to prevent the onset and spread of leaf smut disease of cowpea.