Production of Microconidia byCercospora henningsiiAllesch, Cause of Brown Leaf Spot of Cassava (Manihot esculentaCrantz) and Tree Cassava (Manihot glazioviiMuell.-Arg.)

Abstract

Sporulation of Cercospora henningsii Allesch has been examined under various relative humidities, and in the presence of free water in lesions on leaves of cassava and tree cassava. Mature conidia of the fungus on both cassava and tree cassava do not germinate in lesions but accumulate, and under the optimum conditions of 25–32 °C and in the presence of free water, they bud and fragment into numerous microconidia. Microconidia are cylindrical, mostly one-celled, and measure 7.5–17.5×3.7–7.5 μm. Production of microconidia significantly decreases as relative humidity decreases. Microconidia readily germinate by means of a germ tube at 100% relative humidity on both surfaces of host leaf and on glass slides. Some germ tubes form appressoria and symptoms appear on cassava leaves inoculated with microconidia. The results are discussed in relation to possible modes of dispersal of the spores and control of the disease.