Studies on sugar-cane smut (Ustilago scitaminea) in Kenya

Germination of chlamydospores of Ustilago scitaminea Sydow is typical of the Ustilaginaceae only on rich nutrient media such as potato dextrose agar. On sugar cane surfaces sporidia are rarely produced; the promycelia give rise directly to infection hyphae. This atypical germination sequence also takes place on non-nutrient media and on media containing only sugars (mannose, sucrose, lactose). There is a sharply defined optimum temperature (31°C.) for production of infection hyphae; promycelia and sporidia develop over a broad temperature range. Comparatively high spore concentrations (10 6 –10 7 spores/ml.) were required for severe infection, which is influenced by age of nodal bud or length (age) of tiller; both parts are increasingly resistant with age. Other factors affecting the severity of outbreaks include the rate of smut-whip production in different varieties, the presence of a continuous source of inoculum of high concentration, and the distribution and frequency of rainfall. A reliable method for the laboratory screening of cane varieties for susceptibility is described and analysed.