Towards the management of Ustilago kamerunensis H Sydow and Sydow, a smut pathogen of napier grass (Pennisetum purpureum Schum.) in Kenya
Infection by headsmut, caused by Usti/ago kamerunensis, is the major cause of biomass reduction in Napier grass (Pennisetum purpureum). In Kenya, this grass is the primary source of fodder for zero grazed livestock, which are an important component of small-holder farm incomes in high potential areas of the country. Studies were conducted on the biology and ecology of the pathogen, with the objective of developing technologies for management of the disease. A survey in Kiambu District, central Kenya, of Napier grass plots was used to construct a model of smut incidence. Ten agro-ecological and environmental parameters were factored into the model but only altitude, using a quadratic function, was significant. Napier grass at altitudes between 1800 and 2000masl is particularly at risk from the disease. This was the first smut disease correlative model to make use of environmental factors. The presence of the fungus was demonstrated inside the host, and descriptions of ustilospore and sorus characteristics were made. Ustilospores in soil remained viable for less than eight weeks, and those air dried for less than 14 weeks. The infection court was identified as being restricted to germinating buds, which were the only sites on which ustilospores gave rise to appressoria. On other host surfaces, and on agars, sporidia and hyphae were formed. A disease severity scale was developed, related to biomass reduction in fresh weight of Napier grass stools, that can be used to assess losses in field situations. Using a spore dipping technique, resistance testing of locally available cultivars identified susceptible and resistant types. Evidence was also gained for the possibility of disease remission. These investigations lead to the production of management options for farmers, that are discussed in view of the increasing importance of the disease in Kenya.
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