Tropical Grasslands (2000) Volume 34, 118–124

A novel technique for measuring biomass loss in a diseased tussock grass

G. FARRELL1, S.A. SIMONS2 and R.J. HILLOCKS1

1Natural Resources Institute, University of Greenwich, Chatham Maritime, Kent, United Kingdom
2CABI Africa Regional Centre, Nairobi, Kenya

Abstract

Baseline data were collected on the biomass reduction in napier grass, Pennisetum purpureum, resulting from infection by a smut fungus. The amount of smut infection was assessed in 448 stools in 14 plots, along diagonal transects. A damage class (based on the percentage of inflorescences that were infected) was then assigned to each stool, and the stool was weighed. Models were derived from the weight and damage class data to estimate losses in stands of different heights, and by combining damage classes and the corresponding losses, estimates of total losses in fields were obtained. The damage class scale assumes that the healthy parts of smut-infected plants will be salvaged and fed to livestock, though calculations are also provided to indicate the effect on biomass reduction if diseased plants are totally lost.
The model was applied to other infected stands. In the current outbreak of Ustilago kamerunensis on napier grass, Kenyan small-holder farmers are suffering biomass reductions from 25–46 percent.
It is proposed that this technique be used for measuring losses in other tussock grasses that suffer from systemic infections.

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