1. Itch grass Rottboellia cochinchinensis is a serious weed in the humid tropics. However, the seed bank is short-lived (approximately 3 years) and control methods that effectively prevent inputs to the seed bank should have a long-term impact on the weed. The head smut Sporisorium ophiuri, which forms systemic infections sterilizing the weed, has therefore been proposed as a potential classical biological control agent. 2. A difference equation model of the population dynamics of the weed R. cochinchinensis was developed for a maize cropping system. The model was used to estimate the constant annual infection rate by the S. ophiuri that would be necessary to provide long-term control of the weed. 3. The model suggested that with the smut as the sole control agent, an annual infection rate of about 88% would be require to reduce R. cochinchinensis density to 10% of the level achieved with no control. 4. However, when combined with one or two weedings per year, the level of infection necessary for satisfactory control could be reduced. Since the maximum infection rate achieved in experiments was about 80%, the smut is unlikely to achieve satisfactory control when used alone, but these results suggest that S. ophiuri could be a useful adjunct in integrated control programmes. 5. The extent to which the effectiveness of the smut was improved by combining it with weeding depended on the seed set in the R. cochinchinensis flush emerging after weeding and on seedling mortality. Where seed set or seedling survival were low, the required infection rate could be substantially reduced. 6. These results are discussed in the context of future prospects for biologica control of R. cochinchinensis and areas for further research to improve the utility of the population dynamic model are highlighted.
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