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Modeling of Take-All Epidemics to Evaluate the Efficacy of a New Seed-Treatment Fungicide on Wheat

Take-all, caused by Gaeumannomyces graminis var. tritici, is a damaging disease of wheat that remains difficult to control. The efficacy of an experimental fungicide, applied as a seed treatment, was evaluated in five naturally infested field experiments conducted during three cropping seasons. Plants were sampled and assessed for take-all incidence and severity at different growth stages. Nonlinear models expressing disease variables as a function of degree-days were fitted to the observed data. The incidence equation involved two parameters reflecting the importance of primary and secondary infection cycles. The earliness of infection was identified as an important variable to interpret the effects of the fungicide. In an early epidemic, the fungicide significantly reduced take-all incidence during all or most of the cropping season, whereas in late epidemics, it provided only moderate reductions of incidence. The seed treatment reduced incidence by delaying the primary infection cycle. The fungicide significantly reduced severity during the whole epidemic. It appeared more efficient in limiting root-to-root spread than in slowing down the extension of necrosis on diseased roots.