Stages in the progress of eyespot epidemics in winter wheat crops

The temporal progress of eyespot (Pseudocercosporella herpotrichoides) epidemics in winter wheat crops is considered in four stages: leaf sheath lesion establishment, leaf sheath penetration, stem lesion establishment and stem lesion development. These four stages are described by analyzing the relationship with time, respectively, of the incidence (y) and serverity (s) of leaf sheath lesions and the incidence and severity of stem lesions, with particular reference to data from field experiments at Rothamstedt from 1980 to 1984. Under favourable conditions, the incidence - time relationships for both leaf sheath and stem lesions fit the equation for a monocyclic disease. The severity of both leaf sheath and1498 stem lesions increases linearly with time under favourable conditions and may be a function of accumulated temperature. However, the development of eyespot epidemics is often interrupted. Death of infected leaf sheaths early in the season may cause decreases in the incidence of leaf sheath lesions. Leaf sheath penetration may be delayed by cold weather and if the rate of leaf sheath death exceeds the rate of leaf sheath penetration lesions may be lost at any stage. A crucial stage in the development of eyespot epidemics is the period of stem lesion establishment, when rapid death of basal leaf sheaths may prevent the fungus becoming established in the stem. The stages in the progress of epidemics are discussed in relation to schemes for forecasting the severity of eyespot epide