Agronomic and meteorological factors affecting the severity of leaf blotchcaused by Mycosphaerella graminicola in commercial wheat crops in England

Factors affecting the severity of leaf blotch on the two upper leaves of wh eat plants in crops at the milky ripe growth stage (GS 73-75) were investig ated using survey data from 3513 randomly selected wheat crops sampled duri ng 1985-1996. Year-to-year variation in disease severity was greater than s patial variability at county level, although both showed significant differ ences. The presence of disease above a 5% severity threshold was modelled u sing random effects logistic regression (Generalised Linear Mixed Model), w hich enabled risk variables measured at the field level to be combined with meteorological variables estimated at county level. The final model includ ed terms for the fixed effects of disease resistance rating, date of sowing , high risk septoria periods in May and June, number of fungicide sprays an d number of days with frost (less than or equal to -2 degreesC) in November . The percentage of crops above the threshold decreased with later sowing, increased number of November frost days and increased number of fungicide s prays. In contrast, high risk septoria periods (rain splash events) in May and June showed a positive correlation with the percentage of crops above t he threshold. There were benefits from using resistant cultivars. The model showed that a range of risk variables were of broadly equivalent importanc e in determining the development of leaf blotch. These risk variables shoul d be integrated in any scheme designed to support fungicide use decisions.