FACTORS DETERMINING THE SEVERITY OF EPIDEMICS OF MYCOSPHAERELLA-GRAMINICOLA (SEPTORIA-TRITICI) ON WINTER-WHEAT IN THE UK

Epidemics of disease caused by Septoria tritici were studied in detail in 11 crops of winter wheat cv. Longbow over 4 years. Serious damage to the uppermost two leaf layers was caused by splash-borne infection from lower in the crop early in the life of the leaves, followed by on e or rarely two cycles of multiplication within a leaf layer. Infectio n conditions rarely limited damage, even in a dry year; the timing and , to a lesser extent, amount of initial inoculum movement to an upper leaf layer was of greater importance. Timing of initial infection was determined by when rain splash occurred in relation to emergence of a leaf layer. Occurrence of infections soon after a leaf layer started t o emerge allowed more time for multiplication of disease within that l ayer. These infections tended to be more severe because the leaves wer e closer to inoculum sources within the crop. Slight differences in ph enology between locations explain why initially random disease distrib utions sometimes become aggregated. Early-sown crops are at greater ri sk because they mature more slowly, allowing more disease multiplicati on and better transfer between leaf layers.