Optimising the use of fungicides to control stem canker of oilseed rape

Ascospores of Leptosphaeria maculans infect leaves of oilseed rape to cause leaf spots, from which the fungus can grow to infect the stem. Yield losses due to early senescence and lodging result if the stem infections reach a threshold severity prior to harvest. Host resistance alone has not been sufficient to control the disease in the UK where two forms of the fungus [A group (Tox+) and B group (Tox0)] occur, each with different pathogenicity groups. Currently, approved fungicides do not eradicate mycelium once it is inside the stem, so disease management relies on protection of the leaves. Fungicides on the leaf surface degrade and are diluted by leaf expansion, while new leaves are unprotected. The mild, wet climate of the UK and parts of France promotes the release of a succession of spores from different parts of stem debris. Therefore, one or more well-timed fungicide applications are needed. Weather data from different seasons were compared with the corresponding airborne spore numbers, leaf lesion incidence and canker incidence and severity. At early crop growth stages it is important to apply fungicides very promptly, if infections occur during this period. However, when an epidemic is late and the crop is more mature when leaf spotting first appears, a longer period of time exists in which a fungicide application can achieve economic control. Accurate forecasting of fungicide application can therefore not only improve disease control but also decrease fungicide use when the risk of crop damage is small