Phoma leaf spotting in Britain's oilseed rape is expected to show up a little later this autumn because of drier weather, according to the latest disease forecast from AHDB, the Agriculture and Horticulture Development Board. The forecast puts the start of the fungicide treatment peak around the middle of October.
The model estimates when 10% of oilseed rape plants may develop phoma symptoms. It runs on rainfall and temperature data from 15 July to 26 September to simulate how the pathogen develops, AHDB senior crop protection scientist Catherine Harries explained; damp, warm conditions speed up spore release from infected stubble, infection of the crop and the progress of the disease.
A relatively high number of dry days in many areas has moved the predicted onset slightly later than in recent years. Nearly half of the forecast sites are expected to reach the 10% threshold in the week commencing 18 October and just over a quarter in the week after. A relatively small number, mainly in the western half of Great Britain, will reach it in the first two weeks of October.
Because early phoma epidemics do the most damage to yield, a later start should generally reduce the disease's impact. Left unchecked, however, the disease can form the classic stem canker around the leaf scars at the base of stems, especially in varieties that are relatively susceptible.
Harries advises treating small plants first, along with varieties whose Recommended List phoma resistance rating is relatively low, once 10% of plants show leaf spots. The urgency rises as the rating falls below 7, a group that includes many popular varieties according to AHDB's planting and variety survey. Varieties with high phoma resistance may not need a spray unless the 20% symptom threshold is passed.
Photo: Reaperman / Wikimedia Commons (CC BY-SA 3.0)
Source: Farmers Weekly
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