OILSEED RAPE growers need to monitor crops closely as disease pressure is changing rapidly and is likely to increase, according to the Central Science Laboratory’s first Crop Monitor report of this season.
Most crops are developing well, with mid-August sowings at the 6-8 leaf stage and late August-sown crops about 4 leaves, it said.
But September rainfall at monitoring sites has been higher than recent years which means airborne phoma spores are already producing phoma leaf spots on the oldest leaves and cotyledons in some crops.
“Careful monitoring on a field by field basis is now required as early phoma epidemics cause considerable loss of yield in southern, eastern and central regions,” said the report’s author, Peter Gladders of ADAS.
Moderately resistant varieties with canker resistance ratings of 5 or 6 can lose up to 0.7t/ha, he said. Growers should examine at least 25 plants per crop on a weekly basis and apply a fungicide when 10-20% show phoma symptoms, he advised.
Downy mildew is also present on many crops so growers need to be careful not to confuse symptoms with phoma, Dr Gladders said.
“Typical downy mildew symptoms are yellow blotches with white fungal growth on the underside of the lesions. These become paler and bleached with age and show some irregular black flecking.”
By contrast, phoma leaf spot is usually green on the underside and lacks the white fungal growth, he explained.
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