A wet August has prompted an early warning of phoma to oilseed rape growers, who in many cases have taken advantage of moist seedbeds to get crops drilled and established quickly.
In the South and East, growers were able to get OSR drilled well before the end of August. But with some areas having received half the average monthly rainfall in the first 10 days of the month, growers are now being advised to monitor crops carefully for thresholds being exceeded.
Constant re-wetting of crop debris favours phoma spore development and release and with twelve days with rain already recorded at ADAS sites in Cambridgeshire and Herefordshire, for example, advice is to treat crops against phoma when 10-20 per cent of plants are affected.
ADAS plant pathologist, Faye Ritchie says: “It all depends on what happens from here, but it has been a wet start to August and spores require roughly 20 days with rain to mature. If it remains unsettled then we could expect to see leaf spotting in late September. However, monitoring crops to determine when thresholds are met will be what matters most.”
On the plus side the rain has meant that crops emerged within five days of being drilled and so good growth could limit the disease spreading from leaves to petiole and then stems.
Bayer’s Jack Hill says with many varieties having the RLM7 resistance gene, it does offer some resilience against the disease. But he warns it will not necessarily remove the need for a fungicide. “A good phoma-rated variety will help delay the 10-20 per cent plant affected threshold being reached, but might not mean you can avoid having to treat at all,” he says.