GIVEN current weather conditions, phoma leaf spotting could start to become evident by early October, ADAS' plant pathologist, Faye Ritchie, has predicted.

This year, though, growers can be better prepared to take action, thanks to Syngenta’s new 'Phoma alert' system.

Managed by Faye and an ADAS team, to monitor reference crops in Herefordshire, Yorkshire and Cambridgeshire, it will give a valuable update of pending disease pressure to enable more proactive spray timing and give an early relative value of varietal disease resistance ratings.

“We know that many areas have already received the critical 20 days with rainfall from the August 1, which has primed cankers on last year’s crop trash to release ascospores,” she reported. “With each further rain event, repeated spore releases would be triggered and, with crops infected, the onset of characteristic leaf spotting.”

She warned that, unless leaf spots are controlled, infection will move down the petiole to the stem. The earlier stems become infected, the greater the size of canker that will develop in the spring and the affect on yield.

“Backward crops are more susceptible, since infection will move to the stem faster on smaller leaves; particularly if weather conditions remain mild when disease development is faster,” she advised.

“Varietal resistance appears to help growers by slowing the development of infection on the leaf, which could gain valuable flexibility in fungicide timing. The 'Phoma Alert' will help give practical guidance as to how the resistance ratings could help tailor fungicide programmes.”

First treatments should be applied as soon as leaf spotting is identified on a threshold 10-20% of plants. Small plants, warm temperatures, repeated rain showers and susceptible varieties would all be factors to warrant earlier treatment at lower thresholds, she suggested.

Phoma would typically be the prime target for treatments in early to mid-autumn, though growers may look to introduce some additional light leaf spot activity with later follow-up applications if required, later in November.

Syngenta's technical manager, James Southgate, advised growers to treat with straight Plover in the first instance, to provide cost effective control of the most damaging early phoma.

“Protecting yields on the significantly reduced area of oilseed rape that appears to have been established this season will ensure growers can benefit from any resulting further increase in oilseed values,” he advised. Phoma trials have repeatedly shown yield loss of 0.5 t/ha without treatment – worth around £155 at current prices.