Phoma risk in oilseed rape increased by wet weather

Emerging oilseed rape plants may be under serious threat from phoma this season, a leading agronomist has warned.

Around 20 days of rainfall have fallen since August 1 which is enough to trigger ineffective spore release from phoma cankers in stubbles and trash, warned Ron Stobart, a NIAB TAG agronomist.

September is the time when fruiting bodies are releasing spores which can travel on the wind and infect surrounding fields.

Therefore Mr Stobart encouraged growers to take action at this early stage in the phoma lifecycle.

“Once the disease is deep-seated at the base of the stem there is little you can do to prevent the development of cankers in the spring,” he noted.

With the current conditions being favourable to phoma, Mr Stobart said growers need to consider the risks carefully.

Fungicide timing and targeting could therefore be crucial, even for growers who have drilled varieties with high scores for canker resistance.

“Don’t throw away your fungicide programme simply because you purchased a canker resistant variety,” Mr Stobart advised.

Trials at NIAB TAG have shown that applying fungicides for phoma control still gave yield benefits in canker-resistant varieties.

“Continue monitoring the crop and treat if 1 in 10 plants show signs of infection for low canker resistant varieties and 1 in 5 for the higher resistant varieties,” Mr Stobart added.

Canker-infected stems restrict the function of the plant, which weakens the stem, increases lodging risks and reduces yield.

Nick Myers, agronomy manager at ProCam, has already received reports of early phoma lesions in forward crops.

Crops should be treated with a suitable fungicide at the first sign of infection, he said, and a two-spray programme may be necessary in certain cases.

“If people are spraying in the next week, they may be looking at a follow-up spray 4-6 weeks later,” he explained.

Mr Myers said his phoma control fungicide programme is based around using either Punch C (fluzilazole + carbendazim) or Genie (fluzilazole), mixed with the growth regulator metconazole, if required.

However, ADAS plant pathologist Peter Gladders advised growers to confirm phoma in their crops before resorting to a two-spray programme.

“I would try to counter any pre-emptive strike early in the season and spray later when plants are showing 10-20% infection,” he said.

“Prothioconazole mixes have got a bit more curative activity than some other products, but in some cases, a growth regulator treatment like Prosaro (prothioconazole + tebuconazole) may be necessary.”