Watch out for early phoma infections in oilseed rape crops, growers are being warned. Threshold levels could be reached in some fields in the next fortnight, according to an initial forecast by Rothamsted Research.

“It all depends on local weather,” says Rothamsted plant pathologist Neal Evans. “Where it’s been warm and wet, you can expect the disease to show up any time from now.”

He anticipates that infection will be a long, drawn-out affair. “There’s a small amount of inoculum out there at the moment, but that will increase considerably over the next two to three weeks. So all crops are at risk.”

Signs of the disease have already been observed by company agronomists in Northamptonshire, Cambridgeshire and the south west, confirms ProCam agronomist Nick Myers, but levels are not yet causing concern.

“The weather has been conducive to the disease, with rain throughout August and September, and some crops have been in the ground for a while.

“That means growers should be vigilant, especially where they were able to drill early. Of course, there are still clean crops out there too.”

When 10% of plants are infected with phoma the crop should be sprayed with a triazole-based fungicide, he adds.

The dry autumn in 2009 meant there was hardly any phoma seen in crops before Christmas. “Every year is different with phoma infection timing,” says Mr Myers. “One of the main deciding factors is 20 days of rain from 1 August. And we’ve certainly had that this season.”

South west independent agronomist Steve Harrison has also seen the very beginnings of phoma. “There are some pale, round lesions just appearing in forward crops. It’s no surprise, given the rain that we’ve had.”

Leaf miner damage is also apparent, he adds. “The two can be confused, so make sure that you’ve checked carefully. With leaf miners, the white areas of the leaf are where the damage has been done.”

Mr Harrison reminds growers that some of the leaves affected by early phoma infection are the ones which senesce and drop off, especially with big crops.

“Early infection doesn’t necessarily mean that it’s going to be a bad phoma year. Just make sure that you’re ready to spray at the 10% threshold level.”

Phoma Forecast

The web-based phoma forecast issued every year by Rothamsted Research is up and running this week.

The forecast gives regional information on the date when 10% of oilseed rape plants are expected to show symptoms, so helping growers with the timing of fungicide applications.

In Hertfordshire, phoma is due 15 days earlier than last year, Dr Evans reports. In Oxfordshire, the disease is due to show up 12 days earlier, while Wiltshire crops are expected to show symptoms 11 days ahead of schedule.

“It’s a case of here we go. Even though last year’s epidemic was late and was then curtailed by the very cold weather, things are getting going in the south.”

As a result, he believes all crops are at risk. “The concern is always for the backward crops, but it’s worth checking all crops, regardless of their size, this season.”

The forecast can be accessed at www.rothamsted.bbsrc.ac.uk/leafspot