Phoma prediction for OSR might be early

Phoma sprays could be required in many areas imminently, according to the latest leaf spotting predictions by Rothamsted Research. But dry weather might mean its predictions are slightly early.

The forecast, which is based on rainfall and, to a lesser extent, temperature in August and September predicts when growers might expect to see 10% phoma leaf spot at over 50 locations from Scotland to Cornwall.

In East Anglia and the south-east the prediction is for spotting to occur around 17-20 October, while further north the forecast is anywhere between 10 days earlier in North Yorkshire to late September in Scotland.

Those dates are used by many growers to help pinpoint when to apply their first spray.

But while the dates in this year’s forecast are a good guide to when growers should start looking, the dry weather in many parts in late September and early October might mean they are on the early side, Peter Gladders of ADAS suggests.

“The model uses the underlying assumption that there will be average weather from 26 September, when it makes its prediction,” he explains. “Where there hasn’t been any rain the date will probably creep later.”

In practice thresholds might not be reached until late October or early November in some areas, he believes. “It might a bit earlier in the west and north where it rained more.”

The general guidance is, once it does start to rain, get out there and look, he says. “It is going to be variable on the ground.”

The later start might mean one spray could be enough this autumn. “Some crops are growing quite strongly, so one spray might do it.

“On smaller crops growers might need to watch until Christmas – we’ve had quite big losses from December infections, although it can be difficult to get growers to spray in December though.”

Phoma appears relatively low risk, David Ellerton of ProCam, in Hertfordshire, says. “The prediction for our part of the world is 15 October, so in theory we should see it fairly soon. I haven’t heard any reports yet, although crops are so minute, it wouldn’t be easy to see.”

Those backward crops won’t need a growth regulatory fungicide, meaning flusilazole, prothioconazole or difenconazole are all valid as options, he believes. “I tend to go the flusilazole route as it good on both phoma and light leaf spot and is reasonably priced.”

Light leaf spot is more of a concern in North Yorkshire for AICC agronomist Patrick Stephenson, although neither disease is present currently. “We don’t tend to have a lot of phoma up here, and certainly there is no spotting at the moment.

“We’ve got big plants, so we might be using a growth regulatory fungicide, although the dry weather might keep growth under check and basing programmes on controlling light leaf spot.”

That disease has increased year-on-year over the past five years, Dr Gladders points out. “The big warning is for light leaf spot in early forward crops, and if growers are easing back on phoma control they might need to look again for light leaf spot in January and February. Last year by the time they saw it in March it was too late.”