Phoma is rapidly infecting oilseed rape crops and growers seeing signs of the disease should apply fungicides at the earliest opportunity, say experts.
Infection escalated from almost nothing last week to more than 80% of plants infected with five-six lesions a leaf in some fields, says Pete Gladders from ADAS. “It’s unusual to see such a big increase and it’s the worst I’ve seen for the past three years. We normally get a bit of a run-in.”
He has seen these infection levels in Cambridgeshire and has heard similar reports from Lincolnshire and the West Midlands. There are also signs of phoma B, which has small, dark leaf spots 2-10mm in diameter, he adds.
Mild temperatures and a few wet days earlier this month are to blame, giving phoma spores a chance to form leaf spots, says Dr Gladders. “These spots are still small, at 3-5mm, but as the disease is moving so quickly it needs to be dealt with as soon as possible.”
In some cases, growers will have only a few days before infection reaches the stem, so fungicides should be applied as soon as possible. If infection descends into stems, it could result in yields losses of 0.5-0.7t/ha.
Plants at the two-three-leaf stage are most vulnerable and should be treated at the first sign of the disease, he says. “If there is a lot of disease on smaller plants you may need to go up to three quarter fungicide rates to get knock-down curative effects.”
However, there are generally lower infection levels in small plants at the moment. Larger plants at the five-six-leaf stage have a greater infection risk, but will be more resilient and could be left a week before spraying, he notes.
Strong fungicide products such as flusilazole and prothioconazole should be used for maximum curative effect and growers should expect to apply a follow-up spray in four to six weeks time. “This is when you are likely to see secondary infection. But if temperatures remain high, the fungicide activity could be reduced to three weeks.”
Further north, phoma infection is likely to be lower and growers should wait for phoma to reach the threshold of 10% of plants infected before spraying, he says.
Those who haven’t seen infection should monitor crops closely and keep an eye on Rothamsted Research’s phoma forecast, he says. “If you haven’t seen phoma this week, you probably will next week.”
Peter Riley, agronomist for Prime Agriculture in Norfolk, says phoma is now widespread across East Anglia. He saw the first signs of disease last week, but now it’s in almost every crop. Most are not yet at the 10% threshold, but will be soon, he says. “If you see signs of the disease you need to spray as soon as possible.”
Most fungicides are protectant so it is important to get them on early, he adds. Prothioconazole will be his main active of choice, but flusilazole and metconazole will also have a role.
But in Gloucestershire, Countrywide Farmers agronomist Neil Donkin has seen only pockets of phoma. “Most crops in my area have been planted with resistant varieties such as ES Astrid, which might be why I haven’t seen much. I wouldn’t be surprised if populations exploded in susceptible varieties because conditions are so warm.
• You can find Rothamsted Research’s phoma forecast at www.rothamsted.bbsrc.ac.uk
Dry spell helps East Anglia catch-up with drilling
A week of dry weather has put the East Anglian drilling campaign back on track after a fortnight of deluges brought drills to a standstill.
Clear skies this week mean good progress should be made, but there is a lot of drilling to do and many soils are still wet across the region, says Prime Agriculture agronomist Peter Riley. “Growers have got to make the most of this window in case the weather breaks again.”