Autumn sprays make it pay
Autumn sprays make it pay
Keeping oilseed rape
disease-free now could
provide a significant boost to
yields next harvest. Our
latest baseline advice article
explains what is needed
WITH much of this years oilseed rape crop drilled early into warm, moist seed-beds, weekly monitoring of crops is now needed to ensure disease is tackled effectively, say experts.
Rain over the past two weeks has made phoma a real threat to crops in high-risk areas of the country as spores are spread by rainfall, says Bruce Fitt of IACR-Rothamsted. Light leaf spot is also a concern.
"The timing of the first main flush of phoma ascospores is important, as it determines when leaf spots are seen," says Dr Fitt. "Our work shows leaf spotting becomes visible 14-25 days after spore release."
That was confirmed last year. The main flush of ascospores was detected on Sept 24, with spotting seen on Oct 18. "This year spores were released even earlier, on Sept 19."
Early leaf spots on young, sensitive plants cause the most severe cankers, he adds. "And the warmer the conditions, the quicker the disease will grow and reach the stem. Conditions all appear to favour the disease this year."
Phoma forecasting
According to Judith Turner of CSL, phoma levels have been increasing over the past few years, with 57% of plants affected by the disease in 2000, causing losses of over £44m. "The eastern region has the biggest problem," she adds. "But it also causes regular concern in the north and the south-east."
Work on predicting phoma epidemics aims to produce a forecast similar to that for light leaf spot. To date, key factors have been identified – now they must be matched to agronomic information.
Control strategies
Until phoma forecasting starts growers should look for the first signs of disease before they spray, advises Peter Gladders of ADAS.
"Early disease in small plants poses the greatest risk. So that should be the priority." On high-risk sites yield loss can be as much as 0.75t/ha, so spraying is worthwhile, he says.
Phoma fungicides have limited "kick-back" and persistence, so the life of the first spray may be only three weeks. "An October spray should be followed up four to six weeks later. There will be a greater yield response if light leaf spot is active too."
Punch C (flusilazole + mbc) and Plover (difenoconazole) are the best available fungicides for phoma control, he advises. "But timing is crucial. They must be applied before disease reaches the stem."
He suggests growers identify situations where disease is likely to cause yield loss above 0.5t/ha. That means a spray threshold of 10-20% of plants showing spots this year, he says.
Light leaf spot
Light leaf spot is predominant in the north and west, as it likes cool, moist conditions. It can cause yield losses of 1.5t/ha each year, with growers spending a total of £5m a year on fungicides.
Symptoms are not usually seen until late winter, which can make spray timing difficult, warns Neal Evans of IACR-Rothamsted. "The most effective spray timing is often late autumn, but there is no sign of the disease at that stage.
"Last summer, pod disease levels were lower than we expected in the north. But there has been plenty of rain to allow carryover of inoculum, so we think the risk is quite high for next spring."
To get a forecast growers can visit the light leaf spot forecast, developed with HGCA funding, at www3.res.bbsrc.ac.uk/leafspot. Simply enter details of variety and sowing date to get guidance on local light leaf spot risk. Visiting the site regularly ensures latest rainfall figures are accounted for.
Recognising light leaf spot
lInspect crops monthly from October to March.
lCollect 100 plants in a diagonal across the crop.
lIncubate plants in polythene bags at 10-15íC for four to five days to assess incidence.
lEconomic threshold for spraying is 25% plants infected. *
Act now to stop light leaf spot in its tracks (left) and prevent phoma leaf spotting moving on to stems, where it can cause yield-sapping cankers next summer. That is the consistent message from crop experts.
10-POINTCONTROLPLAN
1Crop progress Most crops drilled early into warm, moist seed-beds.
2Variety Larger variety portfolio than in previous years, but all responsive to fungicides.
3Phoma Phoma predominant disease, especially in east, accounting for losses of £44m.
4Phoma risk First flush of ascospores on Sept 19, so watch for early epidemic.
5Phoma control Spray when 10-20% of plants show spotting and follow up four to six weeks later.
6Plant size Small plants at greatest risk, spray first.
7Fungicides Punch C and Plover preferred phoma products.
8Light leaf spot Worst in west and north.
9Forecast Visit www3.res.bbsrc.ac.uk/leafspot for regional guidance on light leaf spot risk.
10Recognition Incubate plants to assess incidence of disease and spray when 25% plants infected.