Phoma control crucial as OSR area expands

Early infections of phoma in oilseed rape will force growers to treat earlier than usual this season, with tighter rotations and climate change unlikely to ease the pressure in future seasons.


The warning came from a Bayer CropScience symposium last week where Rothamsted researcher Jon West estimated stem canker caused by phoma costs growers £40m a year.


Agronomists, a merchant, a breeder, farmers and the firm’s representatives all agreed that with oilseed rape needing to be viewed as much as a cash crop as a cereal break, growers should combine inbred resistance and fungicides to minimise yield-sapping spring cankers.


Relying on resistance alone was not enough.


Mild winter


Heavy infections last season, partly because of the mild winter, provided plenty of inoculum, and the wet August and warm September meant infective spores were released early to attack this autumn’s sowings, explained Dr West.


“In places leaf spotting is already at [spray] threshold levels and could cause severe cankers if left untreated,” he said. “So we’d advocate that growers be vigilant.” September was 3C warmer than average, he noted.


“This is only the second year that I’ve been finding phoma in September,” said Norfolk-based AICC agronomist Mike Thompson.


The biggest losses come from early infection. That’s because the fungus grows faster in warm weather and can reach the stems more quickly, as the leaves are relatively small.


“The key is to treat infections on the first six leaves,” said Dr West.”Later on you can get 100% of plants infected with no significant yield loss.”


Frontier Agriculture’s Charlie Whitmarsh, who estimated this season’s winter rape area had risen by 5% to about 600,000ha (1.48m acres), reckoned phoma management was becoming increasingly important as growers needed to strive for maximum output rather than cost savings.


“Phoma is still underestimated,” he said.


Treatment


He believed that with the drive for lower seed rates leaving thinner populations, growers might need to consider treating before the current 10% spotted plants spray threshold.


Monsanto’s Geoff Hall, who noted hybrid vigour helped defend against phoma, stressed the need for stewarding varietal disease resistance.


“Disease resistance is fragile. Once it has broken down we have lost it forever,” he said. “Resistance should always be used in conjunction with a robust fungicide programme.”


Dr West agreed. “It’s important to treat new resistant varieties to reduce small amounts of disease on them, because the more there is, the more likely their resistance is likely to break down.”


Lincolnshire farmer James Walgate has been involved in national oilseed rape trials and achieved a yield of over 5.4t/ha (2.2t/acre) last harvest.


But with tighter rotations he believed there was too much canker in stubbles.”We’ve got to take phoma more seriously and clean it up.”


Until recently standing power had been Cereals 2004 host Andrew Ward’s priority when choosing winter rape varieties.


But, with his rotation tightened to growing the crop one year in three, phoma resistance was also becoming increasingly important, he acknowledged.


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