OSR growers warned of high phoma risk next season

Oilseed rape growers are being urged to prepare for a high risk of phoma stem canker in the coming season.

Late plantings of a large area of susceptible varieties in a wet autumn following the highest recorded incidence of spring phoma leaf spot since 2006, add up to a major risk of both early phoma damage and subsequent stem canker lodging and losses, says Philip Marr of Agrii.

“National CropMonitor recording showed 96% of crops and 39% of plants were affected by phoma leaf spot in the past spring,” he reports. “This was the highest incidence since 2006 and infection levels were almost as high in the North as they were across southern England.

“Thankfully, generally tall, well-established plants with very long leaf petioles restricted 2012 disease development into the stem and canker damage. Even so, I’ve seen crops with 80% of stems affected by stem canker this summer. So we know there’s plenty of inoculum about.

“At the same time, we know wet autumns promote the early release of air-borne spores from stubbles – especially so where trash is left on the surface. And late oilseed rape sowing means slower developing plants with smaller leaf petioles that are more immediately susceptible to both early leaf damage and stem canker development.

“Add to this the fact that 55% of all varieties on the current Recommended List and no less than 60% of the top 10 East and West varieties have stem canker resistance scores of less than 5.0 and you can see why I’m worried,” he says.

In the face of this increased risk, he advises growers to be particularly conscious of the stem canker resistance rating of the varieties they are growing this season. “Early winter leaf and plant losses can be severe in slow-developing, susceptible varieties.

“If you’re growing a susceptible variety you’ll need to be right on top of your game with phoma monitoring and spraying to ensure adequate control as soon as phoma thresholds are reached this autumn and winter,” he says.

“I’d be taking the threshold as 10% of plants showing leaf spotting and retreating any reinfections that occur before stem extension as soon as they’re evident.”

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