September 4, 2012
Prepare for Phoma Stem Canker Storm
Prepare for a serious phoma stem canker storm in the coming season, oilseed rape growers have been warned by national crop specialist, Philip Marr of Agrii.
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, he is adamant, add up to a major risk of both early phoma damage and subsequent stem canker lodging and losses.
“National CropMonitor recording showed 96% of crops and 39% of plants affected by phoma leaf spot in the past spring,” he reported. “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 OSR 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 & West varieties have stem canker resistance scores of less than 5.0 and you can see why I’m worried,” he pointed out. “If we’re not very careful indeed, the sort of lodging we’ve seen as a result of the 2012 summer deluge will be as nothing to the crop damage we could easily be facing next year.”
In the face of this worrying position, Philip Marr advises growers to be particularly conscious of the stem canker resistance rating of the varieties they are growing this season.
He stresses that early winter leaf and plant losses can be very severe in slow-developing susceptible varieties. At the same time, he considers good disease resistance every bit as essential with tall, vigorous hybrids to minimise the risk of devastating summer lodging from canker stem weakening.
“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,” insisted Philip Marr. “I’d be taking the threshold as 10% of plants showing leaf spotting and re-treating any re-infections that occur before stem extension as soon as they’re evident.
“Given the likely disease pressure, even varieties like DK ExPower and DK Excellium with exceptional levels of stem canker resistance from the RLM 7 gene will need spraying. But the greater leeway they give you in spray timing means you can safely hold off until 20% of plants are affected.
“With all the concern over new diseases like verticillium in recent years, many people seem to have taken their eye off the phoma stem canker ball in recent years,” he concluded.
“This season, in particular, everyone would do well to remember that phoma remains the most economically important oilseed rape disease in England. Not least with tighter rotations, larger numbers of volunteers and limited peak spraying capacity on so many farms.”