August 1, 2014
Light Leaf Spot Hits OSR Performance
Light leaf spot is the main reason for the disappointing performance of many oilseed rape crops across the country this season, reports Agrii OSR specialist Philip Marr. Indeed, he has found infections causing yield losses of 30-50% in many cases through a combination of very small seeds and premature pod opening.
“Verticillium is being blamed for disappointing yields by some,” he stressed. “But this season has seen some of the lowest levels of verticillium infection in recent years.
“In complete contrast, light leaf spot has become more and more obvious on a national scale as the season has gone on. From widespread leaf spotting as the weather warmed up in the spring, it has progressed into the most severe stem blotching I’ve ever seen on susceptible varieties, coupled with serious pod lesions in many crops.
“Yield losses have been severe in varieties with poor disease resistance ratings. In addition to small seeds resulting from restricted translocation as well as photosynthesis during grain fill, pod infections have led to high levels of shedding ahead of combining, even without the sort of storms many have suffered around this year’s harvest.
“One of the most resistant varieties in our Brotherton trials showed hardly any signs of infection and went on to produce over 5.5t/ha,” Philip Marr noted. “Yet, alongside it we brought in barely 3.5 t/ha from one of the most susceptible which was riddled with stem and pod lesions.
“Varieties without pod shatter resistance appear to be suffering more than most in farm crops as well as in trials. In one case, a grower complained to me that, even with careful harvesting, he left more seed from one leading variety in the field than he put through the combine!”
Under these circumstances, with high levels of light leaf spot incidence recorded across much of the UK and with a far faster apparent decline in LLS sensitivity to triazoles than the much-reported shift in Septoria triciti sensitivity in wheat, Philip Marr urges growers to take a decidely ‘northern’ approach to future crop management.
This includes insisting on a minimum LLS resistance score of 6.0 in the varieties grown and autumn spraying to prevent infections becoming established. “Light leaf spot only becomes evident when the weather warms up in the spring,” he explained. “But by then, unless we have decent levels of resistance in our varieties, the triazole shift means we have precious few defences left these days.
“Increasingly, this means we need to be nipping infections in the bud when they take place in the autumn – adding extra LLS activity to our phoma spraying wherever the disease has previously been evident. As an extra protection, we should also be putting more emphasis on growing varieties with high levels of pod shatter resistance.
“More than half the varieties on the current East & West Recommended List have a light leaf spot resistance score of less than 6.0,” Philip Marr pointed out. “These can clearly perform well where the disease isn’t a problem. However, on the evidence of recent years – and this season’s losses in particular – I consider the majority to be far too much of a risk for most UK growers; especially with every kilo of yield so vital at current crop values and a good selection of varieties with higher levels of resistance so widely available.
“It’s high time we took an altogether better-informed agronomic risk management approach to counter what has fast become the single biggest disease threat across such a large part of the country.”