Light leaf spot is the main reason for thedisappointing performance of many oilseed rape crops across the country thisseason, reports Agrii OSR specialist Philip Marr. Indeed, he has foundinfections causing yield losses of 30-50% in many cases through a combinationof 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 infectionin recent years.
In complete contrast, light leaf spot has become more and more obvious on anational scale as the season has gone on. From widespread leaf spotting as theweather warmed up in the spring, it has progressed into the most severe stemblotching Ive ever seen on susceptible varieties, coupled with serious podlesions in many crops.
Yield losses have been severe in varieties with poor disease resistanceratings. In addition to small seeds resulting from restricted translocation aswell as photosynthesis during grain fill, pod infections have led to highlevels of shedding ahead of combining, even without the sort of storms manyhave suffered around this years harvest.
One of the most resistant varieties in our Brotherton trials showed hardly anysigns 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 mostsusceptible which was riddled with stem and pod lesions.
Varieties without pod shatter resistanceappear to be suffering more than most in farm crops as well as in trials. Inone case, a grower complained to me that, even with careful harvesting, he leftmore seed from one leading variety in the field than he put through thecombine!
Under these circumstances, with high levelsof light leaf spot incidence recorded across much of the UK and with a farfaster apparent decline in LLS sensitivity to triazoles than the much-reportedshift in Septoria triciti sensitivityin wheat, Philip Marr urges growers to take a decidely northern approach tofuture crop management.
This includes insisting on a minimum LLSresistance score of 6.0 in the varieties grown and autumn spraying to preventinfections becoming established.
Light leaf spot only becomes evident whenthe weather warms up in the spring, he explained. But by then, unless we have decent levels ofresistance in our varieties, the triazole shift means we have precious few defencesleft these days.
Increasingly, this means we need to be nipping infections in the bud when theytake place in the autumn adding extra LLS activity to our phoma sprayingwherever the disease has previously been evident. As an extra protection, we should also beputting more emphasis on growing varieties with high levels of pod shatterresistance.
More than half the varieties on the current East & West Recommended Listhave a light leaf spot resistance score of less than 6.0, Philip Marr pointedout. These can clearly perform wellwhere the disease isnt a problem. However, on the evidence of recent years and this seasons losses in particular I consider the majority to be far toomuch of a risk for most UK growers; especially with every kilo of yield sovital at current crop values and a good selection of varieties with higherlevels of resistance so widely available.
Its high time we took an altogether better-informed agronomic risk managementapproach to counter what has fast become the single biggest disease threatacross such a large part of the country.