Ideal conditions for the disease’s formation, enhanced by shorter rotation intervals, has meant light leaf spot pressure has this season been higher than for some time, with effects evident in oilseed rape crops some way south of its usual northern UK environment, report agronomists.
The situation should lead growers in susceptible areas to place resistance to the disease higher up their list of priorities when selecting varieties for next year, believes NIAB TAG’s Dr Jane Thomas.
“Light leaf spot is a problem in the north and Scotland more or less every year,” points out Dr Thomas. “It thrives in the sort of cool and wet autumn conditions that are more common to this region.
“But in recent seasons, and especially in 2010-11, we have seen greater than usual pressure in England – mainly in the west and south, but also in the east to an extent.
“Intensive oilseed rape rotations help to create carry-over of the disease, and there has been plenty of opportunity for build-up. Although infection occurs in the autumn, light leaf spot actually thrives in the sort of cool, wet autumn and cold, frosty winter we had this year – even in the south. And a run of years that favour the disease’s development can build up its presence on crop debris. Severe infestation can mean yield penalties of up to 1.0t/ha, and individual plants can be killed out.”
While the disease establishes itself early on the plant’s lifecycle, in the crown of the plant, the first symptoms often appear only around Christmas, she points out, when a light green discolouration becomes evident, along with leaf crinkling and the formation of small white spore droplets.
“Where the disease has already been an issue, and the focus is on the next rape crop in the rotation, ploughing is probably the best method for incorporating disease-harbouring material from the previous rape crop in order to prevent carryover. But, of course, fewer farms are using that as their favoured method of cultivation.
“Rotations also need to be broadened if the disease is to be managed, with at least a three-year gap between oilseed rape crops if possible. The move on many farms to alternate wheat and rape has helped light leaf spot’s spread and development.
“Without a full fungicide programme, even moderate resistance may not be enough to prevent yield losses, and a two-spray (autumn and spring stem-extension) fungicide strategy is essential even on partially resistant varieties to get the most from them” she suggests.
Craig Padley, head of oilseed breeding at LS Plant Breeding (LSPB), comments that the restored hybrid Palace, Recommended for the east/west region, carries a 7 rating for light leaf spot resistance – the highest of the fully recommended varieties on the 2011-12 Recommended List.
“The higher the pressure, the more help resistance provides, and a variety with a high rating for light leaf spot is essential if rape is to be grown in a known high pressure area,” he adds.