Light leaf spot risk prediction helps growers - farmers weekly


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23 JANUARY 1998 ------------------------- LIGHT LEAF SPOT RISK PREDICTION HELPS GROWERS PROPHETS OF CROP PESTILENCE WERE IN FULL FLOW AT AN ARIA WORKSHOP LAST WEEK. ANDREW BLAKE REPORTS ON


THEIR LATEST EFFORTS TO PREDICT TROUBLE AHEAD MUCH fungicide used against light leaf spot on winter rape in the past was wasted, said the HGCAs Paul Meakin. The latest risk assessment scheme


from IACR, ADAS, SAC and CSL should help change that. Despite fungicide spends of up to £9m a year, national losses from light leaf spot can be considerable, he said. In 1994/95 they were


£45m, but fell to £13m a year later, possibly reflecting increased spraying. "The problem is the disease is so variable between years." Maximum response comes from autumn


treatment. But symptoms rarely show before January and growers cannot afford insurance tactics, said Dr Meakin. The new seasonal risk prediction has operated for two years, with encouraging


results in its first season, he said. Issued in October and based on surveys of the amount of inoculum the previous July, it gives regional forecasts of the proportion of plants affected the


following March. "We have only one years data so far – from the 96 forecasts for spring 97. But the results show a fair correlation between the predicted and actual levels." Last


autumns forecasts suggest less than 10% of plants in the east will be affected by next March, but up to 40% in the north, he noted. New ways of updating the estimates during the season,


including molecular diagnosis to detect infections before symptoms appear, are being explored. These should help growers decide best strategies, perhaps using half-dose treatments initially


and topping up only if required, said Dr Meakin. FORECASTING _&#8226 Pea foot rots: Sound method._ _&#8226 Beet powdery mildew: Proven._ _&#8226 Beet virus yellows: Ongoing._


_&#8226 Pea foliar diseases: Tricky._ _&#8226 Beet rhizomania: Early stages._ _&#8226 Cereal BYDV: Validating._ _Two relatively low virus years have slowed progress with the


HGCA/MAFF-funded BYDV forecasting scheme, admits to Richard Harrington (inset) of IACR-Rothamsted. "We have had an awful lot of zeros in the data, which makes analysis difficult."_


_Fishing for a forecast: This sticky line aphid snare supports suction trap results for BYDV forecasting._