FIELD-EVALUATION OF FUNGICIDE SPRAY ADVISORIES AGAINST LETTUCE DOWNY MILDEW (BREMIA-LACTUCAE) BASED ON MEASURED OR FORECAST MORNING LEAF WETNESS

In a previous field study in coastal California, infection of lettuce by downy mildew (Bremia lactucae) occurred primarily on days when leav es dried late in the morning (at 1000 h or later), suggesting that fun gicide sprays against the disease could be scheduled according to a mo rning leaf wetness threshold. Seven field trials were carried out in 1 993 to 1994 to test whether sprays of maneb or fosetyl Al, applied whe n measured or forecast morning leaf wetness ended at 1000 h or later, would lead to a reduction in the number of fungicide applications and/ or improved downy mildew control compared with a calendar-based schedu le with three sprays. Based on measurements of morning leaf wetness, t he total number of sprays in the seven trials was reduced by 67% relat ive to the calendar-based schedule, with no difference in disease inte nsity. Based on forecasts of morning leaf wetness, which were generate d using a physical dew simulation model with numerical weather forecas ts from the National Meteorological Center as input, about 90% of the days were correctly classified as days with or without prolonged morni ng wet period. However, the occurrence of important wet periods due to fog drizzle was not predicted and the forecasts for the exact time of onset and end of leaf wetness were inaccurate. It is concluded that t he number of fungicide applications against lettuce downy mildew can b e reduced substantially with sprays scheduled according to a morning l eaf wetness threshold of 1000 h and that fog drizzle should be include d in leaf wetness forecasts for coastal California.