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Field Evaluation of Fungicide Spray Advisories Against Lettuce Downy Mildew (Bremia lactucae) Based on Measured or Forecast Morning Leaf Wetness. H. Scherm, Department of Plant Pathology, University of California, Davis 95616. S. T. Koike, University of Cali-fornia Cooperative Extension, Salinas 93901; F. F. Laemmlen, University of California Cooperative Extension, Santa Maria 93455; and A. H. C. van Bruggen, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 79:511-516. Accepted for publication 13 March 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0511.

In a previous field study in coastal California, infection of lettuce by downy mildew (Bremia lactucae) occurred primarily on days when leaves dried late in the morning (at 1000 h or later), suggesting that fungicide sprays against the disease could be scheduled according to a morning leaf wetness threshold. Seven field trials were carried out in 1993 to 1994 to test whether sprays of maneb or fosetyl Al, applied when 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 schedule with three sprays. Based on measurements of morning leaf wetness, the total number of sprays in the seven trials was reduced by 67% relative to the calendarbased schedule, with no difference in disease intensity. Based on forecasts of morning leaf wetness, which were generated using a physical dew simulation model with numerical weather forecasts from the National Meteorological Center as input, about 90% of the days were correctly classified as days with or without prolonged morning 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 the number of fungicide applications against lettuce downy mildew can be reduced substantially with sprays scheduled according to a morning leaf wetness threshold of 1000 h and that fog drizzle should be included in leaf wetness forecasts for coastal California.