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Comparative Study of Microclimate and Downy Mildew Development in Subsurface Drip- and Furrow-Irrigated Lettuce Fields in California. H. Scherm, Department of Plant Pathology, University of California, Davis 95616 . A. H. C. van Bruggen, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 79:620-625. Accepted for publication 6 April 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0620.

Microclimates and downy mildew (caused by Bremia lactucae) disease progress were monitored in neighboring lettuce fields with subsurface drip or furrow irrigation during five trials in 1992 to 1993. The trials included a total of ten irrigation events during which soil moisture, soil temperature, canopy air temperature and humidity, leaf wetness duration, wind speed, and solar radiation were recorded. Disease intensity was assessed at intervals, beginning at thinning of the crop and ending just before harvest. Wilcoxon's Signed Rank Tests were computed to com-pare microclimates between drip- and furrow-irrigated fields, separately for days before or after irrigation. There were no significant differences in microclimate between the two irrigation methods before irrigation. Within 3 days after irrigation, there were significantly longer overall leaf wetness periods (P ? 0.0025) and a trend toward higher daytime humidity (P ? 0.1254) and longer morning leaf wetness periods (P < 0.0863) in fields with furrow irrigation. Air temperature and nighttime humidity were not consistently different between the two irrigation methods. Downy mildew developed in four of the five trials, and disease intensity was always lower under drip irrigation than under furrow irrigation. The magnitude of the differences in disease was small, however. It appears that, on most days in coastal California, mesoclimatic variations outweigh microclimatic modifications that could potentially influence disease development