COMPARATIVE-STUDY OF MICROCLIMATE AND DOWNY MILDEW DEVELOPMENT IN SUBSURFACE DRIP-IRRIGATED AND FURROW-IRRIGATED LETTUCE FIELDS IN CALIFORNIA

Microclimates and downy mildew (caused by Bremia lactucae) disease pro gress were monitored in neighboring lettuce fields with subsurface dri p 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 du ration, wind speed, and solar radiation were recorded. Disease intensi ty was assessed at intervals, beginning at thinning of the crop and en ding just before harvest. Wilcoxon's Signed Rank Tests were computed t o compare microclimates between drip- and furrow-irrigated fields, sep arately 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 less than or equal to 0.0025) a nd a trend toward higher daytime humidity (P less than or equal to 0.1 254) and longer morning leaf wetness periods (P less than or equal to 0.0863) in fields with furrow irrigation. Air temperature and nighttim e humidity were not consistently different between the two irrigation methods. Downy mildew developed in four of the five trials, and diseas e intensity was always fewer under drip irrigation than under furrow i rrigation. The magnitude of the differences in disease was small, howe ver. It appears that, on most days in coastal California, mesoclimatic variations outweigh microclimatic modifications that could potentiall y influence disease development.