The current desert winter vegetable growing season is winding down, most assuredly to the delight of pest control advisors. One big disease pain in the neck this past season, on both sides of the Colorado River, was downy mildew. This disease, which can develop on lettuce, spinach, onions, and cruciferous crops in our area, has been especially severe during the current 2016-17 vegetable growing season as well as in 2014-15. On the other hand, overall downy mildew severity in the 2015-16 production period was considerably lower. Why the difference? Development of downy mildew and the activity of the causal pathogens of the disease are known to be stimulated by the presence of water. One’s first thought might be that the amount of rainfall could be used to predict potential downy mildew epidemics. Rainfall is important, but the critical factors affecting downy mildew development are the number of hours during the late evening and morning that leaves are wet as well as the number of days that these leaf wetness conditions occur. During leaf wetness periods, the downy mildew pathogens produce and release spores, which when landing on healthy plant tissue germinate and cause new infections. So, it is the duration of plant wetness and not just rainfall itself that is the driver of downy mildew epidemics. This fact is demonstrated by comparing rainfall and leaf wetness events for the two seasons with high downy mildew (2016-17 and 2014-15) to the time period when disease intensity was lower (2015-16). The average rainfall recorded at three AZMET weather stations (Yuma Valley, N. Gila Valley, and Roll) from October through February during the high disease 2016-17 and 2014-15 seasons compared to the lower disease 2015-16 season was 2.37, 0.85, and 0.74 inches, respectively. On the other hand, for the same time period and respective seasons, the average total number of nights when relative humidity was at or above 90% (a level at which dew formation would be expected) was 56, 55, and 30. The higher incidence and severity of downy mildew observed during 2016-17 and 2014-15 compared to 2015-16 correlates very well with the number of recorded high humidity nights, but not with average rainfall.

To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.