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Vegetable IPM Updates Archive
Cooperative Extension
The Year of Downy Mildew (March 18, 2015)
The 2014-2015 vegetable production season in Yuma County, Arizona is approaching the finish line, and probably not fast enough for pest control advisors striving to manage downy mildew. The disease has been a tenacious foe this season, even in areas that did not receive appreciable rainfall and normally do not have high downy mildew pressure. The pathogens that cause downy mildew on lettuce, spinach, onions and other crops form spores at night when the relative humidity in the leaf canopy is near or at 100%. Water supplied to crops by furrow irrigation and especially to crops irrigated with sprinkler irrigation is sufficient to create high humidity conditions and facilitate dew formation in the crop canopy. Calm evenings with little or no wind also favor spore production by allowing humid air to remain in the leaf canopy. Four AZMET weather stations are located within the vegetable production region in Yuma County. The maximum relative humidity (RH) measured every evening can be a useful predictor of downy mildew development and severity. A look at the number of nights where the maximum RH was at or above 90% can help estimate the number of potential dew formation nights in the different regions where these weather stations are located. For example, from Nov 2014 to Feb 2015, the average number of nights with RH at or above 90% at the Yuma Valley, North Gila Valley, Roll, and Yuma South AZMET stations was 8, 16, 16, and 19, respectively. For the same time period during the 2013-2014 production season, the average number of nights with RH at or above 90% at the same weather stations was 4, 12, 14, and 14, respectively. The increased incidence and severity of downy mildew observed on lettuce and other crops at the Yuma Agricultural Center this season compared to last season correlates well with the RH levels recorded at the nearby Yuma Valley AZMET weather station during each season. Several other factors, especially a crop’s genetic tolerance or resistance to downy mildew, can impact the final severity of the disease in any given vegetable planting. However, as the number of hours of leaf wetness in the evening and early morning hours increases, the potential for downy mildew development and increased disease severity rises as well.

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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.

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