Upsurge of Lettuce Downy Mildew (March 19, 2014)

Pest control advisors reported recently that downy mildew had become more prevalent in Yuma Valley lettuce fields. This may seem surprising since very little rainfall has occurred in the area. However, as mentioned in an earlier article, rainfall is not an absolute requirement for downy mildew development. Spores of the lettuce downy mildew pathogen form at night when the relative humidity in the leaf canopy is near or at 100%. Ample water is supplied through irrigation to create high humidity conditions and facilitate dew formation in the lettuce canopy as the crop develops. Calm evenings with little or no wind also favor spore production by allowing humid air to remain in the leaf canopy. Data from AZMET weather stations at the UA Yuma Agricultural Center and in the South Yuma Valley show that the average wind speed during January and February this year was 3.9 compared to 5.2 M.P.H. during the same time period in 2013. Also, the mean air temperature during these months was 59°F this year compared to 54°F in 2013. The latent period (the time from spore infection of leaf to first appearance of leaf lesion and beginning of spore production) ranges from 4 to 7 days at temperatures of 68 to 72°F and 24 to 34 days at 42°F. The increased temperatures recorded this year compared to 2013 would favor a shortened latent period, resulting in more cycles of infection and sporulation and a faster rate of disease development. Another factor affecting downy mildew severity is the genetic resistance or tolerance (or lack thereof) of the lettuce cultivars in fields at this time. Several factors impact the final severity of downy mildew in any given lettuce planting; however, air movement and temperature recorded in January and February of this year were more favorable to downy mildew development compared to the same months in 2013.

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