Environmental studies have been essential to understanding the distribution and spread of lettuce mildews in California. Downy mildew has been found to increase with cooler temperatures of the late fall, winter and spring in the Salinas Valley, while summer temperatures restrict it primarily to the coastal areas. Powdery mildew, however, is not found in the cool, northern and coastal portions of the valley, but rather increases toward inland areas and with warmer spring and summer temperatures.

A. S. Greathead, Farm Advisor, Monterey County, the Ferry-Morse Seed Co. and Major Farms, Inc., Salinas, cooperated in these studies.