Three ingredients are required for a plant disease to develop: a susceptible host plant, a pathogen capable of infecting that plant, and a favorable environment. Temperature and moisture are aspects of the environment that critically affect the development and severity of diseases caused by bacterial and fungal pathogens. A plant disease caused by these types of pathogens will not occur if temperature and/or moisture levels prohibit pathogen and host plant interaction. This explains why some diseases only appear during a particular time period during the growing season of a particular crop. For example, Fusarium wilt of lettuce in the desert is found primarily during the fall, but not during the winter months. Why? Because soil temperatures in the fall, but not the winter months, favor the growth of the pathogen and thus disease development. Another example; downy mildew on winter vegetables such as lettuce, cruciferous crops, onions, and spinach is usually a concern in late autumn, winter, and early spring, but only when periods of leaf wetness caused by rainfall, dew, and sprinkler irrigation are present for the required amount of time. For this disease, periods of high humidity and leaf wetness are essential for the downy mildew pathogens to grow, produce spores, infect plants and cause disease. The generally dry conditions prevalent in the desert benefit growers by inhibiting foliar diseases caused by bacteria and many fungi. On the other hand, these organisms can flourish in regions receiving abundant rainfall. Growers can’t control the weather; however, they do have control over irrigation practices, which in some cases can influence the severity of vegetable crop diseases. For instance, the severity of Sclerotinia drop of lettuce can be increased by over-irrigation, especially if this results in prolonged wetting of the bed top. Also, during periods of rainfall and high humidity, sprinkler irrigation can extend the duration of high foliar moisture and thus increase the severity of downy mildew.