The three ingredients required for development of most plant diseases include a susceptible host, a pathogen capable of infecting that host, and a favorable environment. Two components of the environment, temperature and moisture, are critical factors affecting 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 the temperature and/or moisture levels prohibit the pathogen from interacting with the host to cause disease. This explains why some diseases only appear on a crop during particular times during the growing season for that crop. For example, Fusarium wilt on lettuce in the desert is found primarily during the fall, but not during the winter months, as soil temperatures in the fall, but not the winter months, favor the growth of the pathogen and disease development. Also, downy mildew on winter vegetables such as lettuce, cruciferous crops, onions, and spinach is usually a concern in the winter and early spring, but only when periods of leaf wetness caused by rainfall and dew are present. Periods of high humidity and leaf wetness are essential for the downy mildew pathogens to grow, proliferate, and cause disease. The generally dry conditions prevalent in the desert benefit growers by restricting foliar diseases caused by bacteria and many fungi. The same 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 on 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.