Infectious diseases are caused by fungi, bacteria, viruses and nematodes, which all require prolonged periods of free moisture to spread and infect plants. Dry weather conditions normally prohibit their activity. Most of the diseased that drought inhibits attack leaves and stems of plants, such as fireblight of crabapple and pyracantha, scab of crabapple and mountain ash, leaf spot of rose, and needlecasts of evergreens. Home owners can encourage these disease during drought by wetting the foliage and stems of plants during irrigation. On the other hand, drought favors some diseases.
Notable examples are Dutch elm disease.
Verticillium wilt of maple and Armillaria, Rhizoctonia and Phytophthora root rots. Stress makes plants more susceptible to all of these diseases. So, the primary control of these diseases is the elimination of stresses that encourage their activity. If trees or shrubs die from disease, it is a good idea not to replace them with susceptible plants. During drought, watering to minimize moisture stress will help plants resist disease attack.