Trees, especially fruit trees, are susceptible to diseases. There are many diseases that attack fruit and fruit trees. This apple tree exhibits telltale brown spots on the leaves that may indicate rust, a fungal disease. Other signs of disease on foliage include a powdery white, gray or silver coating, which is powdery mildew. This may attack trees, shrubs, annuals or perennials, especially during very moist times of the year. Fungi love moisture and flourish during periods of rain or from improper watering. Many gardening experts recommend watering in the morning so that the sun can dry plant leaves.