ABSTRACT
The powdery mildew fungi in the phylum Ascomycota cause easy to recognize diseases. The fungus grows across the surface of the host in conspicuous colonies, creating whitish circular patches that sometimes coalesce until the entire leaf surface is white (see Plate 5B following page 80). The colonies may form on either the upper or lower surface of leaves, as well as on stems, flower parts, and fruits. Because energy from photosynthesis is diverted into growth of the pathogen, infected plants may be stunted and produce fewer or smaller leaves, fruits or grain. The impacts of powdery mildews on their hosts may be mainly aesthetic or may reduce yield or quality. Many floral and nursery crops (Figure 14.1) and also fruit, vegetable, and field crops are affected.
