Podosphaera leucotricha (Ell. & Ev.) Salmon
Occurrence and importance
Powdery mildew occurs in nearly all parts of the United States where apples are grown but is usually controlled by the scab sprays applied in eastern orchards. It is generally more of a problem on the Pacific Coast than elsewhere.
On the market it is found chiefly on apples grown in Washington, Oregon, and California. The varieties most often affected are Jonathan, Grimes Golden, Winesap, Gravenstein, Yellow Newtown, Ben Davis, and Rome Beauty.
Powdery mildew is a fungus that attacks the foliage, twigs, blossoms, and fruits of apple and pear. On all these parts it shows first as small grayish or white feltlike patches of fungus growth, which by enlargement may in time entirely cover the part affected. A later symptom is stunting and russeting of the fruits. The fungus may become established on fruits either by spore germination on the fruit surface or by growth down the stem from infected twigs.
Mildew rusting on the apple fruits, in its commonest form, has a peculiar lacy or netlike appearance (see photos), which distinguishes it rather definitely from rusting caused by spring frosts or spray injuries. In the more unusual form of solid patches in which it sometimes occurs, it might be mistaken for one or the other of the two injuries just named, except for the fact that there is always more or less of the lacy russeting around the edges of the larger affected areas. Furthermore, the surface of mildew-affected areas is smooth, not roughened as in frost and spray injury russeting.
Control of powdery mildew must be done in the orchard through proper spraying. Recommendations of the State agricultural extension service or experiment station should be followed.