Powdery Mildew on Strawberries

Dan Egel, Vegetable Pathologist, SWPAC, Botany & Plant Pathology, Purdue University

The photos that accompany this article are of powdery mildew of strawberry. There are powdery mildew diseases of many different ornamental, vegetable and field crops. For the most part, a species of powdery mildew fungus that affects plants in a particular plant family will not affect plants in a different family. Thus, the powdery mildew on this strawberry plant will not affect cucumbers.

A fungus known as Podosphaera aphanis causes powdery mildew of strawberries. The fungus may infect leaves, fruit or stems of strawberries under conditions of high humidity. Unlike most other pathogenic fungi, powdery mildew does not need leaf wetness for infection.

The first photo below shows the bottom of a strawberry leaf with the talc-like growth of the powdery mildew fungus (Figure 1). The lower side of leaves often becomes infected before the upper side because the relative humidity is often greater under the plant canopy. Figure 2 shows the upper side of a leaf with a few colonies of powdery mildew. Note that Figure 2 also shows white residue on the outside of the leaf that has resulted from water of guttation <https://en.wikipedia.org/wiki/Guttation>. This is not to be confused with the more diffuse appearance of powdery mildew. The final photo (Figure 3) is of a strawberry fruit with a bit of powdery mildew. Note that the stem or petiole that attaches the fruit to the plant also has powdery mildew.

Normally, I confine my writing to traditional vegetable crops. However, my colleague at the Southwest Purdue Agricultural Center, Dr. Wenjing Guan, has started a strawberry variety trial in a high tunnel (a greenhouse without heat). While growing strawberries in a high tunnel offers an opportunity to harvest strawberries later in the fall and earlier in the spring, the increased humidity of the high tunnel may explain why powdery mildew has appeared. Dr. Guan has found that the varieties in her trial differ in resistance to powdery mildew; it is possible to select a variety with good resistance. There are systemic fungicides that may lessen the severity of powdery mildew.

Click image to enlarge

Figure 1: Powdery mildew on the underside of a strawberry leaf.

Figure 2: A few white talc-like powdery mildew colonies on the upper side of a strawberry leaf.

Figure 3: Powdery mildew on a green strawberry.