PPDL Picture of the Week
November 9, 2015
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.