(Formerly Sphaerotheca macularis f. sp. fragariae)
Powdery mildew is a fungal disease that infects leaves and occasionally fruit. Powdery mildew thrives under conditions of low light intensity and warm humid weather. As a result powdery mildew is common in greenhouse-grown plants but occurs only occasionally in gardens.
White patches of fungal growth develop on the lower leaf surface. In some cultivars, this fluffy white growth is thick, abundant and can cover the entire leaf surface. In other cultivars, the fungal growth is thin and difficult to see. Some cultivars develop purple to red blotches when infected. The leaf margins of infected plants frequently roll upwards. Infected fruit have raised seeds, a bronze cast to the fruit, and have patches of fluffy white fungal growth.
These fruit symptoms are similar to symptoms caused by thrips. Use a hand lens to look for white fungal growth around the seeds, which is characteristic of powdery mildew. Bronzing on the underside of calyx tissue would indicate feeding by thrips. Some day-neutral cultivars are susceptible to fruit infection in fall even though leaves may appear healthy.
The powdery mildew fungus is most commonly introduced into a garden on contaminated transplants but can be present on wild strawberries as well. The fungus overwinters in live infected plant tissue. Spores are carried short distances by wind and, unlike most fungal diseases, can germinate on dry leaf surfaces given high humidity. Powdery mildew is favored by warm (60 - 80°F) weather with high humidity. Frequent rain, dew or irrigation slows the progress of disease. The disease can establish in the spring if there are extended periods of warm weather and high humidity. More often the disease establishes in mid to late summer where cooler nights lead to high humidity conditions favorable for infection.
- Although some cultivars have been reported to show resistance to powdery mildew, these have not proven to be reliably disease-free in Minnesota.
- Purchase plants from a reputable supplier. Inspect all plants for symptoms of disease. Plant only healthy symptom free plants.
- Renovate strawberry beds every year after harvest. The process is described in Strawberries for the Home Garden.
- Following renovation, rake and remove old leaves.
Fungicide sprays may be necessary to protect plants after summer renovation if powdery mildew has been identified in the patch before renovation. Several fungicides are available to control powdery mildew. Choose products with an active ingredient of myclobutanil, sulfur, potassium bicarbonate, or horticultural oil. Apply products according to the label instructions. Repeated applications will be necessary as long as conditions favorable for disease continue. Do not use sulfur on fruit intended for canning.