By: Caio Correa, Pathologist-Horticulture, OMAFA; Erica Pate, Fruit Crop Specialist, OMAFA
Published March 2025
Powdery mildew is one of the most widespread plant diseases, and in strawberries, it can be a persistent challenge. Caused by the fungus Podosphaera aphanis, this disease can significantly reduce fruit quality and yield by distorting leaves, stunting plant development, and covering berries with an unsightly white fungal coating. In severe cases, powdery mildew can make strawberries unmarketable, leading to economic losses. Good management is essential to reduce the losses, and understanding the disease is the first step in getting better control.
When and where powdery mildew thrives?
The pathogen prefers warm temperatures (15-27°C, 60-80°F) and high humidity, but unlike many other fungal diseases, it doesn’t need free moisture on the leaves to infect plants. In field production, outbreaks usually peak in mid-summer, but a warm spring with cool nights and dew can trigger early infections.
Rain can wash off some of the spores and temporarily slow disease spread, but it won’t eliminate the fungus—it can quickly regrow and continue infecting plants.
How does it spread?
The fungus overwinters in plant debris but needs living tissue to grow and reproduce. In the spring, wind-borne spores kickstart new infections. The fungus then spreads across plant tissues, forming white powdery patches and releasing fresh spores every 72 hours if conditions are right. These spores are carried by the wind to new plants, allowing the disease to spread quickly.
While powdery mildew affects many crops, each host is infected by a different fungal species. So, an outbreak in your neighbor’s pumpkin field won’t spread to your strawberries.
For more information on powdery mildew identification and disease cycle, make sure to check OMAFA’s Crop IPM website (ontario.ca/cropipm).
Managing powdery mildew
Fungicides are still the go-to method for controlling powdery mildew. Products with tetraconazole (Mettle), myclobutanil (Nova), or trifloxystrobin (Luna Sensation, Flint Extra), are commonly used, but timing is key. Spray as soon as you notice early signs of the disease — such as leaf distortion or discoloration — to prevent the disease from spreading.
Beneficial microorganisms, like certain strains of Bacillus subtilis, can help control powdery mildew by attacking fungal structures or boosting plant immunity. Some commercial products already include these beneficials in their formulation, offering good options for both conventional and organic growers.
Since powdery mildew can develop resistance to fungicides, rotating products with different modes of action is crucial for maintaining effectiveness. You can check which products are registered for strawberries and their modes of action on the Crop Protection Hub (ontario.ca/cropprotection).
Prospects for High Tunnel and Greenhouse Growers
Protected strawberry production (greenhouse and high tunnel, table top systems) is gaining popularity, but the same protected environment that helps the plants thrive also creates the perfect conditions for powdery mildew. Outbreaks of powdery mildew are making it difficult to manage protected strawberry production in Canada.
On the bright side, new integrated pest management (IPM) strategies are being tested, and one of the most exciting developments for powdery mildew control comes from research into UV-C light technology. UV-C light works by disrupting the DNA of fungal spores, stopping them from germinating and spreading. USDA researchers in Kearneysville, WV, validated the efficacy of UV-C for control of powdery mildew on strawberry, and application of UV-C in field experiments in Florida provided equal or better control of powdery mildew compared to weekly applications of conventional fungicides. In addition, panels equipped with UV-C lights can be mounted on robots to automate the treatment. Here are a few benefits:
- Less Chemical Use: UV-C treatment can reduce the need for fungicide applications, cutting down on the risk of resistance and chemical residues.
- Eco-Friendly: UV-C light is residue-free and non-toxic to plants, making it a great option for organic production systems.
- Efficient: Robots can work overnight when UV-C light is most effective, so workers are not exposed, and the plants’ natural cycles aren’t disturbed.
- Multiple pests: UV-C treatments have activity against multiple strawberry pests, including botrytis grey mould, anthracnose, and two-spotted spider mite.
Commercial UV-C robots are already on the market for greenhouse growers, and while the initial cost may be high, the long-term benefits—like lower fungicide costs and better yields—could make it a smart investment down the road.
The Berry Growers of Ontario (BGO) is actively working with a robotics company on adapting UV-C technology for table top strawberry growers and is working on testing treatments tailored to the crop. With trials underway this season, there’s more to come soon — so stay tuned!
References
Janisiewicz, J.W., F. Takeda, B. Nichols, D.M. Glenn., W.M. Jurick II, and M.J. Camp. 2016. Use of low-dose UV-C irradiation to control powdery midlew caused by Podosphaera aphanis on strawberry plants. Can. J. Plant. Pathol: 38, 430-439.
Onofre, R.B., D.M., Gadoury, A. Stensvand, A. Bierman, M. Rea, and N.A. Peres. 2019. USe of Ultraviolet light to suppress powdery midlew in strawberry fruit production fields. Plant Disease.