›› Alysha Punnett
Powdery mildew crops up at this time of year and is often perplexing to gardeners. Isn’t it too hot and dry for fungi to thrive?! Well, while the weather may feel hot and dry, there actually is enough moisture in the air for mildew to get around. This is why fungal issues are common in late summer when the days heat up fast and the morning dew or a light rain provides enough moisture for them to spread. Add to this the fact that many plants are weak and susceptible to pests and disease from drought stress in this driest and hottest part of the growing season, and you have a perfect recipe for powdery white squash leaves. Plants can still grow relatively unaffected if the infection is mild, however if it becomes so severe that the majority of the plant is covered in powdery white, this can inhibit the plant’s ability to photosynthesize and grow. Not to mention it makes the plant unappetizing. As with pest management, prevention is still our best friend here, and it’s important to be patient with treatments as they usually work best when applied frequently.
Identification: Powdery mildew is commonly seen on squash and cucumber leaves at this time of the year, but it can show up on pretty much any kind of green leaf. It starts as downy white spots on the undersides or tops of leaves and gradually spreads to take over the whole leaf surface.
Management: Trim plants throughout the season to maintain airflow throughout your garden beds. Ensure plants are receiving enough water. Overhead irrigation can cause fungal spores to spread as drops of water splash onto leaves, bouncing them into the air and onto surrounding plant material. Try drip irrigation in perennial beds instead, or try to limit overhead watering to the morning so leaves will have time to dry during the day. Make and spray compost tea every 2 weeks throughout the growing season to outcompete the bad fungi with good fungi. Other DIY sprays include a 1:10 dilution of milk to water or a diluted horseradish tea.
Happy zucchini bread, cake, muffin, and noodle making with your harvest!
Alysha Punnett is the Site Manager and Community Education Coordinator at the Compost Education Centre.