By Scout Nelson
Many gardeners leave perennial plants standing through winter because the stems provide shelter for insects and help collect snow for winter protection. However, plants with serious disease problems may need to be cut back during fall cleanup. Peonies affected by powdery mildew are one example.
Carrie Knutson, Horticulture agent, NDSU Extension, Grand Forks County, shared important information about managing powdery mildew and keeping garden plants healthy. Powdery mildew is a fungal disease that affects many plant species and is often seen in shady areas or places with limited air movement.
The disease appears as white or gray powdery spots on leaves and other plant parts. Young leaves that become infected while growing may look distorted. In many cases, powdery mildew mainly affects the appearance of a plant. However, severe infections can cause leaves to drop, lead to dieback, and weaken plants when the problem happens repeatedly.
Some plants are more likely to develop powdery mildews than others. Gardeners should watch plants that have shown signs of the disease in previous years. Early signs are easier to manage than an infection that has already spread across the entire plant.
Powdery mildew spreads through spores carried by wind. Unlike many other fungal diseases, it does not need water on leaves to infect plants. Weather conditions can also affect disease development. Cool nights, high humidity, and dry daytime conditions can create an environment where powdery mildew grows well.
Fungicides can help prevent and manage powdery mildew when they are used at the right time. They work best before the disease becomes widespread by protecting healthy plant tissue and helping prevent new infections. When an entire plant is already covered, spraying may provide little benefit.
Because many plants can tolerate powdery mildew, gardeners do not always need to use fungicides. Fungicides may be considered for valuable plants or plants that develop the disease often. Anyone using these products should always read and follow the product label.
Good planting practices can also reduce powdery mildew problems. Plants that are likely to develop the disease should be placed where air can move freely around them. Avoiding crowded planting areas helps leaves dry and improves airflow.
Choosing resistant plant varieties is another useful option. Gardeners can also improve air movement through proper spacing and careful pruning. These steps can make growing conditions less favorable for powdery mildew.
Fall cleanup is especially helpful when plants have heavy infections. Gardeners can cut back severely affected plants after a hard freeze and remove infected leaves and stems from the garden. This reduces the amount of infected plant material left in the growing area.
Heavily infected debris should not be placed in a backyard compost pile when possible. Home compost piles may not reach temperatures high enough for a long enough period to destroy disease-causing pathogens. Gardeners should follow local rules when disposing of diseased plant material.
Gardeners who normally leave plants standing through winter may need to make an exception for plants with serious powdery mildew. Healthy plants can provide winter benefits, but removing heavily infected material can support better disease management.
Regular scouting is also important. Checking plants early in the next growing season can help gardeners notice new signs before the disease becomes widespread.
Powdery mildew does not always seriously harm plants, but repeated heavy infections should not be ignored. A combination of good airflow, proper spacing, resistant varieties, early monitoring and fall cleanup can help gardeners protect plants and maintain healthier gardens.
Photo Credit: North Dakota State University
Categories: North Dakota, General