By Jessica Carlson

Otter Tail County Master 

Gardener Volunteer

When a garden plant develops spots, white powder, or suddenly starts turning brown, most gardeners often ask themselves, “Will it go away, or will I be dealing with this again next year?”

Let’s start with leaf spot. “Leaf spot” is a general name for a group of diseases that can affect many different plants. Trees and shrubs, tomatoes and many ornamental plants can all develop leaf spots, although the specific disease may be different from one plant to another. Leaf spots may be brown, black, yellow, orange, or reddish. They often begin on lower or inner leaves where there is more shade and humidity. Many leaf spot diseases are caused by fungi, although some are caused by bacteria or other pathogens.

The important thing to know is that many leaf spot diseases do not simply disappear when the leaves fall. The pathogens can survive the winter in infected leaves and other plant debris. Some can also survive in infected buds or young twigs. 

This is why fall cleanup matters. If a plant has suffered from a serious leaf spot problem, remove fallen leaves and other infected plant debris. Major pruning on woody shrubs and trees should wait until dormancy as this creates stress for the plants and creates environments to spread the disease further. Improve air circulation by pruning crowded plants. Lastly, avoid pruning while leaves are wet as moisture allows fungal and bacterial leaf spot pathogens to reproduce and stick to your hands and tools, in turn spreading the disease to healthy parts of the plant. Water at the base of plants rather than regularly wetting the leaves. On tomatoes, staking and mulching can help keep soil from splashing onto lower leaves. When choosing new plants, look for disease-resistant varieties whenever they are available.

In Minnesota gardens, powdery mildew is a common occurrence. It appears on plants such as phlox, bee balm, peony, zinnia, squash, cucumbers, pumpkins, and other members of the squash family. Powdery mildew looks like someone sprinkled baby powder over the leaves. It spreads by tiny spores that are easily carried by wind, splashing water, and direct contact. Unlike many other fungal diseases, powdery mildew does not need water sitting on the leaves to infect them. In fact, on cucumbers and squash, new spores form and spread easily during warm, dry, and even in humid conditions. Powdery mildew fungi often survive the winter. When spring arrives, spores are released and begin new infections on young growth. The disease can then produce more spores and becomes more visible during the growing season, spreading from plant to plant. Prevention starts with choosing resistant varieties when possible.
Give plants enough room for air movement, thin crowded foliage and keep weeds from creating a dense canopy around plants. Avoid overfertilizing with nitrogen, which encourages lush new growth that powdery mildew can easily infect.

Here’s some good news, powdery mildew is often more of a cosmetic problem than a serious threat to plant health. UMN Extension recommends tolerating mild infections that do not reduce plant growth or flowering. If a valuable plant has a history of powdery mildew, fungicides work best when applied to healthy tissue early, as a preventative measure before the disease becomes widespread. Keep in mind when most of the leaves are infected, fungicides will not cure or remove the existing mildew.

Some plant diseases are mainly a problem during one growing season. Others can survive Minnesota’s winter and start the cycle all over again the following spring.

Understanding where a disease spends the winter is one of the best ways to prevent it from becoming a yearly problem.

For more information on diagnosing an issue in the garden visit the University of MN Extension diagnostic tool  https://apps.extension.umn.edu/garden/diagnose/