Powdery mildew in the flower garden
Top five things to know about powdery mildew
- Powdery mildew is a common fungal disease that does not cause significant damage to plants.
- Tolerate low levels of powdery mildew that do not reduce plant growth or flowering.
- Look for powdery mildew resistant varieties in seed catalogs, on seed packages, and on plant labels.
- At the end of the growing season, cut infected plants at the soil line and compost them.
- Fungicides should only be used to protect high-value plants with a history of disease. Fungicides will not cure or remove existing powdery mildew infections.
Powdery mildew is a common fungal disease that occurs on many different plants in the flower garden. Although powdery mildew can cause plants to look less attractive, it rarely results in significant damage to the plant. When severe, the disease may reduce plant growth and flowering.
Powdery mildew occurs on over 10,000 plants. Many commonly grown annual and perennial flowering plants, as well as ornamental grasses, can be infected by powdery mildew. Zinnia, phlox, bee balm, and peony are a few of the plants regularly infected by powdery mildew in the flower garden.
M. Grabowski, UMN Extension
Black, round fungal resting structures of powdery mildew on a phlox stem
- Infected plants may appear to be sprinkled with baby powder or covered in cobwebs.
- White to gray powdery spots, blotches or felt-like mats on leaves, stems and buds.
- In some plants, leaves turn purple to red around the infection.
- If young leaves are infected, they may become distorted or twisted as they grow.
- Severely infected leaves may turn yellow and fall off.
- Tiny, round orange to black balls may form within white fungal mats often at the end of the growing season.
- This disease is most severe on plants or plant parts in shaded areas with poor air movement.
Powdery mildew is caused by a group of related fungi in the Erysiphaceae family.
Powdery mildew spores are easily carried by the wind to neighboring plants or to plants hundreds of miles away. Once a spore lands on a host plant, it will quickly germinate and start a new infection. Unlike other leaf spot fungi, powdery mildew fungi do not need moisture on the leaf from rain or dew to infect. Some powdery mildew fungi require high humidity but others can germinate even when humidity is low.
Powdery mildew fungi produce a mat of fungal growth on the surface of the plant. Specialized structures penetrate the plant tissue to take up nutrients. Spores are produced in long chains rising up in a vertical column perpendicular to the leaf surface. These spores break off and are spread by the wind. Powdery mildew fungi thrive with cool humid nights that stimulate spore production and warm (70 to 80° F), dry days that allow for spore spread.
Powdery mildew fungi survive winter in several ways. Some infect buds of woody plants. Powdery mildew fungi can also create a dark, round, hard resting structure known as a chasmothecia. These resting structures contain and protect spores during harsh weather. Spores are later released when the weather becomes favorable for disease development. Some powdery mildew fungi survive on leaves that remain green throughout the winter. In Minnesota, this may mean that the powdery mildew survives on greenhouse plants or new spores are carried by the wind into the state after surviving the winter on plants in southern states.
- Tolerate powdery mildew infections that do not reduce plant growth and flowering.
- Replace severely infected plants with a resistant variety or with a plant from a different family. Infected plants should be buried in a compost pile that heats up to 148° F or taken to a municipal compost facility.
- Powdery mildew resistant varieties are available for some flowers. Look for powdery mildew resistant varieties in seed catalogs, on seed packages, and on plant labels.
- Reduce humidity in the plant canopy.
- When planting, space the plants to allow adequate air circulation through them.
- Increase the airflow and light throughout the plant by pruning to thin the foliage.
- Mulch the soil around the plants with woodchips or other organic mulch.
- Do not overfertilize plants. Apply fertilizer according to soil test results or as required by the particular plant species. Using too much nitrogen may result in young, green, lush growth, which has been shown to promote a powdery mildew infection.
- At the end of the growing season, cut infected plants at the soil line and place all plant debris in a compost pile that heats up to 148° F. Or, take plant debris to a municipal compost facility.
- Fungicides are rarely necessary to manage powdery mildew in a home garden and should only be used to protect high-value plants that cannot be replaced and have a history of severe infection.
- Fungicides must be applied to healthy green tissue early in the growing season before infection begins.
- Examine plants with a history of severe powdery mildew once a week. When the first leaf spot is observed, pinch off the infected leaves and begin fungicide sprays to protect healthy tissues.
- Repeat applications are often necessary throughout the growing season and should be applied according to label instructions.
- Fungicides will not cure or remove existing powdery mildew infections. Once the majority of leaves have leaf spots, it is too late to treat.
- Many different fungicides are effective in protecting plants against powdery mildew if applied correctly. Low impact fungicides like sulfur, potassium bicarbonate, and horticultural oils are recommended.
Caution: Read all label directions completely before buying and applying fungicides. Follow all instructions. The label is the final authority for use of the product.