Idaho Falls
clear sky
humidity: 18%
wind: 7mph NE
H 88 • L 87

Preparing for powdery mildew in your yard or garden

In the Garden

Share This

Powdery mildew in eastern Idaho is most common in the cucurbits — cucumbers, squash, pumpkins, melons, and gourds.

This past year was the first time I had a significant infestation of powdery mildew in my tomato plants. You will often see it in roses, lilacs, and apples. A couple of years ago I had powdery mildew on the grass beneath a spruce tree. There is a species that infects field bindweed.

It usually shows up in ornamental and vegetable plants in late summer and fall. So why worry about it now? Because it’s best to be prepared for prompt action as soon as it shows up, and I have seen it on pansies in late spring.

There are many different powdery mildew species, and the individual species are specific to a limited family of plants. For example, the powdery mildew that infects cucurbits, does not infect tomatoes. However, the spores and overwintering bodies of various species are pretty much everywhere. So, when conditions promote one powdery mildew species, they will also promote species that infect other plants as well.

Powdery mildew02
Ron Patterson,


Powdery mildew requires live tissue to grow and develop, but unlike most fungal infections it does not require free liquid. Conditions that favor powdery mildew are:

  • High humidity at night
  • Low humidity during the day
  • 70 – 80F temperatures

It is most common to experience powdery mildew in late-summer and fall growing conditions when plant canopies are large and airflow is limited, increasing humidity.

Powdery mildew04
Ron Patterson,

Signs and symptoms

Powdery mildew starts as small (less than ¼ inch) white spots on the leaves. It is most common on the upper leaf surface but will often move to the underside as well. As the fungal spots become larger, photosynthesis is reduced, yield is reduced, and fruit quality may also be affected. A heavily infested plant will have a white, powdery substance covering the leaves, and perhaps the petioles, stems, and fruits (tomatoes have distinct yellow spots).

Control options

Cultivar selection — plant-resistant cultivars. If you have consistent powdery mildew infections every year, consider selecting a different cultivar when you shop in your seed catalogs. The descriptions will usually indicate resistance to various diseases, including powdery mildew. This is especially true of cucumbers, squash, and melons.

Sanitation — till in or clean up plant material that has been infected with powdery mildew. If you compost your garden refuse, be sure the temperature gets above 140F for several days, turn the pile and increase the temperature again. Plant material that has not been properly composted can host the spores that will reinfect your garden the next season.

Airflow — allow more space between your plants for better airflow to reduce canopy humidity.

Irrigation — sprinkler irrigation should be done in the morning so canopy humidity will be less by evening. Drip irrigation will reduce canopy humidity.

Fungicide — apply an appropriate fungicide as a preventive method, or as soon as the first symptoms show up to reduce the spread to new leaves and plants. Be sure to rotate between fungicides with different modes of action to reduce the risk of resistance.

Powdery mildew in trees and shrubs is not usually fatal, but heavy infestation every year will reduce plant vigor. Apple fruit quality is often affected by this disease.

So, select resistant plants and be prepared to act when powdery mildew shows up in your yard and garden.

Powdery mildew03
Ron Patterson,