Powdery mildew

Powdery mildew is visible on these leaves.

It is rare to hear a horticulturist say “Rain, rain go away,” unless she has yet to plant her garden. But this horticulturist is going on record: Illinois gardeners need a break.

This much rain in the spring means our Master Gardener help desk will be busy with disease symptoms and perplexed homeowners wondering what is growing on their landscape’s leaves. We can’t tell the rain to stop, but we can do a few things to prevent the spread of disease in our gardens.

Most plant diseases are prevented, not cured. Once symptoms are noticeable, they are unlikely to be reduced in the present growing season. Infected tissue should be removed, and in severe cases, plants should be culled (but absolutely not thrown in the compost pile!). In most cases, fungicides are only effective the following spring as a preventative measure.

The best defense starts at planting: choosing resistant cultivars and proper placement. In current landscapes, good sanitation practices help: avoid mowing the lawn and other garden work when plants are wet. Reduce the humidity in your garden by only watering in the mornings, and thinning and pruning to increase airflow around your plants.

One such disease that will benefit from all of this moisture is powdery mildew, which looks like the dusting of a doughnut, and can rub off in your fingers. Most plants are susceptible. The fungus favors shade, high humidity, and moderate temperatures. Most of the time, powdery mildew is only an aesthetic issue, but in severe cases, it can cause leaf distortion and defoliation. Fertilizing creates an even better environment for powdery mildew to spread — new growth is its favorite.

In this case, a fungicide can be used when symptoms first appear. But cultural practices are important, too: prune the diseased portion of the tissue, improve air circulation, and cull the most symptomatic plants.

Many new cultivars in the industry have more resistance to powdery mildew than old favorites. Purdue University suggests the below:

Bee balm: ‘Jacob Cline’ and ‘Marshall’s Delight’; phlox: ‘David’ and ‘Miss Lingard’; zinnia: ‘State Fair’ and ‘Profusion series.’

Botrytis (bo-TRY-tis), also known as “gray mold” for its fuzzy appearance, is another disease that thrives in high humidity and warm temperatures. This fungus shows up as web-like growths after several days of rainy weather, and releases a powder (spores) when disturbed.

Botrytis is more prolific in shaded areas with dense vegetation and can commonly be seen in roses, strawberries, petunias, peonies, etc. In severe cases, open cankers show up on stems, which leads to plant death. Fungicides are not effective after symptoms have appeared.

For resistant varieties, try ‘Tidal Wave Pink’ and ‘Fantasy Blue’ petunia, and the ‘America’ and ‘Buckeye Belle’ peony.

For a more detailed list of disease resistant cultivars from Purdue, please visit https://www.extension.purdue.edu/extmedia/id/id-414-w.pdf

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Kelly Allsup is the University of Illinois Extension horticulture educator in Livingston, McLean and Woodford counties.


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