Oh, No! There's Powdery Mildew on My Plants!

Powdery mildew is rampant in my garden this year. I believe I've managed to control the problem enough that you may not notice it as you walk around, but in spite of my best efforts, I haven't eradicated it completely. Powdery mildew didn't attack last year, although there was no shortage of other troubles, as you will learn if you click on this posting:  Garden Pests and Other Problems in Paradise.  So what is powdery mildew? what are the causes? and what control measures are available?

Symptoms of Powdery Mildew

Powdery mildew is a fungal disease that causes white spots on the leaves of some plants. The infected plants in my garden this year are phlox, bee balm, perennial sunflower, and zucchini.  It is a destructive, disfiguring disease. The grayish white, powdery patches appear on the foliage, starting at the bottom of the plant, in summer. The blotches, usually circular in shape, grow together into a continuous matt of mildew. The lower leaves often turn yellow and drop prematurely. The fungus removes nutrients from the plant during infection and may result in the decline of the plant.

Powdery Mildew on my new bee balm, Monarda 'Raspberry Wine'

Powdery mildew on zucchini foliage.

Causes of Powdery Mildew

 The main causes are:

1. warm temperatures
2. high humidity
3. poor air circulation between plants*
4. shading increases the incidence of the disease

Wind spreads the spores of the fungus. The fungus can survive during winter on plant parts and debris where tiny, black, fruiting bodies known as cleistothecia develop.

*One of the characteristics of a cottage garden tends to be close plantings, so I have to be very attentive to point number three.

Two types of phlox with other cottage garden plantings

Prevention

Plant sun-lovers where they will get at least six hours of sunlight each day. Plant in open areas away from fences or walls where air circulation is poor. Because I plant fairly close together, I remove all but five or six stems of the susceptible plant when it is about six inches tall to improve air movement. I divide my plants every two-to-four years for continued air circulation. Water regularly, but soak the soil instead of the plants to prevent disease development. In the fall, remove and destroy all mildew-infected plant debris to reduce the likelihood of disease the next season.

I systematically remove stems of bee balm, 'Jacob Cline,' to aid air circulation.

Chemical Control

I hate using chemicals of any sort, but there are some organic fungicides for controlling powdery mildew. To be on the safe side, I spray only in the very early morning before the pollinators are active. Also, check that the particular fungicide is safe on the plant you are treating. Begin a fungicide program when the first white spots appear. Spray the upper and lower surfaces of the leaves, covering thoroughly. Repeat every seven to fourteen days. Follow all directions on the label. Contact your local Cooperative Extension Office for more information.

Perennial sunflower, Helianthus sp.

Just a few small white spots have appeared on the perennial sunflower plant.

Resistant Varieties

For the best long-term solution choose plant cultivars resistant to powdery mildew. Plant breeders have developed many resistant varieties. This does not mean the plant will not become infected, but the disease will be less severe or spread more slowly.

Phlox 'Bright Eyes' acquires but suffers little from powdery mildew because it is resistant to the disease.

Powdery mildew did not prevent zucchini production or the quality of the fruit.

What garden pests and/or diseases did you contend with this summer, dear gardening friends?

Pamela x

This posting was adapted from an article I wrote for eHow called My Phlox Has White Spots on the Leaves.

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