Tuesday, September 9, 2014

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

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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  1. Thank you for sharing me this lesson. I often find this case in my garden, and it usually be a serious problem.
    Btw, your perenial sunflowers look so lush so beautiful. Happy gardening!

  2. I had a fair number of Japanese beetles, mostly on some potted Canna lilies and the Zahara zinnia. I used a certified organic spray on them once a week through July and much of August, and it really helped.

    I believe powdery mildew has decimated the morning glories growing in the containers on my porch. I plant to dump all the soil from those pots and start fresh next year. What I don't know is whether it's safe to save the seeds from the infected plants.

    I've read that keeping plants watered helps fight powdery mildew. If they're allowed to dry out, they are weaker and less able to fight it.

    August was just a bad month, with all the humidity, cooler temps, lack of sun . . .

    Keep fighting the good fight!

  3. Hi Pam!! We who live on Vancouver Island off the west coast of Canada are oh so familiar with your mildew issues. We get it pretty much every year. All the squashes. Roses. Darn stuff. I have heard that if you spray milk on it it will cure the mildew. I did try it on my apple tree and it worked. But, we have also had a much hotter and drier summer than usual. Next year will be the test. Our real problems here are Peach Leaf Curl and Tomato Late Blight. You cannot grow either without shelter from the rain. There you have it. Gardening Canadian style!!

  4. Hi Pam. I've had more earwigs in the garden this year. I haven't really taken the time to figure out what I should do about them. I have a little of the powdery mildew on my bee balm and am glad to know more about it now, because of your post. I will check that link you put up as well.

    Your gardens look beautiful!

  5. Very informative Pam ! Every year I combat this due to the wet humid summers....my main pest this year was the voles in the pea and bean bed again...grrrr!

  6. Despite your mildew, your garden is still looking gorgeous.
    Earwigs, slugs & snails have been a problem for us this year....grrrr.
    But nature always fixes things :)

  7. Cottage style gardens are my favorite, and I think your's is lovely! Since I live in a climate with hot dry
    summers (and it's especially dry with the extended drought), we are not affected by powdery mildew. But even with the drought, the snails live on! And in our small garden on the coast, cucumber beetles are running (crawling) rampant, eating the roses and brugmansias. It's always something!

  8. I do have a bit of powdery mildew at the moment. I put it down to the high humidity. Lucky enough, it is never a significant enough of a problem that I have to take any action.

  9. My Joe-pye Weed got powdery mildew quite terribly this year. I didn't think it was packed in that closely, but I guess since they are planted against my greenhouse wall, they didn't get as much air circulation. We had quite a few humid days at the beginning of summer, so I'm surprised more plants didn't get it!

    I've had some problems with slugs this year, and I'm also trying to figure out what is killing off my rhubarb. Very frustrating, as rhubarb isn't supposed to have too many problems, and I really really want a rhubarb patch for pie! :)

  10. some black spot on the roses, but those that aren't tough enough to survive, have quietly left the garden. And something upsetting the leaves of one aloe, of which I have MANY plants - but the other succulents are all fine. I can live with my garden as it is.

  11. I usually get powdery mildew on my courgettes but I just let it be as it never seems to affect the plant or the production of fruit. I've heard that a milk and water solution sprayed on to plants can treat it but I've never tried it myself. My worst problem is slugs, every year without fail they'll do some damage.

  12. Pam, My crab apple tree fell prey to some disease after giving an amazing show of Spring blooms the leaves started falling off and looked sickly all Summer. Not really sure what the problem is, but I will give a Winter wash and see how it is next year.
    We also had powdery mildew on Monarda. Ah well, like others have said, your garden still looked terrific.

  13. Pests were of the 4-legged variety with the exception of the slugs. Even after shaking Sluggo around, they still got my strawberries. Drat!

    Christine in Alaska