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Country Gardens: Powdery mildew

     Powdery mildew is a very recognizable and common fungal disease found on several plants in Minnesota. On trees and shrubs, powdery mildew rarely causes serious damage to its host. The disease can significantly reduce the ornamental value of plants grown for their appearance, like roses and purple-leafed ninebark shrubs.

    The fungi that causes powdery mildew is usually on a certain type of genus or family of plants. There are many species of powdery mildew that affect thousands of plant species. If you have mildew on one plant it usually does not affect all the plants you have. Only a few species of powdery mildew fungi can cause the disease in many different types of trees and shrubs. What usually happens is that one tree or shrub gets several different species of powdery mildew fungi.

    Powdery mildew can seriously impact yield on flowering crops, such as squash, pumpkins, cyclamen and reiger begonia, but on other plants, such as lilac and oak, the mildew is unsightly but does not severely harm the plant.

    Powdery mildew is a growth that appears as white to gray powdery spots, blotches on the leaves, stems and buds.  You might look at the plant and think it was sprinkled with a powder. The younger leaves and sprouts are usually the most affected. Some plants may even turn purple to red around the infection.

    It the late summer or early fall, tiny round orange to black balls form on the white powdery sections.

    Those areas that are shaded or where there is very little air movement, such as the inside or lower branches of the plant, are where it is usually bad.

    Powdery mildew thrives in humid condition but does not do well if leaves are wet from frequent rain or irrigation. When there are cool nighttime temperatures and warm day temperatures seems to be when it is most likely to occur.

    When you see mildew you might think it will kill the plant, but is does not really affect the health of the tree or shrub and does not require management. Choose mildew-resistant cultivars for new plantings or those you need to replace.

    Keeping space between plants so there is some air movement around them will keep them healthy. When it is spotted on a tree or shrub prune it so there is more air circulation throughout the tree, but do not prune it more than one third or even less.

    Any plant or shrub that has mildew should not be fertilized and a soil test is recommended to correct any nutrient deficiency.

    Avoid overhead watering to help reduce the relative humidity or water in the early morning to let the tissue dry as soon as possible.

    Remove only severely infected and damaged shoots in the summer to reduce the spread and overwintering of the fungi within the tree or plant.

    You can use a fungicide to protect plants that seem to get powdery mildew or a very precious ornamental shrub, such as ninebark or roses. Fungicides will prevent infection and some will get rid of a small infection that has started. Check your plants regularly. As soon as you see some of the leaf spots appear, apply the fungicide to protect the remaining leaves. Keep repeating as necessary on the instructions of the product used.

    Here are some you can find at your local garden store: Thiophanate methyl, Chlorothalonil, Sulfur – do not use on a sensitive plants like viburnum, Potassium bicarbonate.

    An alternative nontoxic control for mildew is baking soda combined with lightweight horticultural oil. Researchers at the University of Rhode Island have confirmed that a combination of 1 tablespoon baking soda plus 2.5 tablespoons oil in 1 gallon of water is effective against powdery mildew on roses. Use of this combination on other crops is still experimental.

Send your gardening questions to Christie Schlueter at

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