Powdery Mildew

How to Prevent and Control Powdery Mildew

Powdery Mildew2
Photo by Pollinator: Wikimedia Commons

Wondering about that white fungus on your plant? The fungal disease powdery mildew is a common problem in gardens, infecting a wide variety of plants and reducing the quality and quantity of flowers and fruit.

What Is Powdery Mildew?

Powdery mildew is a fungal disease that affects a wide variety of plants. There are many different species of powdery mildew, and each species attacks a range of different plants. In the garden, commonly affected plants include cucurbits (squash, pumpkins, cucumbers, melons), nightshades (tomatoes, eggplantspeppers), roses, and legumes (beanspeas).

When the fungus begins to take over one of your plants, a layer of mildew made up of many spores forms across the top of the leaves. These spores are then carried to other plants by the wind. Powdery mildew can slow down the growth of your plant and, if the infection is severe enough, will reduce fruit yield and quality. 

How Does Powdery Mildew Spread?

Powdery mildew spores typically drift into your garden with the wind, but if you’ve had powdery mildew occur in the past, new outbreaks may also come from dormant spores in old vegetative material or weeds nearby.

Unlike many other fungal diseases, powdery mildew thrives in warm (60-80°F / 15-27°C), dry climates, though it does require fairly high relative humidity (i.e., humidity around the plant) to spread. In cooler, rainy areas, it does not spread as well, and it is also slowed down by temperatures higher than 90°F (32°C). It tends to affect plants in shady areas more than those in direct sun, too.

 

    Identification

    How to Identify Powdery Mildew Damage

    • Plants infected with powdery mildew look as if they have been dusted with flour.
    • Powdery mildew usually starts off as circular, powdery white spots, which can appear on leaves, stems, and sometimes fruit.
    • Powdery mildew usually covers the upper part of the leaves, but may grow on the undersides as well.
    • Young foliage is most susceptible to damage. Leaves turn yellow and dry out.
    • The fungus might cause some leaves to twist, break, or become disfigured.
    • The white spots of powdery mildew will spread to cover most of the leaves or affected areas.
    • The leaves, buds, and growing tips will become disfigured as well. These symptoms usually appear late in the growing season. 

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    Powdery mildew first appears as small white spots on the upper part of the leaves. Photo Credit: The Regents of the University of California, UC Davis.

    Control and Prevention

    How to Prevent Powdery Mildew

    As with all pests and diseases, the best means of controlling powdery mildew is proactive prevention. 

    • Choose plants for your garden that are resistant to powdery mildew. Many mildew-resistant varieties of cucurbits (melons, cucumbers, squash, etc.) have been developed and can be bought from major seed suppliers.
    • Plant in sunnier spots, as powdery mildew tends to develop more often in shady areas.
    • Selectively prune overcrowded areas to increase air circulation around your plants; this helps to reduce relative humidity.
    • Watering from overhead can help to wash spores off leaves. Note, however, that wet foliage can often contribute to the development of other common diseases, so it’s best not to rely on this as a prevention tactic.

    How to Control Powdery Mildew 

    • Consider spraying infected plants with protectant (preventative) fungicides. Effective organic fungicides for treating powdery mildew include sulfur, lime-sulfur, neem oil, and potassium bicarbonate. These are most effective when used prior to infection or when you first see signs of the disease.
    • If you don’t want to use chemical fungicides, try spraying your plants with a bicarbonate solution:
      • Mix 1 teaspoon baking soda in 1 quart of water. Spray plants thoroughly, as the solution will only kill fungus that it comes into contact with.
    • Once plants are heavily infected, it’s very difficult to get rid of the disease, so focus on preventing it from spreading to other plants. Remove all infected foliage, stems, and fruit and destroy them, either by throwing them in the trash or by burning. Remember, do not compost any infected plant, as the disease can still be spread by the wind and persist in the composted materials.

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    Reader Comments

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    I have two sun flowers one

    I have two sun flowers one about 6 feet the other about 4and a half the small one all the leafs started to wilt couple of days ago now the tall one is doing the same the smaller one large flower three buds and the tall one has a very large flower what could be causing this any help would be appreciated thank you

    Usually, sunflowers wilt

    The Editors's picture

    Usually, sunflowers wilt because they need more water. Water every day to every other day and see if they perk up.  Sunflowers also naturally drop as their seed heads get heavy and they protect the seeds from the sun; this is a natural part of the cycle.

    I just got my phlox in

    I just got my phlox in July.Planted it now all the leaves have brown spots. Asked nursery told me it has that mildew.How can I tell if the whole plant is dead? I trimmed it as much as possible. Thank you for ur time.

    Yes, powdery mildew is a

    The Editors's picture

    Yes, powdery mildew is a well-known nuisance on phlox. Usually, this is not a serious enough problem to kill the plant in its first season, but cut back all the disease and remove the stems this fall (after frost) s the fungus does not survive the winter. In the spring, watch for new development and spray the foliage with a fungicide or dust with sulfur. Also, make sure the plants are well spaced to allow good air movement throughout the foliage. Water plants early in the day so leaves dry quickly.
    Do not over fertilize (especially with nitrogen).

    Please help.--My home is in

    Please help.--My home is in Turkey, (ex pat).Earlyier this year i returned from the uk bringing with me two 5ft conference pear trees (my favorite). Reading all the info about pear trees, i purchased another euoropean tree to cross polonate.--well i waited ,and in the begining of June the buds started to open NO flowers, It seemed happy with the position i planted it in,leaves and shoots started to appear . Now they have stopped growing the leaves a little curlled but still very green. Q. We have another 8 weeks of hot sunshine 35c untill it starts to cool down, could that be the case of it being to hot. All my other fruit trees do not seem to be affected --Apple,Peaches, lemon, & orange. ----PS, It would be an good idea to include the temptures that most plants can endure in artcles writen . Regards Michael. Trudgeon.

    Rose Red77-I live by the

    Rose Red77-I live by the seaside and have finally given up on having roses because of blackspot.The best treatment is to spray very frequently (weekly),rake under the roses getting all the infected stuff away from the plant,and buying roses that are blackspot resistant. There are websites that teach about blackspot.You have to do a little searching for them though. Hope This Helps!

    Oh, dear. Black spot is

    The Editors's picture

    Oh, dear. Black spot is common to roses the world over and there may be as many opinions on this as there are rose varieties. Some say that cleaning the ground of leaf debris at the end of season and then again in spring is the solution, as well as pruning the infected canes, avoiding wetting canes on gray (cloudy) days, and avoiding planting roses too close together. Others suggest that in combination with control agents such as the fungicide chlorothalonil (Daconil), reapplying it after rain and applying it to all new leaves (applications in 7 to 10 day intervals have been proposed). Another suggests Funginex. Some say fertilizing roses with Epsom salts helps. And then there is choosing plants that are resistant to black spot.
    Finally, you might also consult your local agricultural extension service for local advice and/or notice where roses thrive in your vicinity and ask the property owner how they handle or avoid this plague. Best wishes!

    This link said

    This link said BlackSpot/Powdery Mildew. Are these different? I have black spots & yellow leaves on nearly every rose I plant in my garden but I don't have anything that looks like the powdery mildew description. Where can I find the best help for getting rid of Black Spot? My Hybrid Teas only last a maximum of 3 years before they succumb to this disease, it eventually ruins the canes and rots the root-stock. Thanks.

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