Powdery mildew is a widespread plant fungal disease that affects both edibles and ornamentals, but when you find that your squash vines or rose bushes appear to be covered in white dust, all hope is not lost. You can control or eliminate powdery mildew, and there are preventative measures you can take before it appears.
Powdery mildew thrives on dry foliage in high humidity and moderate temperatures, especially in low-light conditions, or when plants are crowded. But if you have powdery mildew in your vegetable garden or on your shrubs and trees, here’s some good news for you: Powdery mildew is host-specific and will not jump from one type of plant in your garden or landscape to another.
Though there are several species of fungus that cause powdery mildew, the symptoms are the same. Powdery mildew presents as white or gray spots on a plant that often cover most, if not all, of the leaf surface. But powdery mildew is not limited to leaves. It can also be found on the stems, flowers and fruit of the host plant.
Fortunately, the symptoms of powdery mildew look worse than the actual damage caused. Powdery mildew is rarely fatal to the plant it infects, though in advanced stages the fungus will cause plant foliage to yellow, curl or turn brown and eventually causes the plant to defoliate prematurely. On flowering plants and trees, the fungus can lead to early bud drop or reduce the flower quality.
How to Prevent Powdery Mildew
- Pick resistant plants: If you select plant varieties that are bred to be powdery mildew-resistant, you may never have a powdery mildew problem. Read the plant tag before you buy to learn the diseases a plant is resistant to, or contact your local county extension office for help picking the right varieties and cultivars.
- Provide air circulation: Adequate air circulation will deter powdery mildew. Providing air circulation is accomplished by spacing plants properly. When planting seeds or transplants, be conscious of how much space a plant or tree will need when it is fully grown. Crowded plants will get less air circulation.
- Provide light: Pick planting sites where plants will get at least six hours of direct sunlight each day. Prune trees or shrubs that are blocking light to powdery mildew-susceptible plants.
- Don’t over-fertilize: New growth is more susceptible to powdery mildew, so a plant growing vigorously — too vigorously — is vulnerable. A slow-release fertilizer that continues to provide plants with a small amount of nutrients over time will provide for more controlled growth.
Ways to Control Powdery Mildew
- Early Detection: Monitor your plants for signs of powdery mildew. The sooner you can catch it, the better your chance of containing or eliminating the problem before further spread.
- Chlorothalonil: Among the off-the-shelf powdery mildew controls are broad-spectrum fungicides containing chlorothalonil (commonly sold as Daconil). These products are effective at controlling — not eliminating — powdery mildew, and they require coating the leaf surface with a quite noticeable white milky film.
- Baking Soda: Normal household baking soda (sodium bicarbonate) provides for a homemade, organic powdery mildew solution. However, studies indicate baking soda is not all that effective on its own. Baking soda’s efficacy comes when it is combined with horticultural grade or dormant oil and liquid soap, and when applied as a preventative or early on. Mix 1 tablespoon of baking soda with 1 teaspoon of dormant oil and 1 teaspoon of insecticidal or liquid soap (not detergent) and combine with 1 gallon of water. Spray on plants every one to two weeks.
- Potassium bicarbonate: Potassium bicarbonate is a contact fungicide that kills mildew spores quickly, and it’s also approved for organic growing. Oil is not necessary when using potassium bicarbonate for mildew control. Just mix half a teaspoon of liquid soap with a gallon of water, then stir in 1 tablespoon potassium bicarbonate and spray lightly on leaves.
- Mouthwash: The fungal spores of powdery mildew are no match for germ-fighting mouthwash. Jeff Gillman, Ph.D., the director of the University of North Carolina Charlotte Botanical Gardens, tested it, and he found that one part ethanol-based mouthwash to three parts water is an effective powdery mildew control. But be careful when mixing and applying mouthwash as it can damage new foliage.
- Vinegar: The vinegar in your pantry is mainly diluted acetic acid, which can control powdery mildew. A mixture of 2 to 3 tablespoons of common apple cider vinegar, containing 5% acetic acid, mixed with a gallon of water does the job. Higher concentrations of acetic acid are more effective, but vinegar can also burn plants, so test first and apply with care.
- Sulfur: Disease spores cannot develop in direct contact with sulfur. Sulfur fungicide, which is approved for organic gardening, can be dusted on dry or mixed with water and sprayed on.
- Milk: Applying diluted milk is a novel method for controlling powdery mildew. Why it works so well is not clear, but it is believed that naturally occurring compounds in milk work to combat the powdery mildew while boosting the plant’s immune system. Apply a weekly dose of one part milk to two parts water.
Powdery Mildew Controls That Do Not Come Recommended
- Water: Dry conditions and high humidity provide the most favorable environment for powdery mildew to form in, so using water to combat powdery mildew is counterintuitive. However, water works because it washes spores off leaves before they can embed. But I do NOT promote water for control because wet foliage makes plants susceptible to many other diseases that are far more of a concern than powdery mildew. If you do go this route, do so early in the day so foliage has time to dry out before the sun goes down.
- Neem oil: This organic option for plant disease and pest control is extracted from the neem tree, native to India. Neem oil is an effective and broad-spectrum insecticide that kills soft-bodied insects — including some beneficial insects. As for controlling powdery mildew, results are moderate at best. There are better options with fewer drawbacks.
Disposal and Composting of Powdery Mildew Affected Leaves and Plants
When leaves afflicted with powdery mildew drop or when infected plants die, you should practice good garden sanitation by removing all of that plant debris.
Understand that powdery mildew gets on leaves through airborne spores that can travel a great distance. Once airborne, the spores can only survive for a few days without a host plant, yet that can be more than enough time to infect susceptible plants. If you leave infected plant debris in your garden over the winter, the spores will have a much shorter distance to travel to find host plants and continue the cycle.
I am often asked if it is safe to compost leaves that have powdery mildew, and the answer is: It depends.
If your compost pile remains cool, the leaves may decompose slowly while powdery mildew fungus and other plant pathogens survive. If you then apply that compost as a top dressing or soil amendment in your garden the following season, you may be introducing powdery mildew to susceptible plants.
To ensure powdery mildew does not overwinter in your compost pile, you should practice hot composting. Not only does hot composting turn inputs into humus much faster, the process also kills plant diseases and neutralizes weed seeds. This requires getting the temperature up to 140° Fahrenheit or higher for a prolonged period — about 10 days.
To achieve those temperatures, your compost will need to have the right balance of nitrogen-rich “green” ingredients, such as vegetable scraps from your kitchen, grass clippings and used coffee grounds, and carbon-rich “brown” ingredients, such as fall leaves and shredded newspaper. By volume, your compost bin should have a 3-1 ratio of brown inputs to green inputs. You will need to keep the compost moist by adding water when it becomes drier than a wrung-out sponge. You should also keep it aerated by turning compost weekly or whenever the temperature exceeds 165°. (A long-stemmed compost thermometer is a useful tool here.)
For comprehensive instructions on composting, you can get my free resource, The Complete Guide to Home Composting. It will tell you everything you need to know to create quality compost that you can add to your garden confidently.
If hot composting proves too difficult because of your climate — or any other limitations — the conservative approach is to put leaves and plants afflicted with powdery mildew out with the trash.
However, if your municipality picks up fall leaves and other yard debris for composting, it’s even better to put the affected leaves and plants in there. Municipal composting is done in such tremendous volume that getting the compost hot enough to kill pathogens is never a challenge. And if you can avoid sending plant material to a landfill, you should.
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