As you look over your garden, something catches your eye. There’s a powdery grayness on the leaves of your lilac and maybe your peonies. Maybe your roses or dogwoods have it too. You have powdery mildew.
What is powdery mildew, exactly?
Powdery mildew is a fungal disease that shows up between late June and September when it’s hot and humid. Shade and poor air circulation contribute to its establishment and reach. The fungus is mainly spread by wind-borne spores; however, some varieties can overwinter in leaf litter.
There are many different types of powdery mildew, each specific to plant families. The powdery mildew on your peony won’t infect your crepe myrtle. Common ornamental plants that are susceptible to powdery mildew are roses, crepe myrtle, zinnias, phlox, the aster family (over 300 plants), monarda, and legumes. In your edible garden, grapes, beets, cucurbit family (pumpkins, cucumbers, and melons), legumes, tomatoes, and peppers are susceptible.
Powdery mildew starts out as white spots on leaves that grow and soon cover the tops of the leaves with a white or gray powdery film. It can look like someone had a flour fight in your yard. On some plants, it’s merely cosmetic and doesn’t harm the plant. On others it can weaken the plant and lead to other opportunistic infections.
Don’t get powdery mildew confused with downy mildew. Downy mildew loves cool temperatures of spring and needs wet leaves to infect. Powdery mildew can sometimes be controlled by water, because water can stop the spores from germinating. With downy mildew, water will make it spread. Make sure you know which one you have.
How do you prevent powdery mildew?
The best way to treat powdery mildew is to not let it get a toehold in the first place since it can be hard to treat and eradicate. This is a cornerstone of integrated pest management. Sunshine and good air circulation are key. Prune, at the appropriate time for the plant, to open up and allow good air flow. Don’t plant susceptible varieties too close together. Trim back tree branches where needed to allow the sun to shine on susceptible plants. Plant resistant varieties when adding to your landscape or garden. There are resistant varieties of crepe myrtles, roses, and zinnias for ornamentals. In the vegetable garden there are resistant squash, tomatoes, and legumes.
In the winter, clean up diseased foliage. While there seems to be a debate as to whether or not you can compost the affected leaves, err on the side of caution. While powdery mildew spores can live only on live plant material and won’t spread once composted, you may have other co-infections that aren’t killed by the composting process and can overwinter. When in doubt, throw it out.
How do you eradicate powdery mildew?
To treat powdery mildew, determine first how big of a problem you have. On peonies and lilacs, assuming the plant is otherwise in good health, the infection is mainly cosmetic. On roses and ninebarks, though, powdery mildew can overwinter on the plant, so you’ll have to prune the affected branches and buds.
In all other plants, start with the least toxic options first. If the timing is right, prune to increase airflow. Mist or spray with water to wash off spores. Adding moisture to a mildew may seem counterintuitive, but researchers have found that the powdery mildew spores may like humidity but not rain or water. This method can be used on cucumbers, lettuce, melons, pumpkin, and squash. But be sure to confirm the plants don’t have any co-infections that can be spread by water. For instance, roses can be infected with black spot, another fungus infection, which loves moisture.
If you still have a problem, the next step is applying a nontoxic biofungicide, which uses microorganisms, like bacteria, to prevent and control fungal infections. Follow directions exactly.
Or, you can try homemade solutions. Do an Internet search and you’ll find recipes featuring baking soda, milk, yogurt, or other household ingredients. One of the more well-known DIY recipes is the Cornell powdery mildew spray. Attributed to Cornell University, the spray was formulated initially to control black spot, but it’s said to combat powdery mildew as well. (There doesn’t seem to be a consensus on whether the spray truly came from Cornell, and you won’t be able to find confirmation on their website.) The recipe is simple and you may have all the ingredients at home.
Cornell Powdery Mildew Cure
- 1 gallon of water.
- 2 teaspoons of light horticultural oil. (This is a lighter version of dormant oil. Choose one that is made from plants and not petroleum. Neem, soybean, or cottonseed are good choices.)
- 1 heaping teaspoon of baking soda.
- 1 teaspoon of mild dishwashing liquid (example: Castile soap).
Mix everything together and pour into a spray bottle. Spray after sunset, as spraying during the day can burn the foliage. Make sure to coat the top, underside, and the stems until wet and running off.
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Frequently asked questions
What is powdery mildew?
Powdery mildew is a fungal disease that affects many plants, including roses, lilacs, cucumbers, and squash. It appears as a white or gray powdery coating on the leaves, stems, and sometimes flowers of infected plants.
How does powdery mildew spread?
Powdery mildew can spread through wind-dispersed spores or by direct contact between infected plants and healthy ones. It thrives in high humidity and moderate temperatures, with optimum growth at around 60-80°F.
What are the symptoms of powdery mildew?
Symptoms of powdery mildew include the powdery white coating on leaves, stems, and flowers, as well as distorted growth, yellowing or browning of leaves, stunted or deformed fruit, and premature leaf drop.
How can I get rid of powdery mildew?
There are several methods to control powdery mildew. These include removing and destroying infected plant parts, improving air circulation around plants, avoiding overhead watering, applying fungicidal sprays or sulfur-based products, and using biological controls such as neem oil or compost tea.
How can I prevent powdery mildew?
Preventing powdery mildew involves selecting resistant plant varieties, providing adequate spacing between plants to allow for air circulation, avoiding overcrowding, watering at ground level, practicing good garden hygiene by cleaning up plant debris, and applying preventive fungicidal sprays.
Is powdery mildew harmful to humans?
Powdery mildew does not generally pose any significant health risks to humans. However, it can cause allergic reactions in some individuals, and the presence of powdery mildew on edible plants may affect their taste and quality.
Can powdery mildew be treated organically?
Yes, powdery mildew can be treated organically. Natural remedies such as baking soda solution, milk spray, or diluted vinegar can be effective in controlling powdery mildew. Additionally, improving overall plant health through proper care and maintenance can help prevent and reduce powdery mildew infestations.
Are there any chemical treatments for powdery mildew?
Yes, there are various chemical treatments available for powdery mildew control. Fungicides containing active ingredients like sulfur, potassium bicarbonate, or neem oil can be applied following label instructions to combat and prevent powdery mildew.