Most gardeners in the US end up with a powdery mildew issue on one plant or another. For me, it's zinnias and my pumpkins and squash that get it the worst. But what is it, and should you panic when you see it? 

What is Powdery Mildew?

Powdery mildew is a fungal disease. There are thousands of species of fungi in the powdery mildew group, and most are host-specific, meaning they won't grow on just anything. The powdery mildew that attacks your pumpkins is normally not the same species that wants to colonize your bee balm.

From zinnias to asters, and snap peas to artichokes, and shrubs like lilac and roses, there's a powdery mildew to worry about. These fungal organisms feed on the plant, growing on leaf surfaces and stealing some of the nutrients from the leaf cells below. But, the good news is that it usually doesn't kill the plant, but will slow it down and can be unpleasant to look at. 

Where does powdery mildew come from?

Powdery mildew spores drift on the wind in spring and summer, released from fungal structures on the previous year's plant growth. They land on new susceptible plants and begin growing. Spores can arrive in your garden on the breeze, be carried by insects, or splashed up from old plant material on the soil surface, which has not been removed. 

All types of powdery mildew require living plant tissue to grow and reproduce, but many can remain dormant over winter on dead plant material and recolonize in spring by releasing a fresh batch of spores. Once the spores land on a leaf, they begin to grow, releasing more spores as they reproduce and spread.

What does powdery mildew look like?

This might be the most easily identifiable plant problem you'll encounter. While there are many different species, you don't need to get down to that level. Identifying it as powdery mildew is enough.

Powdery mildew-infected leaves appear to have been dusted with fine white flour. The infection begins as small white spots, typically on the lower leaves. Then the spots expand until the entire leaf is often covered. If it looks like someone was playing with a flour sifter over the top of your plant, that's powdery mildew. 

Some plants are more tolerant of it than others. Milder infestations may not significantly slow the plant's growth, and it will retain its leaves. Once a leaf is so covered that the white spores are starting to block light from reaching the leaf surface, the plant may withdraw nutrients (causing it to yellow) and drop the leaf. On shrubs, especially, the leaves may twist and look sick before they wither. 

Ways to Control Powdery Mildew

Gardeners want a way to prevent powdery mildew, but given the way that it spreads via wind and insects, you really can't stop the spores from landing in your garden if they are already nearby. What we can do is reduce how appealing our plants are to the spores, and limit the paths it has to enter our garden.

Think of combatting powdery mildew as being in a rowboat with a bunch of holes leaking water. Even if you can't get to all the holes, plugging most of them will give you time to row to shore. It's the same thing with powdery mildew. Slowing it down gives the plant time to fruit, flower, or store energy for the year.

Powdery mildew can be managed through cultural controls (things we do as gardeners) and by selecting disease-resistant varieties (like this gorgeous pumpkin). Most powdery mildews in the garden can be managed without chemical applications, although a few are available for use. Later, we'll discuss some DIY home remedies as well. 

One important cultural control for powdery mildew is pruning plants to maintain good airflow throughout the foliage, which also lowers humidity. Pruning improves light penetration into the plant canopy, which also reduces humidity and provides a more vigorous plant better able to withstand the fungus. 

A second cultural control is proper spacing. When transplanting nursery stock or seed starts, maintain adequate spacing between plants not only for improved airflow, but also to give the plants room to grow and thrive. Healthy plants are better able to withstand all manner of pests and diseases, just like healthy humans. When plants are weakened or stressed, they become more susceptible to disease. 

Third, while we like to leave some plant material on the soil for beneficial insects to have habitat over winter, don’t do that with infected plants. Regardless of what the disease was, remove any infected plant material promptly in fall (or earlier) and dispose of it, don’t compost it. Good garden sanitation of diseased material will reduce the opportunity for reinfection next year.

Once you see powdery mildew in the garden

The first step is to remove the worst of the infected leaves before they drop off, and disposing of them will remove part of the spore source for later in the season and next year. Don't compost them, bag them up, and toss them in the trash or burn them. While it won't stop powdery mildew from spreading, it can slow its spread somewhat. It will also keep them from being released in spring to start the cycle over. Of course, you aren't going to get all the spores, and you can't prune every leaf off the plant. It's merely reducing the source of inoculum. 

For most gardeners and plants, the above cultural controls, planting resistant varieties, and pruning off the worst of it are all that are needed. However, plants in the cucurbit family, such as cucumbers, squashes, and pumpkins, are especially susceptible to powdery mildew and may warrant a fungicide application. 

While most chemical products on the market are not available to homeowners, several horticultural oils, neem oil products, and fungicides are. Always take precautions with any chemical, and try to avoid using those which are harmful to pollinating insects. Alternatively, try one of the home remedies below. 

DIY remedies for powdery mildew

While oftentimes the gardening DIY home remedy ideas floating around the web are bogus, there are a few home remedies you can use in addition to the cultural practices mentioned above. Since these are inexpensive and you may already have them at home, consider giving these a try before purchasing a commercially-made product. 

Milk

Milk appears to be an effective treatment for powdery mildew when diluted and sprayed onto the plants. Several studies in Brazil and Italy have found that milk is effective in treating diseases on cucurbits. However, the mechanism by which it works is still unknown. 

To try it yourself, mix two parts milk with three parts water and spray on both the upper and lower sides of the leaves. Spray it on during the day so it can dry, and don't worry, it won't make your garden smell like old milk. Repeat again a few days later.

Baking soda 

Baking soda helps control powdery mildew by raising the pH level on the leaf surface, making it less hospitable for the spores to grow. It is most effective at low disease levels as a controlling agent rather than a cure.  

Mix one tablespoon of baking soda with one gallon of water, and add a tablespoon of cooking oil, such as vegetable oil, and a few drops of liquid dish soap. Shake it all up, and spray the tops and bottoms of the leaves. 

Test your mixture first on a single leaf and wait 24 hours. Too much baking soda in the sprayer can turn this mixture into an herbicide. Avoid spraying on drought-stressed plants, and apply the spray on a sunny day so the mixture can dry on the leaves. You'll need to reapply after a heavy rain.

Vinegar

This idea gets a lot of traction, and indeed, vinegar does kill some molds and fungi. However, it is also an herbicide, so use care or avoid it when trying to treat powdery mildews.

Ultimately, the best course of action for annual flowers and vegetables is to plant disease-resistant varieties. Powdery mildew will always be floating around, so a combination of resistant plants and good cultural controls is your best bet.