Powdery Mildew: Treatment and Prevention

Growing your own veggies and herbs is always a challenge, but nothing gets me more frustrated than when I have to battle powdery mildew.

It's one of the most common diseases that will afflict your plants, especially if you've been gardening for a while. It's caused by many different types of fungus that cause white, powder-like spots to appear on the leaves and stems of your plants.

If you leave it unchecked, it can quickly affect the health of your plants, reducing their vegetative growth and inhibiting their ability to fruit or flower.

Fortunately, you can treat it in a variety of ways, which I'll cover below.​

First: What Is Powdery Mildew?​

Powdery mildew completely covering this leaf. It will spread quickly to the rest of the plant if left unchecked. Photo by Pollinator

Powdery mildew completely covering this leaf. It will spread quickly to the rest of the plant if left unchecked. Photo by Pollinator

When people refer to "powdery mildew", they could mean many different types of fungus-related plant diseases, However, the one that is suspected to have caused the rest of the other fungus' to develop is Erysiphe cichoracearum, a fungus that primarily attacks squash plants.

Regardless of the specific type of fungus that afflicts your plants, they all act in a similar way. The fungus will spread over the vegetation of your plants and prevent photosynthesis and the plant's ability to utilize nutrients. Without catching it early, the damage may be too far along to stop, and you will have to remove the plant from your garden completely.

How To Identify Powdery Mildew​

The most obvious sign of this disease is the distinct powdery, white dust that covers a plant's leaves. These start out small, but grow in size as the disease progresses.

Downy vs. Powdery Mildew

The difference between downy mildew and powdery mildew. source

It can be easy to confuse powdery mildew with a similar disease, downy mildew. To tell which type of mildew you have, look at where it appears on the plant. Downy mildew only grows on the underside of plant leaves, whereas powdery mildew will grow anywhere. Downy mildew also lacks the powder-like appearance:

Symptoms of Powdery Mildew

As the disease progresses, the small white spots begin to form a root-like structure that enters the plant's leaves and saps them of nutrition. Leaves will become starved and begin to turn yellow.

If the disease continues to progress, leaves will turn brown and die. This is a bad enough problem on its own, but the dying leaves also open your plant up to sun damage, a malformation of buds and fruit, as well as potentially failing to fruit altogether.

How To Prevent and Treat Powdery Mildew​

Complete prevention of powdery mildew outbreaks can be difficult. The spores of the fungus are carried through the air on gusts of wind. The spores then fall on plant surfaces and begin to reproduce if the conditions are right. In moist conditions, powdery mildew may not be a problem. However, other types of plant diseases can grow in damp conditions.

The best ways to prevent powdery mildew are:​

  • Choose plants that are resistant to powdery mildew
  • Avoid planting vulnerable varieties in the shade
  • Manage aphid problems, as they can carry the spores into your garden
  • Provide moisture to leaves on a regular basis
  • Remove dried or diseased plant matter immediately upon seeing it
  • Use a variety of home or professional treatments if your plants have a serious mildew problem

Best Products to Treat Powdery Mildew​

A variety of commercial products are available to cure powdery mildew. Many of these are fungicides that contain copper to kill off the powdery mildew spores.

However, a number of other treatments can provide good results at lower cost, such as:​

Baking Soda

Baking Soda for Powdery Mildew

Common baking soda that you keep in your kitchen for cooking and baking can be used to prevent the spread of powdery mildew in your garden. Just dilute one tablespoon of baking soda in one gallon of water.

Add 1/3 teaspoon of dish-washing liquid to help the solution stick to the leaf surfaces of the plant. Do not save leftover solution. Make a new solution each time you provide treatment for the plants.​

Potassium Bicarbonate

Potassium Bicarbonate for Powdery Mildew

Potassium bicarbonate is a powdery compound that has a number of uses in food processing, in medicinal products and for wine-making. This compound can also be used in solution as a treatment for powdery mildew problems on garden plants.

The advantage of using potassium bicarbonate is that the compound is effective against powdery mildew that is already established, instead of a preventative measure.​

Mouthwash

Mouthwash for Powdery Mildew

Common mouthwash that you have in your medicine cabinet or on your sink can be used to treat powdery mildew in your garden. The mouthwash should be ethanol-based and should be mixed one part mouthwash to three parts water.

Because mouthwash is formulated to kill germs, it can be used as a powdery mildew spray to eliminate the spores that will continue to reproduce and damage your plants.​

Vinegar

Common household vinegar can also be diluted and used a powdery mildew treatment. Mix four tablespoons of vinegar in one gallon of water and spray onto the plants every three days. The solution can be used safely for gardens with edible fruits & vegetables.

However, vinegar is an acidic substance and repeated use can negatively impact the condition of your plants.​

Sulfur and Lime / Sulfur

Sulfur Spray for Powdery Mildew

Sulfur and sulfur spray have been used for many years to prevent and eliminate molds on plants. It can be used to treat powdery mildew and a variety of other plant diseases. It can be found at your local garden center or plant nursery.

Some products include both sulfur and lime, which is thought to be even more effective against powdery mildew. However, these compounds can burn delicate plant tissue, so use them only as directed and space applications a sufficient amount of time apart to avoid harming the plants.​

Milk

Milk has been recommended for powdery mildew for generations, but only now is the science behind it being investigated. A study found that a 10% solution of milk was as effective against the mold as other methods of treatment. Milk is an inexpensive and completely organic way to fight this plant disease and can be used safely in any planting area.​

Water

Powdery mildew spores require a hot, dry environment in order for the spores to spread and proliferate. When conditions are moist, the spores cannot multiply. Keeping your garden plants slightly moistened will help to prevent the spread of powdery mildew spores that are carried on the wind.​

**Note: Many other plant diseases thrive in a damp environment, so using water is not a long-term solution.​

Neem Oil

Neem Oil for Powdery Mildew

Neem oil is an extract made from the fruit and seeds of the neem tree, which is native to India. It has been used as an insecticide and anti-fungal for thousands of years, and it is still useful today as an organic compound to eliminate garden pests.

Neem oil is used to remove powdery mildew by reducing the ability of the spores to reproduce. It's often recommended as a garden spray against plant insects, but it is also effective as a powdery mildew spray.

However, it may be more effective as a preventative than as a treatment when the problem has already occurred.​


Although powdery mildew can be a stubborn and frustrating problem, good gardening practices can reduce the likelihood of developing this troublesome plant disease.

Try a few of the treatments above to see which works best for your garden.

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