Every seasoned gardener has had at least one bout with powdery mildew. It is annoyingly pervasive, and no one wants to see those tell-tale spots on their plants. It seems that everywhere we look—trees, landscaping, gardens—there is no escaping it. So what exactly is it, and how can you keep it from stunting your plants’ growth and destroying your yield?  How can you avoid what seems to be everywhere?

What is powdery mildew?

Powdery mildew is a generic term that refers to a group of related fungi that are plant-specific and share common symptoms. Because they are plant-specific, the powdery mildew that you have on your ornamentals will not necessarily spread to your vegetable garden. Despite this, powdery mildew does spread easily from plant to plant and spores can easily travel through the air. They can even move through screen windows, so your indoor plants are at just as much risk as your outdoor ones.

Although each type of powdery mildew is tailored to thrive on only one or two types of host plants, no type of plant is safe. This nasty fungus attacks every type of plant from vegetables to fruits, flowers, trees, and house plants. Powdery mildew may just be the most common garden pest of all. Despite its pervasiveness, some plants are less susceptible. Don’t be discouraged! There are things you can do to protect your garden and your yield.

A Hardy Invader

Powdery mildew produces enzymes that help break up the plant cells of its host. This allows the fungi to penetrate its host plant’s cells with root-like structures. These structures, called hyphae, begin to create a microscopic web-like structure across the plant. This larger web-like structure is called mycelium. The mycelium blocks out essential light from the sun and cripples the plant’s ability to breathe.

Once established, powdery mildew spreads quickly across a plant and through a garden. The hyphae rapidly-produce conidium or asexual spores. This rapid reproduction means that infections can get out of hand quickly, so prevention is essential.

Why Should You Worry About Powdery Mildew?

Powdery Mildew Weakens Plants

Although a mild powdery mildew infection is not generally considered fatal to plants, it is an extremely tenacious fungus. It can kill plants and will hurt your harvest if not properly treated. Powdery mildew spores easily go dormant. Many strains of powdery mildew can survive the winter on host plants, so if it is not treated properly the infection will redevelop every spring.

Because the fungi penetrate host cells, they can extract nutrients directly from the plant. This causes an overall decline in plant health and vitality; and, can affect the quality and quantity of flowers, fruits, and vegetables.

Fast-moving fungus

No gardener wants to deal with a well-established powdery mildew infestation. It may seem like a relatively mild garden invader, but it can dig in deep and spread quickly. Although spores generally spread through the air, they can also be spread via animals, clothing, birds, insects, rain splatter, and through their ever-extending mycelial network. These tiny spores can go just about anywhere without you even realizing your garden has been exposed. It is safer to assume you are always at exposure to powdery mildew. Act as if it is always in the air around you.

Where Do Powdery Mildew Symptoms First Appear?

Keep your eyes out: small, round, white patches

Powdery mildew shows up as small, round, white, or gray patches. These patches often look like a fine fuzz or powder, giving it the name “powdery mildew”. These powdery patches are the thread-like mycelium that pierces the plant’s leaves.

These patches will usually appear on the tops of leaves first and move their way down to the lower leaves of the plant. As the infection progresses, the mycelium will spread across the top of the leaf area, and onto the undersides of leaves, down stems, and can even penetrate fruit or flowers. This can happen quickly, and what started small can feel like it is spreading before your very eyes. As the fungi spread, you will notice the leaves begin to yellow, wilt, and sometimes brown.

Prevention is your best pest/disease management tool. For the best results, you should be treating your plants before you ever see a spot.

When Are Plants Most Vulnerable to Powdery Mildew?

Mild times of the growing season

Plants are most vulnerable to powdery mildew when temperatures range between 60°F and 80°F. Powdery mildew thrives in shady areas and does not need humidity to germinate. Many strains prefer a dryer climate and growing area since water or rain can wash away spores.

The spring is an especially sensitive time for powdery mildew infestations because as soon as temperatures are steadily above 60°F, spores start releasing rapidly.

Although powdery mildew can thrive throughout the outdoor growing season, it is sensitive to sun and heat. It tends to be less of a problem in the really hot days of summer unless you have a well-established infestation. For outdoor growing, prevention is most important during the beginning and end of the season.

For indoor gardens, prevention is crucial any time you have plants going. Greenhouses and indoor grows create the perfect environment for powdery mildew.

How do you prevent and treat Powdery Mildew?

Preventative Sprays

Sprays are generally either preventatives or eradicant fungicides. There are not any good treatment measures that do both well. Be sure to use a method that is appropriate for what you need. When using sprays, coat the entire plant including the undersides of leaves. Applying regularly is important for prevention and treatment. If you are not seeing any signs of powdery mildew, apply once a week as prevention. If you are already treating an infection, thoroughly spray your plants every 3-4 days with the spray of your choice.

Milk and Whey

Milk has been studied as far back as the 1950s as a preventative and eradicant for powdery mildew. It can reduce instances of powdery mildew by up to 70% in outdoor gardens. There is a possibility that is even more effective in greenhouse settings.

Whey is also significantly effective in treating powdery mildew. In greenhouse settings, whey spray has been shown to reduce instances of powdery mildew by 71-94% in cucumbers and up to 90% in zucchini. These figures are comparable to common fungicides.

There is speculation that milk and whey help to raise the pH on the leaf surface, which helps prevent powdery mildew. However, a study in the journal Australasian Plant Pathology suggests that there are more complex reasons milk is so effective against powdery mildew. This study suggests that in natural light, multiple parts of milk produce oxygen-free radicals. These free radicals work to kill and prevent powdery mildew. A mix of both milk and whey damaged the spores and hyphae within 24 hours of treatment. The study isolated different components of milk, such as lactoferrin, whey, and lactoperoxidase. The milk and whey were the fastest-acting and most reaching. They damaged both spores and hyphae within 24 hours. The other methods, including just hydrogen peroxide, took at least 48 hours to see any effect. Hydrogen peroxide has been shown to also use free radicals to damage powdery mildew. But as this study shows, the proteins in milk and whey were even more effective.

Although milk appears to be an effective eradicant, it is most effective when used as a preventative. Higher concentrations of milk have shown to be most effective. Use at least 60% milk to 40% water with a few drops of dish soap.

Bicarbonates

Sodium bicarbonate and potassium bicarbonate can both be used as preventative solutions. However, sodium bicarbonate can have adverse effects on your soil structure, and potassium bicarbonate can be damaging to plants if not diluted properly. Powdery mildew cannot survive in an environment with a pH above 8. However, extremely alkaline treatments can damage your plants. When combined, sodium and potassium bicarbonate can cause serious damage to plants, so they should be used sparingly. If you do choose to use bicarbonates, be sure to mix and apply properly.

Green Cleaner or Alcohol Spray

Green Cleaner is a natural fungicide, miticide, pesticide, and insecticide. It can be used as an eradicant. It helps dehydrate powdery mildew and coats spores to help prevent them from furthering the infection. If using a spray containing alcohol, such as Green Cleaner, only use it in the evening. If used under direct sunlight, the combination of light and alcohol will damage your leaves, so make sure you spray your plants as the sun is going down.

Sulfur

Sulfur is a preventative that has been used for centuries and is most effective if you use it as a foliar spray. Sulfur should not be used in temperatures over 90°F or within two weeks of an oil spray. It can also damage ornamental plants, so be careful if using it outdoors near landscaping.

Commercial Antifungal Sprays

Serenade is a biological fungicide that acts as a preventative. It contains a bacterium that helps prevent powdery mildew infections from taking hold. SaferGro Mildew Cure can also work to prevent an infection.

Oil Sprays

There are a variety of natural oils that work well as eradicants after an infection is present. These include neem oil or oil-containing products that are specially formulated for powdery mildew.

Additional Measures

Removing Infected Leaves

Regularly check your leaves for the telltale circular spots of fungus and yellowing. If you see any signs of powdery mildew, remove all infected leaves immediately. Make sure you do not disturb or shake plants when trimming infected leaves. When you are removing leaves, make sure that you do not contaminate yourself or your surrounding area with spores. You want to minimize spore release! Put all trimmed leaves into a sealed bag and dispose of the removed leaves far away from your garden. Do not ever put contaminated leaves in your compost. This is true with any type of infestation. Wash your hands with a solution containing alcohol and spray your entire garden immediately. After you have noticed an infection, spray your garden every 3-4 days. Check all leaves and plants regularly for signs of powdery mildew.

Healthy Rhizosphere

Compost teas and beneficial microbes help act as an immune system for your plant. The healthier your beneficial microbial activity is, the less likely an infestation is to occur. All plants need a healthy rhizosphere to thrive.

Good Air Circulation and Low Humidity

Do not let humidity get trapped in your canopy. This creates an ideal environment for powdery mildew. You can avoid this with proper plant spacing, good pruning practices, and using drip irrigation rather than overhead spray irrigation. Be sure to use good air circulation and ventilation practices indoors. Check back next month to learn more about proper ventilation practices! 

Companion Planting

When planning your garden, keep in mind that certain plants are more susceptible to powdery mildew than others. Mix up your beds! Put plants that are more susceptible near other plants that are less likely to contract powdery mildew. This will help minimize the spread of spores throughout your bed. When doing this, remember to maintain other proper companion planting practices. In the vegetable garden, cantaloupe, cucumber, melons, peas, pumpkins, and squash are most susceptible to infection.

Temperature

Because powdery mildew is somewhat temperature sensitive, you can use this to your advantage if you are growing indoors. According to a study published in the journal Phytopathology, powdery mildew germination ceased at 92°F. 

As you can see from the data, controlled temperature environments have the opportunity to utilize temperature to kill active colonies of powdery mildew. Some gardeners say you only need to keep your room above 100°F for an hour to kill a powdery mildew infection. According to this data, however, it would be more thorough to keep it above 100°F for 6 hours.

Most other treatment methods are strictly preventative, so this is important information to consider. If you are thinking about using the heat method to kill powdery mildew, remember that high temperatures can also stress your plants. Consider raising the heat in your room just before lights out and making sure that your plants are well-watered.  If you are growing hydroponically, also think about keeping your water temperature closer to 60°F to help your plants deal with the heat stress.

Resistant Strains

Some plants have strains resistant to powdery mildew. If powdery mildew is a major problem in your climate, then planting resistant strains might be worth a consideration.

There is a reason we tell you so many times that prevention is important. To avoid powdery mildew, prevention is key. Powdery mildew is extremely common and a big headache, but there are ways you can treat it. However, any treatment you try may be less effective after powdery mildew is on your plants. You know the saying- “An ounce of prevention is worth a pound of cure.” Do not let it get to that point and you will be a much happier gardener.

If you have questions or tips about dealing with powdery mildew, feel free to contact us!