Powdery mildew related diseases are some of the most widespread and common plant diseases worldwide. They can affect a wide variety of hosts, including agricultural crops and ornamental flowers. Almost all types of plants are affected by powdery mildew. It is a very economically important disease on grapes, strawberries, grass, cucurbits, tomatoes, apples, cherries, pears, rose, hemp, and more. 

Though there are many different species of powdery mildew fungi, and many hosts have a specific species that affects them, there are similarities with all types of Powdery Mildew. In this guide, we will look at the common symptoms, conditions, disease lifecycle, and control methods that can be used on a wide variety of crops affected by Powdery Mildew. 

Powdery Mildew Symptoms

Powdery Mildew symptoms can vary by species and plant host, but all are recognizable for the white or gray, powdery patches or spots. These patches can spread to stems, leaves, flowers or fruit. Small pin-like structures called cleistothecia first appear white, before turning brown, then black. These structure are how the fungi overwinter, and are not always present.

In most cases, the disease is first observed on the leaves, (it could be undersides or upper sides depending on plant/ powdery mildew species). Usually, newer, tender leaves are infected first. After the initial infection of leaves, the white powdery spores then spread to young stems, buds, flowers, or fruit. Areas of the plant that are infected eventually die and leaves can become distorted or discolored. Infected flower buds may die before opening.

Conditions For Disease Infection

The risk of powdery mildew infection can be infected by many things: the host plant variety, growth stage and condition, the presence of the disease, and the weather conditions. For some crops. plant susceptibility can differ greatly between cultivars. If Powdery Mildew is a persistent problem and there are more resistant cultivars available, these should be chosen. Usually younger leaf or stem tissues are more susceptible than older tissues, and plant stress due to poor irrigation or fertilization can also make plants more susceptible to powdery mildew infection.

Plant Pathology disease triangle. From aps.net
Plant Pathology disease triangle. From aps.net

Powdery Mildew are usually more present in warm and dry climates. While the fungus does require higher humidity for spore germination, free water such as rain or overhead irrigation can disrupt the reproduction of the disease. When plants are overcrowded, or in shaded areas, humidity is usually increased, and disease risk is higher.

For several crops and powdery mildew species, there are Disease Risk models that can help inform when plants may be at higher risk for powdery mildew infection or outbreak. Grapes, Hops, and Cherry Powdery Mildew disease risk index models are available on the Pest Prophet app, or they can be calculated manually using daily weather data.

The output of these models presents risk as Low, Moderate, or High and can be a useful tool for timing preventative fungicides, or for determining when to begin scouting for powdery mildew. Because powdery mildews of different plants typically favor similar weather, the Grape Powdery Mildew Risk Index model may be a useful tool for other crops, such as hemp.

Powdery Mildew Model Pest Prophet

Powdery Mildew Life Cycle and Growth

Powdery mildew fungi are host specific, so they can only survive on their specific host plant, and there is no risk of cross contaminating different plant types. For example, Grapes infected with powdery mildew are not risk of infesting nearby strawberries. However, different powdery mildew species may be favored by similar climates, and thus appear at the same time.

Powdery Mildew Lifecycle. (APS)

Mycelium are the main part of the fungi that observed on plants. These are white powdery threads that grow on the outside of plants. Haustoria are root-like structures that grow into the plant tissue. Cleisothecia are the overwintering bodies that can grow on the plant or plant debris. 

In spring, cleistothecia rupture and release spores that are spread by wind, insects, or other methods. These spores find new plant tissues to infect. The fungus then reproduces asexually, growing the white mycelium on the plant tissue. In this phase, the plant disease can spread rapidly. How fast the fungus grows and reproduces depends highly on weather conditions. 

Powdery Mildew Control

Cultural Practices

There are several practices that can be implemented to prevent (or at least reduce) powdery mildew. As mentioned above, planting disease resistant cultivars if available can greatly help with powdery mildew control. If multiple cultivars are planted, the more resistant varieties should be planted in areas where there might be the most risk of powdery mildew infection, such as shadier areas. 

There are a few other growing practices to keep in mind. Plant stress can makes plants more susceptible to powdery mildew, so it is important to keep plants irrigated and fertilized. However, over using nitrogen fertilizer can produce more new, succulent, plant tissues which are more susceptible to powdery mildew infection. Also, overwatering can produce excess humidity which encourages the growth of powdery mildew.

After infection occurs, infected plant parts (stems, leaves, flowers) should be pruned and removed from the field. Infected plant debris should be left in the furrows or used for compost. These will become inocolum sources and can help the fungus overwinter. Pruning can also be used to increase air circulation if the plant needs it, which reduces relative humidity.

Powdery Mildew on Cucurbits

Chemical Control

If cultural control methods are not adequate to prevent disease infection, chemical fungicides may be needed. There are many different types of fungicides that work against powdery mildew. Some will be more successful against specific species/ hosts. It is important to always carefully read the label of any fungicides when deciding if its use is appropriate. Powdery Mildew fungi can also develop resistance to some active ingredients, so rotating fungicide types is an important part of any management program.

Nearly all chemical fungicides for powdery mildew are more effective when used as a preventative spray, before symptoms occur. A 7 to 14 day interval between sprays is usually recommended throughout the growing season. The Powdery Mildew Risk Index can be used to adjust spray intervals for disease risk, based on the weather. If disease risk is high, shorter intervals should be used, and if disease risk is low, longer intervals can be considered.

Sulfur is the most commonly used product against Powdery Mildew in agricultural settings. It can be effective as a preventative spray, but over using sulfur can sometimes have harmful effects on plant health, or because it is also an insecticide, on natural predator populations. It can also be moderately toxic to humans and animals, so it should be rarely used in home gardens.

Neem oil can also have some effect on Powdery Mildew, in addition to its use a broad spectrum insecticide. Like sulfur, it can ham harmful effects on beneficial insects or plant health, if not used properly. 

Potassium bicarbonate based products can also be used preventatively against Powdery Mildew infection and they are relatively safe to use. There are products that use Potassium Bicarbonate as an active ingredient available for agricultural use, and these work as preventative sprays, but can also be used after powdery mildew spores are observed. It only works through direct contact with spores, so adequate spray coverage is needed to reach the undersides of infected leaves.

For home gardens baking soda (sodium bicarbonate) can be used. 1 tablespoon of baking soda for gallon of water can be effective for many home gardens on a variety of powdery mildew species. It is more effective when combined with a horticultural oil, and in some cases might not work when used alone. 

Biological fungicides are commercially available products that use bacteria or other organically derived compounds to effectively kill powdery mildew. For example, one product Serenade, has an active ingredient of Bacillus subtilis that can damage powdery mildew spores. These products can vary in effectiveness, but as part of a rotation they can be every helpful in powdery mildew prevent and are usually safe for the plant and beneficial insects.

Biological Fungicides

Biological fungicides (such as Serenade) are commercially available beneficial microorganisms formulated into a product that, when sprayed on the plant, destroys fungal pathogens. The active ingredient in Serenade is a bacterium, Bacillus subtilis, that helps prevent the powdery mildew from infecting the plant. While this product functions to kill the powdery mildew organism and is nontoxic to people, pets, and beneficial insects, it has not proven to be as effective as the oils or sulfur in controlling this disease.

Home Remedies for Powdery Mildew

Even though higher relative humidity is beneficial for the growth of Powdery Mildew, spraying water can actually be effective to slow the spread of mildew, if spores are present. This is because water can wash off the spores. So in some cases, spray from a water bottle or sprinklers can be used to slow powdery mildew. This can be risky, however, because most other fungal plant pathogens require water, so spraying water on plants can make other diseases worse. It is recommended only when the plants will dry quickly, with warmer weather and/ or wind is present.

Milk is another commonly used home remedy against powdery mildew. Scientists are not completely clear on the exact mechanism, but something in milk does seem to affect powdery mildew spores. A weekly dose of one part milk to two parts water is recommended to try for home gardens.

Mouthwash can also kill fungal spores on plants, and has been shown to be effective against powdery mildew. Any ethanol based mouthwash can be effective. 1 part mouthwash to 3 parts water is recommended as a starting point. Using too much mouthwash may have a toxic effect on newer foliage of  some plants.

Vinegar is also an effective home remedy to control powdery mildew. 2-3 tablespoons of apple cider vinegar in a gallon of water is usually enough to be effective. A vinegar concentration of over 5% can burn some plants, so it should be used cautiously.


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