Powdery Mildew

Last updated: June 14, 2021

What Does Powdery Mildew Mean?

Powdery mildew is a common fungal disease that affects many different plants. It is caused by a wide array of fungi of the Erysiphales order. Pososphaera xanthii (aka Sphaerotheca fuliginea) is the most common reported cause.

Powdery mildew is easy to spot by the white, powdery residue that covers leaves and stems on the infected plant.

Cucumber plant infected with powdery mildew


Maximum Yield Explains Powdery Mildew

Powdery mildew grows best in high humidity and moderate temperatures, with greenhouses providing the most optimal growing conditions. Powdery mildew can infect everything from cannabis to common houseplants to crops such as squash, beans, cucumber, and legumes.

Mildew spores are carried by the wind, air ventilation systems, clothing, pets, and just about any living thing that moves about. These mildew spores will remain dormant until environmental factors offer the optimal conditions for them to come to life.

Powdery mildew is most likely to occur when humidity levels exceed 55% and temperatures are warm. Once awakened, the powdery mildew spores are likely to attack the young plants first and then will spread over the entire plant, infecting stems and buds, in addition to leaves.

Some risk factors for powdery mildew include overwatering, getting the leaves of the plant too wet, improper ventilation that results in high humidity, and rain. Powdery mildew can also occur in young plants that aren’t yet well-established, and when plants are overcrowded in general and end up trapping moisture within the plant canopy.

Chemical methods, genetic resistance, and careful farming are the common methods of controlling powdery mildew in agribusiness and professional horticulture. If not monitored and treated, powdery mildew can lead to low yields or even crop failure.

There are many home remedies for the average gardener, as well. Milk is a common remedy popular with gardeners when it is diluted in water and sprayed on plants at the first signs of the disease. A weekly application is suggested thereafter. Exactly why milk is an effective treatment isn’t known, but it is believed to be the result of the protein in whey that produces free radicals when exposed to sunlight. Contact with these radicals is toxic to the powdery mildew fungi.

Another popular treatment is a careful weekly washing. This works best on plants with glossy or waxed leaves, but not on textured leaves.

There are also commercial fungicides available, but these are generally used only in an agricultural setting and not recommended for use on certain crops such as cannabis.

Because powdery mildew is so widespread and can infect so many species of plants, a grower must remain vigilant to the early signs of the fungal disease.


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