Protect Against Powdery Mildew

By Russ Landry
Published: November 1, 2015 | Last updated: April 23, 2021 02:12:42
Key Takeaways

Powdery mildew is a common name for many species of fungi that infect the leaves of both indoor and outdoor plants. While it is difficult to eradicate completely, it is one of the easiest plant leaf diseases to recognize.

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Powdery mildew is often seen covering the tops of plant leaves, stems and flower petals. It appears as white or gray spots with a powdery coating of thread-like fuzzy patches. The cause of powdery mildew is an infection of a common fungus resulting in a range of problems including reduced visual appeal, plant health and vigor and fruit yield reductions at harvest.


An unsightly blotch of powdery mildew drastically lowers the visual appeal and value of indoor flowering plants. Although powdery mildew rarely destroys affected plants, it can wield an uncertain outcome and wreak havoc upon them.

The powdery mildew fungus is an obligate intracellular parasite, meaning it grows and completes its life cycle only on living plant tissues. Powdery mildew is widely prevalent, affects numerous plant specific species and families and attacks both indoor and outdoor plants.


There are many types and varieties of powdery mildew fungi that produce similar symptoms in host-affected ornamental and vegetable plants. Most are problematic, difficult to eradicate and difficult to treat after an infection has begun. Fortunately for growers, powdery mildew is one of the easiest plant leaf diseases to diagnose.

Indoor and greenhouse climates are often ideal breeding grounds for powdery mildew. Powdery mildew can grow indoors at any time under favorable conditions. It can be spread from previously infected cuttings or transplants. Outdoors, it usually appears in mid-to-late summer, proliferating as spores.

Powdery mildew can often overwinter on protected decaying plant mater that is located in sheltered compost piles. Spores do not require water or moisture for germination and can be spread by splashing water, circulating fans or summer’s drying winds.


Newer plant growth is frequently more vulnerable than older plant tissues. Powdery mildew may be observed in the both vegetative and flowering stages, including bud tips, and on younger leaves soon after they spread out. Leaves start to become afflicted when cool nights and warm days provide an optimal growing environment on leaf surfaces. Dry spores begin to grow on leaves as they scatter amongst an unsuspecting grower’s crop.

Crowded leaves are the first to suffer from powdery mildew’s infecting spores. Often these are lower areas of the leaf canopies – shaded, warm areas that support poor air circulation and harbor moisture. Cool nighttime temperatures greater than 60°F with relative humidity above 90% present optimal conditions for spore germination. Hidden from the sun’s warmth, sheltered leaves are in danger of spore infection at daytime temperatures of up to 80°F, with humidity in the 40 to 70% range.


Powdery mildew is a common name for many different species of fungi that infect the leaves of countless types of plants. As the spores germinate, the rooting tips of a newly spreading hyphal organism is formed—a short appendage known as a haustorium. This hyphae network of appendages is elongated and multi-branching, appearing as a filament, thread-like architecture of interlinking and connected formations.

The haustoria penetrate into the cell’s epidermal walls, drawing nutrients from them. From the time of germination, new spores can begin to form in as little as 48 hours. High humidity results in optimal spore formation, and low humidity with drying winds promotes spore dispersal.

Infected leaves might start to wilt, bend, curl or buckle. They deform, and then gradually begin to yellow, and then they either die or fall off. The powdery mildew fungus usually does not grow on the surface of vegetable fruits, but flowers may be severely affected. Infected plants have greatly reduced aesthetic worth, lower yields, reduced harvest periods and the fruit often have little flavor. Plant senescence is also hastened as the stress is translated into a shortened and compressed growth cycle.

Simple disease prevention, monitoring and early detection of powdery mildew is critical to maintain adequate yields -- waiting to apply preventive foliar fungicide sprays once the disease is detected often results in more extensive and prevalent infections.

Detection of powdery mildew on crops and leaves indicates an infection that has already progressed enough to have potentially damaging effects. At this initial stage, many leaves might have been assailed by the powdery mildew spores but do not show any visible disease symptoms. It is important that powdery mildew never gets out of control.

Once it is established on leaves, powdery mildew infections are difficult to deal with. It will most likely continue to flourish and spread on plants. Consequently, seemingly minor infections will continue to expand and proliferate. Left unchecked, crop yields can be severely reduced and ultimately destroyed in short time periods.

If signs of powdery mildew infection are present, growers are advised to avoid or reduce overhead watering. Lowering relative humidity around the plant leaves helps in slowing spore germination and spreading of the infection. Removal and destroying infected leaves and plant materials is important to reduce maladies. Diseased and infected plant parts should not be composted.

Powdery mildew becomes a problem for a grower when it affects photosynthesis and begins to severally reduce the vitality of new growth. Expansion and size gains of the plant’s sinks might be slowed or entirely halted. Plants can grow weaker as the infection covers an ever larger surface area of the leaves, but usually survive in less, albeit in a less robust state.

Environmental controls that improve greenhouse conditions are an excellent way to pre-empt and passively manage the damage caused by powdery mildew. Monitoring humidity levels to avoid prolonged episodes of high humidity levels above 50 to 60% is most advantageous. Nighttime humidity is best kept below 70% to limit spore germination.

Ventilation and continuous air movement helps to inhibit mildew spores by lowering humidity. Using oscillating fans to circulate the air and external ventilation periodically during the night cycle also reduces humidity from irrigation and transpiration. However, fans might begin to spread spores throughout the area if powdery mildew has already been established.

Traditionally, prevention techniques include frequent foliar applications of fungicides. These products are generally effective when used in moderation and rotated in use to avoid resistance. Organic and biological controls can protect plants, although in some conditions fungicides might be needed. Fungicides function either as protectants to prevent infections, or as eradicants to halt an existing infection. Protective fungicides must be applied before the disease appears to be effective.

Many preventative organic foliar management measures include the use of phosphite, a salt of phosphorous acid. Phosphite controls the disease by inhibiting spore formation and growth. Usually phosphite is processed into liquid and its formulations offer increased mobility in plant tissue and soils. Phosphite is easily absorbed and distributed through the xylem and phloem and can be applied to all parts of the plant to elicit or enhance systemic acquired growth responses.

Sprays of stylet or neem oils, along with 10% milk solutions and baking soda sprays, also work somewhat effectively as controls of powdery mildew. Baking soda mixtures include 1 tbsp. baking soda, 1 tbsp. liquid soap and 1 tbsp. horticultural oil mixed in 1 gal. of water. Apply the homemade powdery mildew foliar spray immediately after all irrigation events and early in the morning before the sun comes up to lessen leaf burning.

Other alternatives to harsh chemical fungicides are found in misting plant leaves with potassium bicarbonates, a safe, organic measure that can be used on most ornamentals and vegetable plants. Potassium bicarbonates are a contact fungicide that results in desiccation while also inhibiting spore germination of powdery mildew’s spidery hyphal network. They clearly outweigh the disadvantages of using costly synthetic chemical control methods and work better than traditional organic controls.

Newer bio-control preventative fungicides have emerged in recent years that growers might recognize as bacterial and fungal products. They include Streptomyces lydicus used as a foliar spray; the root fungus Trichoderma harzianum sprayed as a contact killer;and a fungus-like bacteria, Bacillus subtilis, used as a spray inhibitor or contact fungus killer. The fungus Ampelomyces quisqualis is a parasite of powdery mildew. It infects and grows pycnidia or fruiting-like bodies inside the hyphae. This parasite reduces powdery mildew growth and might eventually kill the mildew colony.

Whichever contact, systemic preventative or foliar control measure is used on plants, growers are wise to ensure applications of some of these measures are in place well before an outbreak of powdery mildew occurs. Prevention early in the growth cycle is key to ensuring the biggest yields possible are harvested.


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Written by Russ Landry | President

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Russell Landry is the former vice-president of the Great Pumpkin Commonwealth and its many competitive weigh-off sites held worldwide. He is now the current president of the Giant Vegetable Growers of Ontario. Russ publishes the GVGO Growers’ Vine newsletter.

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