Problem Identifier / How to Treat and Prevent Powdery Mildew (PM)

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What is Powdery Mildew?

Powdery Mildew Squash
Powdery mildew on squash.

Powdery mildew, also referred to as PM, is classified as a fungal disease, impacting a wide range of plants. There are numerous different species, with each one attacking specific types of plants.

According to the most recent estimate, there are almost 700 different species of powdery mildew. When PM is present, the plant’s growth rate is slowed, with severe infections decreasing both yield and fruit quality.

Once powdery mildew infects your plants, it will appear as though flour has been dusted over the leaves. The first symptom is powdery white, circular spots. This can appear on the leaves, fruit and stems.

In most cases, you will see PM on the upper areas of the leaves, but sometimes it grows on the undersides. Younger foliage is the most vulnerable to damage. The leaves begin turning yellow, then start to dry out. Look for twisted, broken or disfigured leaves. Powdery mildew leaves white spots eventually spreading until almost the entire surface of the leaves are covered in the infected areas.

The growing tips, buds and leaves will also become disfigured from this disease. Symptoms are commonly seen toward the end of the growing season.

What Causes Powdery Mildew?

The main causes of powdery mildew include:

  • PM is carried with the wind
  • Warm, dry climates
  • High levels of humidity
  • Shady areas

The spores from powdery mildew are often carried to your plants by the wind. If you have experienced this issue during the past, new outbreaks of PM can result from dormant spores hidden in nearby weeds or aged vegetative matter. Powdery mildew is different from many fungal diseases because it thrives between 60- and 72-degrees Fahrenheit.

PM prefers dry climates with a fairly high level of humidity surrounding the plants. Under these conditions the disease can spread rapidly. The plants most likely to be infected are in shady areas as opposed to under direct sunlight.

How to Treat Powdery Mildew

Powdery Mildew Tomato
Powdery mildew on a tomato plant.

The best solutions for eliminating powdery mildew include:

We recommend using Trifecta Crop Control for the treatment of powdery mildew. It is extremely effective when the first symptoms of the disease appear. Use a dilution of two ounces per gallon of Super Concentrate at 72-hour intervals until the PM is gone.

Getting rid of PM can be extremely difficult if your plants have become heavily infected which is why we always recommend using Trifecta Crop Control as a preventative.

If your infection is more developed, you will need to prevent it from spreading to surrounding plants. Remove all the infected fruit, stems and foliage. Everything must be destroyed by burning or discarding in the trash.

Infected plants must not be composted, or PM can be spread by the wind and remain in your composted materials.

Powdery Mildew Prevention

The best solutions for powdery mildew prevention include:

Powdery Mildew PM Gourd
Powdery Mildew (PM) on a gourd leaf.

No matter what type of disease or pests you have, your best option for control is always prevention! Select plants with higher resistance to powdery mildew whenever possible.

One of the best preventative measures for powdery mildew is using Trifecta Crop Control. Apply a preventative dilution of 1oz per gallon. An application once each week will prevent growth on your plants.

You can generally find varieties of plants resistant to PM including squash, cucumbers and melons at your local garden center or major seed suppliers. These types of plants were developed specifically for PM resistance.

Any overcrowded areas should be pruned to improve air circulation near the plants. This also helps decrease humidity levels.

Do not overwater your plants or numerous common diseases can develop including PM. Powdery mildew frequently develops due to a combination of high relative humidity and a dry climate.

What is the Life Cycle of Powdery Mildew?

Both sexual and asexual reproduction is part of the complete life cycle of powdery mildew. Asexual spores referred to as conidia are produced during the asexual state. The sexual state of PM includes chasmothecia, cleistothecia and perithecia containing ascospores and asci. Ascocarps are generally important for both states during perennation and disease establishment.

Powdery mildew is spread and intensified during the conidial state. Conidia are produced by PM during the growing season in enormous numbers. If the conditions are conducive to PM, this usually occurs within three to seven days once the plant has been infected. The asexual rate of reproduction increases rapidly causing growth. The result is often a population leading to an epidemic.

PM sexual reproduction is usually triggered in a wide range of plants towards the end of the growing season or when flowering. Asocarps are naturally resistant to drought and lower temperatures enabling them to survive through even the harshest conditions. Sexual reproduction can result from genetics resulting in new genotypes with resistance to fungicides.

Powdery Mildew Flower
Powder mildew infestation on a flower.

The new powdery mildew is more virulent than the original genotypes. New epidemics of powdery mildew are often produced during the spring after the growing season when they were initially formed. Ascospores are discharged by powdery mildew after irrigation or a rainstorm. The spores land on unprotected and vulnerable plants to establish an infection that may cause an epidemic.

PM is capable of surviving during the winter by becoming dormant within infected plant buds. If you live in an area with milder winters such as portions of the Pacific Northwest, the sexual states occur far less frequently including during the winters.

If you live in an area with a milder climate and are growing deciduous plants, powdery mildew will most likely be a serious issue. PM will hide in plant tissue during the winter months, then remerge during the spring.

What Plants are More Susceptible to Powdery Mildew?

  • Squash
  • Cucumbers
  • Cannabis
  • Tomatoes
  • Peppers
  • Pumpkins
  • Melons
  • Roses
  • Eggplant
  • Legumes including peas and beans
  • Begonias
  • Dahlias
  • Zinnias
  • Sunflowers
  • Lettuce
  • Potatoes
  • Certain tree fruits
  • Parsley
  • Grapes

Is Powdery Mildew Harmful?

In most instances, PM will not totally kill your plants, but you will have an aesthetic issue. If you do not properly treat powdery mildew, nutrients will eventually be drained from your plants. The leaves will yellow, then eventually wither. Your fruits and vegetables will become susceptible to sunburn, with blooms becoming unsightly.

The production capacity of your plants will eventually be decreased by powdery mildew, with the flavor of your vegetables and fruits negatively impacted.

How Long Does It Take for a Plant to Recover from Powdery Mildew?

With proper treatment and a controlled environment, PM should disappear within one to two weeks.

Defeat Pests, Mold and Mildew… NATURALLY!

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