Knowledge To Grow – Powdery Mildew

I would venture to guess that every gardener, at one point or another, is going to have an infestation of powdery mildew.  It is one of the most widespread and easily identifiable plant fungal diseases out there.  Fortunately, the symptoms of powdery mildew are usually worse than the actual damage and is rarely fatal to a plant.

This fungus is host specific, meaning out of the many different species, each specie only attacks certain plants.   For example, the lilac powdery mildew pathogen cannot infect the honeysuckle plant next to it.

Infected plants will first show signs of infection with small, white, powder-like dots on the leaves.  If not treated, the powdery coating will gradually spread over a large area of the leaf and stem.  Leaves can become discolored, die and fall off, flowers may become deformed and shoots may be disfigured.  Severely infected vegetable plants may not produce as heavily, have a shortened production time and less taste.  Brownish spots on pea pods can mean an infection of powdery mildew is present.

Previously infected plant material contains cleistothecia (overwintering bodies of the fungus) that will rupture and release fungal spores in the spring.  These spores are carried with the wind to newly emerging, susceptible tissues where they start growing.  Powdery mildew is most prevalent when temperatures are on the cool side and humidity is high or in weather that is very dry.  These pathogens are some of the only fungal organisms that can germinate and infect when there is no free water.

One of the best ways to avoid powdery mildew infections is to purchase plants that are resistant to the disease.  When purchasing, check the plants tag for PM resistance.  In the process of planting, choose a site that is appropriate for the plant, allowing adequate spacing between plants in order to increase air circulation and enough sun to thoroughly dry plant parts.  Some pruning and thinning will help existing plantings by increasing air circulation and light penetration.

If you prune infected plant parts, destroy them, do not place them in the compost pile.  Temperatures are sometimes not hot enough to kill the spores.  In the fall, remove as much of the plant debris in the garden as possible so overwintering spores will not cause infection the following year.

If an application of a fungicide is needed, those with a low environmental impact are suggested.

Horticultural oils, neem oil, wettable sulfur or antitranspirants can prevent infection when applied and can last up to 30 days. Keep in mind that some of these fungicides need to be applied before or at the first sign that the disease is present.   Do not apply these products when the air temperature is above 85 degrees.  Check the label to make sure the plant you will be using it on is listed and follow the instructions for rates to be used, timing of applications and waiting periods before harvest.

As always, Happy Gardening!

More information about gardening and related subjects can be found online. The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co., 636-2111 in Noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.

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