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How to Treat Powdery Mildew

Powdery Mildew Close Up

Powdery mildew, or Erysiphe graminis, is a lawn and garden fungus that affects cool season grasses like Kentucky bluegrass, fescues and perennial ryegrasses. If you look closely, the disease presents itself as a white substance that coats turfgrass blades. 

From a distance, it can look almost like a layer of frost on the lawn during the winter. As a disease that attacks cool season lawns, powdery mildew is often spotted between April and September in shady locations when it’s humid and temperatures are warm. 

In this article, we’ll walk you through the symptoms of powdery mildew on grass and lawn disease treatment options, as well as the best fungicide for powdery mildew disease.

What does powdery mildew look like?

Powdery mildew looks like a white, grayish spot easily found on the surface of plants. It often looks like a powdery substance found on the blades of turfgrass. Given the right amount of time, the areas of diseased grass can expand and look like large white or gray spots throughout the lawn. 

What causes powdery mildew?

Powdery mildew usually occurs because of a fungal buildup in the turfgrass level. On turfgrass, it is mainly because of the fungus Erysiphe graminis. As an obligate parasite, it can only survive if it’s living on a host or turfgrass plant.

Here’s a list of some conditions that often contribute to powdery mildew’s presence.

Reduced Air Circulation

Grasses with low air circulation usually have a higher chance of developing powdery mildew. Aerating your lawn regularly prevents this issue. Pruning back trees and shrubs also improve air circulation.

High Relative Humidity

Higher humidity levels and dampness can make the turfgrass susceptible to powdery mildew. Thus, areas with higher humidity levels face this problem.

Low Light Intensity

Turfgrass located in areas that receive a lot of sunlight don’t face fungal disease growth as much because the grass dries out at higher temperatures. Areas on your lawn under shade have a higher chance of getting this disease.

Air Temperatures

The temperature is a leading concern with powdery mildew because it prevails in temperatures ranging between 60 and 70 degrees Fahrenheit.

Pictured above: Powdery mildew grass.

When does powdery mildew appear in lawns?

Unlike other grass diseases, powdery mildew has high propagation during humid weather conditions. It grows better in warmer weather but it won’t typically grow when temperatures reach 90 degrees Fahrenheit or more. 

In the spring when temperatures approach 60 degrees Fahrenheit, powdery mildew begins to produce spores. These spores are then carried into the air and settle on turfgrass blades. 

Purdue Extension says “Prolonged periods of dew or wet weather are not needed for disease establishment and spread, although periods of high humidity favor disease development.”

What types of grass does powdery mildew affect?

Powdery mildew host turfgrass species include Kentucky bluegrass, fine fescues and perennial ryegrasses.

The turfgrass disease is usually spotted between spring and fall, specifically during moderate temperature and higher humidity levels.

Powdery mildew can also infect a wide variety of other plant hosts like crape myrtles, roses, cucumbers, squash, lilacs, azaleas and more.

How does powdery mildew spread?

These fungi are obligate parasites and require tissue to grow and reproduce. Powdery mildew produces spores that usually drift into other surrounding turfgrass with the wind. 

However, turfgrass with a history of the disease can face the same problem again. Dormant spores are also a possible reason for the disease and may cause the powdery mildew to actively grow again once the environment is optimal.

How do I treat powdery mildew?

The good news is, there are several different strategies that can be approached to treat powdery mildew. See below for a few maintenance tips and chemical solutions.

Cultural

The best thing you can do to culturally treat powdery mildew is to maintain a strong, healthy lawn. Grass should be mowed at the proper height, watered appropriately, fertilized and planted in areas that receive sunlight. 

For example, your grass may require 1 inch of water per week including irrigation, but if it’s located in shade, it typically requires less water, less nitrogen and does not require you to mow your lawn too often.

Make sure you avoid watering the lawn right before sunset. Disease grows better in areas without sunlight and this may contribute to the spread of the powdery mildew. 

Providing your lawn with essential nutrients is vital to maintaining a healthy, strong lawn. Sod Solutions recommends the Lawnifi® Fertilization Program. Lawnifi gives your lawn what it needs in the form of nano-sized nutrients, exactly when it needs it in the form of easy-to-apply monthly applications from March through November.

It’s important to note that even with the use of an improved fertilizer program, the timely application of a lawn fungicide is still necessary for overall lawn health. If you fertilize your lawn with nitrogen before applying a fungicide and presently have a lawn fungus, that fungus will expand exponentially, which can be disastrous for your lawn.

As previously mentioned, pruning trees and other plants to reduce shade increases the amount of light that reaches your lawn, thereby reducing the chance of powdery mildew spreading. This also improves air movement and lower humidity.

Keep these strong maintenance strategies in mind the following year because powdery mildew spores may remain dormant and return again in the future.

NC State Extension recommends selecting a turfgrass that has a higher shade tolerance. A good example is fine fescues (hard fescue, chewings fescue or red fescue). A combination of Kentucky bluegrass with grasses like tall fescue and fine fescue would also be ideal. This helps to prevent severe problems.

In addition to NC State’s recommendation, overseeding your lawn with shade tolerant Kentucky bluegrass cultivars also seems to slow powdery mildew expansion.

Chemical

Below are some of the most effective chemical control products for treating powdery mildew. Lawn disease control products with Triadimefon (Armada 50 WDG), Chlorothalonil (Southern Ag Liquid Ornamental & Vegetable Fungicide), horticultural oil (Southern Ag ParaFine Horticultural Oil) Propiconazole (Spectracide Immunox) or copper (Liquid Copper Fungicide) seem to be the most effective for treating powdery mildew. 

If problems persist, consider rotating fungicides with different active ingredients as the powdery mildew may have built up an immunity to one active ingredient over time. Be sure to read lawn fungus treatment labels thoroughly before application.

How do I prevent powdery mildew?

Preventing powdery mildew from the get-go is the easiest way to avoid all these complicated control methods. We suggest the following preventive measures for it.

  • Maintain a strong, healthy lawn with our care guides,
  • Choose varieties of turfgrass with higher powdery mildew resistance,
  • Reduce shade and humidity levels by pruning back trees and shrubs,
  • Increase air circulation by pruning back trees and shrubs,
  • And apply a disease control product preventively to keep powdery mildew from infecting your lawn

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