Powdery mildew

Departments - Weed/Disease/Insect Control

Keep grass long and dry to help fight this fungus.

March 16, 2011

Tom Isakeit, AgriLife Extension-Texas A&M.Definition:
Powdery mildew is a fungus that attacks leaves and grass blades and is caused by the fungus blumeria graminis. The fungus is usually not very aggressive unless the right conditions are met. However, severe outbreaks may occur, says Ronald D. French, extension specialist in plant pathology at Texas A&M University.

“Once it’s there, it’s very easy to diagnose because of the fluffy growth,” French says. “The fungus is actually growing inside but also outside the leaf. At the beginning, you might only see it as a yellow spot. You’re not going to see that fluffy growth when the fungus first attacks. But, later on, you’ll see that fluffy growth turn more into a grayish color and then potentially, if it’s really warm, it will actually produce some fungal structures that allows for the fungus to survive from one year to the other.”

Where and when:
Powdery mildew is very common on many plants, but these fungi can be so specialized that if you see powdery mildew on roses, it will not go to your lawn, and vice-versa, French says.

“The ideal scenarios for powdery mildew development are cool temperatures in the mid-60s, poor air circulation, shady areas and high humidity.

“You are more likely to find powdery mildew in cool season grasses and the spring, summer – in some locations – and fall, when temperatures are in the low 40s Fahrenheit, and as high as in the low 70s Fahrenheit,” he says.

The type of grass you have may also factor in to the likelihood of a powdery mildew attack. “Although all turfgrass species can be hosts to this fungus, some Kentucky bluegrass varieties, perennial ryegrass, bermudagrass, zoysiagrass and some fescues tend to be more prone to the attack by the powdery mildew fungus,” French says.

“Since powdery mildew is usually more of an issue on shaded turfgrasses, re-seeding with shade tolerant grass specie or tolerant varieties is a feasible strategy,” French says.

In locations where there is too much shade for grass to do well, shade-tolerant ground covers, such as hostas may be planted.

Other management strategies suggested by French include: providing good air circulation by pruning shrubs and trees, increasing sunlight by pruning shrubs and trees, avoiding excess nitrogen fertilization and watering deep and not too frequent while making sure the grass is not drought-stressed.

Your mowing practices can also help with managing the disease. “A lawn that is too short can stress plant growth while a lawn that is too high can provide the right niche for humidity and lack of aeration,” he says. “Fungi thrive under high moisture, dew (and) condensation because that promotes fungal growth and germination of spores.”
When you cut your lawn, French says to mow only one-third of its leaf length because drastic cutting can stress the plant, making it more susceptible to other diseases as well. The height of the turf depends on the specific species you have, he says.
Chemicals can also be used to control the disease as well, but French says they’re usually not needed.

If you choose to use fungicides look for products that contain propiconazole, myclobutanil, triadimefon, fenarimol or manozeb. If you’re looking for an organically labeled fungicide, there are products that contain potassium bicarbonate.

“Whenever you use a chemical product,” French says, “make sure to follow label directions for use and make sure that it is labeled for turfgrass.”

The author is an associate editor at Lawn & Landscape. He can be reached at bhorn@gie.net.