Kentucky bluegrass shaded by structures may be perpetually bothered by powdery mildew.

Powdery Mildew on Kentucky Bluegrass

by Angela Ryczkowski

All species of turfgrass are potentially affected by powdery mildew, but Kentucky bluegrass (Poa pratensis) is among the species that are most severely affected by this fungal disease. Kentucky bluegrass is best suited for lawns in U.S. Department of Agriculture plant hardiness zones 2 through 7. Understanding the preferences of the pathogen responsible for powdery mildew and adjusting care practices and the growing environment accordingly will often eliminate the need for fungicide applications.

1. Disease Description

Powdery mildew first appears on infected turfgrass as isolated tufts of fine, whitish, cobwebby growth on leaf blades. As the disease progresses, the white fungal growth becomes denser and spreads so that entire leaves and areas of turf are completely covered with the powdery white growth. Where an infection is advanced, affected grass blades turn yellow and wither while the entire area looks pale green or dull white and thins out. This disease is typically most prevalent when weather is mild and humid.

2. About the Pathogen

Powdery mildew on Kentucky bluegrass is caused by the fungus Blumeria graminis, or Erysiphe graminis. This pathogen overwinters in dormant plants or on debris and spores spread by wind. Powdery mildew development on turf is favored by high relative humidity, poor air circulation, shade and air temperatures between 60 and 70 degrees Fahrenheit. Powdery mildew on Kentucky bluegrass will not harm children or pets that use the lawn area. Although other types of plants can host powdery mildew, the various powdery mildew pathogens are fairly host-specific, so the disease on Kentucky bluegrass will not spread to other desirable vegetation nearby.

3. Cultural Control

Selectively pruning off overhanging branches and thinning out adjacent shrubs and other dense vegetation will increase the amount of sunlight reaching the grass and improve air circulation around the turf, making conditions less favorable for powdery mildew development. Ensuring that drainage is good and avoiding overwatering grass in shady sites is also helpful. Overseeding affected areas with grass seed from other grass species or Kentucky bluegrass cultivars that offer greater resistance to powdery mildew will help to minimize problems with this fungus. In general, Kentucky bluegrass cultivars that offer a greater shade tolerance are less susceptible to powdery mildew. Excessive nitrogen fertilizer application encourages a flush of tender new growth particularly attractive to powdery mildew.

4. Chemical Treatment

Fungicides to address powdery mildew on Kentucky bluegrass are generally not necessary or recommended and typically are warranted only in highly visible, managed areas. Various fungicides can prevent or control powdery mildew, including thiophanate-methyl, myclobutanil and propiconazole. Recommended application rates vary between products, but most call for blending 1/2 to 2 ounces of concentrated product with a gallon of water and thoroughly spraying the vulnerable or affected turf. Repeated applications every few weeks while conditions still favor powdery mildew development may be necessary. Always follow manufacturer recommendations for safe and effective use. As a general rule, wear goggles, gloves, pants and long sleeves. Do not apply chemicals on windy days or when rain is anticipated for at least 24 hours, and do not permit any activity in the area until the fungicide has dried completely.

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