Pythium diseases

Features - Weed/Disease/Insect Control

These water-mold diseases spread fast, but can be managed when correctly identified.

June 16, 2011

Photo by: William M. Brown Jr., Bugwood.orgPythium diseases are caused by fungi in the genus Pythium and are commonly referred to as “water molds.” There are several Pythium diseases that affect turfgrasses and have names such as Pythium blight, Pythium crown and root rot, cottony blight, root dysfunction, grease spot, snow blight and seedling damping-off.

Pythium blight is caused by several species of Pythium, such as Pythium aphanidermatum, and can be damaging on several turfgrasses, including perennial ryegrass, creeping bentgrass, colonial bentgrass, annual bluegrass and rough bluegrass. Although these cool-season turfgrasses may be more vulnerable to Pythium blight, warm-season grasses such as St. Augustinegrass, bermudagrass and zoysiagrass, can be targeted by Pythium root and crown rot.

Identifying the disease

Pythium blight is a disease that spreads fast, can cause damage on crown tissue and leaves and could potentially kill the plant.

The disease first can be observed as small, round to irregular patches of water-soaked leaves that look and may feel greasy. Leaves will then dry, turn light brown and collapse. Small patches can turn into large patches in a matter of a few days. A slight reddish discoloration may be observed on dead tissue. Under high humidity, leaves may be covered by a whitish cottony growth, which is a sign that the fungus is actively growing.

Pythium blight usually occurs under humid and hot conditions during the day, when night temperatures reach the mid-60s or higher, and when leaves are wet for more than 12 hours during consecutive days.

Pythium root and crown rot may first be observed as small, yellow patches of turf that may appear stunted and having leaves that are usually yellow but may also look reddish brown in appearance. Unlike Pythium blight, these symptoms may be visible under cool and humid conditions during spring and fall.

Pythium root rot will also be active with warm weather, high humidity and wet soil.
Root and crown tissue may be water-soaked and will have a brown discoloration as tissue rots. If a patch of turf is dug out of the ground, roots will be thin and less vigorous in growth and length.

Managing the disease
Since Pythium is a water mold, avoid excessive watering and provide good drainage so that root and leaves are not waterlogged and moist. Watering late in the day or at night should be avoided.

Other management strategies against Pythium blight include: providing good air circulation so that leaves and stems can dry quicker, keep soil pH neutral to slightly acidic, and avoid excessive nitrogen rates.

Mowing practices can also help with managing Pythium diseases. Mowing when leaf and stem tissue are dry is best, less fungal growth will be spread by mowing equipment, shoes and clippings.

Against Pythium root rot, increasing the mowing height and mowing frequency and practices that promote good root growth and plant health can lessen the damage.

Because species of Pythium can overwinter from year to year in plant debris, plant material and as hardy spores known as oospores, it is important to properly identify a disease issue as soon as possible so that management practices can be carried out and a preventive disease management strategy can be done the following season when Pythium is active again.

Although fungicide applications are usually the last line of defense, there are several fungicides that can be used alone or in combination. Fungicides used to control or manage Pythium contain: mefenoxam, pyraclostrobin, cyazofamid, propamocarb, azoxystrobin, fluoxystrobin, Aluminum tris, as well as other chemistries labeled against Pythium diseases for home or commercial use.

The author is an extension plant pathologist at Texas A&M University’s AgriLife Extension Service.