DESCRIPTION OF THE DISEASE
Pythium root rot causes poor growth as a result of rotten roots. Small, bleached patches develop in the turf that may progress to large dead areas. Affected roots of plants can appear necrotic, or seemingly normal, although incubation under laboratory conditions at high humidity will cause the fungus to emerge from the tissue. Foliage can appear cholorotic or necrotic, wet and greasy. The fungus survives as thick walled resting structures (oospores) in old roots and in the soil and thatch.

SUSCEPTIBLE TURFGRASSES
All turfgrasses except for creeping bentgrass and annual bluegrass are susceptible to Pythium root rot.

CONDITIONS FAVORING DISEASE
There are both cool-season and warm-season Pythium species that cause root rot. Both groups can cause disease when turf is overrrigated and soil drainage is poor. Cool-season Pythium root rots usually occur at air temperatures of 55° to 70°F and generally are slow moving, causing small thinned areas of turf. Warm weather Pythiums are most active at air temperatures above 86°F and can cause explosive, rapid disease overnight under favorable conditions.

MANAGEMENT
Irrigation management is key to managing this disease, but fungicide applications can be made as needed.

Cultural Control
If pythium root rot is a problem in turfgrass, improve drainage and do not overwater. Increase mowing height as feasible to reduce plant stress. Manage the thatch layer to allow for proper water penetration into the soil. Irrigate as needed occurring to evapotranspiration rates.

Treatment Decisions
For warm-season Pythium diseases, time fungicide applications preventively or at the very first onset of disease symptoms. Make fungicide applications for cool-season Pythiums when symptoms develop.

Resistance has developed to mefenoxam for Pythium in a number of locations in the United States. Practice resistance management by alternating the use of fungicides from different chemical classes. In cases where mefenoxam no longer provides control, switch to a fungicide in a different chemical class.

Common name Chemical class

Many of these products are available for use by licensed applicators only and cannot be used by home gardeners.

A. FOSETYL-AL Organophosphate (multi-site)

B. MEFENOXAM Phenylamide

PUBLICATION

UC IPM Pest Management Guidelines: Turfgrass
UC ANR Publication 3365-T
Diseases
F. Wong, Plant Pathology, UC Riverside
M. A. Harivandi, UC Cooperative Extension, Alameda Co.
J. Hartin, UC Cooperative Extension, San Bernardino Co.
Acknowledgment for contributions to the disease section:
M. E. Grebus, Plant Pathology, UC Davis

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