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How to Manage Pests

UC Pest Management Guidelines


Pythium Root Rot

Pathogen: Pythium spp.

(Reviewed 9/09, updated 9/09)

In this Guideline:


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.


Creeping bentgrass and annual bluegrass grown on golf greens are particularly susceptible to Pythium root rot, but the disease can affect all other turfgrasses as well.


There are both cool season and warm season Pythium species that cause root rot. Both groups can cause disease when turf is overirrigated 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.


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 Example trade names Ag Use
NonAg Use
(trade name)   (hours) (hours)

Calculate impact of pesticide on air quality
When choosing a fungicide, consider general properties as well as information relating to environmental impact.
A. FOSETYL-AL Aliette, Prodigy, Chipco Signature  
  MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33) 12 until dry
B. MEFENOXAM Subdue Maxx, Apron  
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4) 48 Subdue Maxx: until dry
Apron: —
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Agricultural use applies to sod farms and commercial seed production.
Indicates use is not listed on label.



[UC Peer Reviewed]

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

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