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How to Manage Pests

UC Pest Management Guidelines

Pythium grease spot damaged spots surrounded by healthy grass in annual bluegrass turf.


Pythium Blight (Grease Spot)

Pathogen: Pythium spp.

(Reviewed 9/09, updated 9/09)

In this Guideline:


Pythium blight, also known as grease spot, kills turf in small, roughly circular spots (2 to 6 inches) that tend to run together. Blackened leaf blades rapidly wither and turn reddish brown. Leaf blades tend to lie flat, stick together, and appear greasy. Roots may be brown and rotten.

When dew or high humidity is present, especially in the early morning, white, cottony mycelia can be seen in diseased areas. These areas may have an orange to purplish border or smoke ring. Diseased areas may "run" with the pattern of water drainage, or mowing. The symptoms may appear rapidly under favorable conditions, sometimes killing large areas of grass within 24 hours. This is the foliar form of Pythium; the pathogen may also cause seedling damping off or root rots.


All grasses are susceptible to Pythium blight, which can be caused by several different Pythium species. Certain Pythium species attack some grasses more preferentially, although the distribution of these species in California is not known at this time.


Infection and disease development is associated with daytime air temperatures above 86°F, night temperatures above 68°F and 15 or more hours of relative humidity above 90%. Lush grass with high nitrogen nutrition appear to be more susceptible to the disease as are young/germinating seedlings, so care must be taken during overseeding or establishment in hot weather. The fungus forms thick-walled sexual spores that enable it to survive in soil and plant debris for long periods.


Proper irrigation is the most important factor in the management of this disease, especially during periods of high nighttime temperatures.

Cultural Control
Avoid overwatering; irrigate only when needed to a depth of 4 to 6 inches. Avoid mowing wet grass or applying high levels of nitrogen during hot, humid weather. Reduce shading and improve soil aeration and water drainage. Time establishment of new turf from seed or overseeding for periods when environmental conditions are not favorable for the development of this disease.

Treatment Decisions
Due to the speed at which this disease can destroy grass, use fungicides preventively when environmental conditions favor Pythium development, or soon after symptoms are first evident. Apply a fungicide during overseeding or establishment if temperatures are high and the established area is being heavily irrigated. Fungicide treated seed is available.

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 of a different chemistry.

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.
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) 4 until dry
B. FOSETYL-AL Aliette, Prodigy, Chipco Signature  
  MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33) 12 until dry
C. MANCOZEB Fore, Dithane M-45  
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3) 24 until dry
  COMMENTS: For pythium blight control but not pythium root rot. Dithane M-45 registered for use on sod farms only.
D. MEFENOXAM Subdue Maxx, Apron  
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4) 48 Subdue Maxx: until dry
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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