PITHIUM BLIGHT

• Pathogen:  Pithium aphanidermatum, P. arrhenomanes, P. graminicola, P. myriotylum, P. ultimum.
• Grasses Affected:  Annual bluegrass (Poa annua), Kentucky bluegrass (Poa pratensis), roughstalk bluegrass (Poa trivialis), bermudagrass (Cynodon dactylon), bentgrasses (Agrostis spp.), tall fescue (Festuca arundinacea), red rescue (Festuca rubra), annual ryegrass (Lolium multiforum), perennial ryegrass (Lolium perenne), St. Augustine grass (Stenotaphrum secundatum).
• Season of Occurrence:  Late spring, summer and early fall.
• Symptoms and Signs:  In an overall view, Pythium blight is first seen as small, irregularly shaped purplish areas ranging from 1 to 4 inches in diameter.  The individual leaves in these patches have a dark, water-soaked appearance.  As colonization by the fungus progresses, they become soft and slimy, and when they are in contact with each other, they mat together.  In the early morning hours, or if conditions of high humidity exist throughout the day, the leaves of diseased plants may be covered with the white, cobwebby, mycelium of the pathogen.  Also during these times the older patches often develop dark purplish borders up to 1 inch wide.  The color of the affected leaves soon changes to light brown or reddish brown and they become dry and shriveled.    In the event the growth of the pathogen is checked before the entire leaf is colonized, distinct straw-colored lesions of varying size will develop.  Generally, these lesions will not have distinct between healthy and diseased tissue, in contrast to Sclerotinia dollar spot or Rhizoctonia blight.  Blighting of the foliage within the developing patches may be uniform or the affected areas may develop as frogeyes – circles of blighted grass with centers of green, apparently healthy plants.  Individual patches of affected grass frequently coalesce to envelop sections of turf ranging from 1 to 10 feet in diameter.  At times the affected areas may develop as elongate streaks.  Development of this pattern of blighting is apparently the result of the pathogen being washed over the surface of the soil or “tracked” by mowers.
• Conditions Favoring Disease Development:  The pathogen is spread to new locations by transport of diseased leaves on mowing and cultivation equipment and walkers’ shoes, and fungal spores and diseased leaf fragments in flowing surface water.  The highest frequency of infections occur during extended periods of leaf wetness brought on by either rainfall, high atmospheric humidity in the leaf zone, dew formation, or night and morning ground fogs. Weather conditions that normally precede outbreaks of severe Pithium blight are (a) daytime air temperatures of 86°F or greater and (b) nighttime air temperatures of 68°F or above in combination with 15 or more consecutive hours in which the relative humidity is 90 percent or higher.
• Control (1) Cultural Practices – Maintain satisfactory, but not luxuriant plant growth through the use of balanced fertilizer applications.  Also decrease the length of time the leaves are wet by poling or dragging a water hose across the turf in the early morning.  Duration of the periods of daily leaf wetness can also be reduced by 2 to 4 hours by programming a nighttime watering schedule in which the irrigation begins at least 3 hours after sunset and is completed before sunrise.  (2) Call Empire Tree and Turf for a free estimate for a fungicidal program for your turf grass.