You Are Here: Management Updates 2006 Archive August 2, 2006

Date: August 2, 2006
Category: Diseases
Subject: Hot Weather Diseases & Disorders

The recent hot, humid weather and warm nights has resulted in the hot weather diseases of Summer Patch, Brown Patch and Pythium Blight. Many specimens coming into the UMass Extension Plant Diagnostic Lab are suffering less from disease than from adverse environmental conditions such as heat stress and “wet wilt”; as well as physiological syndromes like Summer Bentgrass Decline.

Summer patch caused by Magnaporthe poae, is a serious disease of bluegrass and to lesser extent, also attacks fine fescues. It first appears in the warm weather of summer, usually following heavy rain or irrigation. The other factor for symptom development is a soil temperature above 70˚ F. The initial infection occurs in the spring and only outer cortical tissue is infected. High temperatures and excess water push oxygen out of the soil, weakens the roots of the grass plant, and allows invasion into the vascular tissue where the fungus interferes with water and nutrient uptake as well as movement of sugars from the foliage to roots. Adequate levels of nitrogen are essential for managing summer patch; without proper levels of nitrogen, fungicide treatments will be ineffective. To manage summer patch, apply one half pound nitrogen per 1000 square feet every three weeks; slow release forms of nitrogen (TurfRestore™) are more effective than quick release forms. Where possible, maintain soil pH of 5.5-6.0 to reduce disease severity. Annually, apply manganese sulfate at a rate of 2 lb/acre in the spring. Aerifying, improving drainage, raising mowing height, and reducing compaction will reduce disease.Water deeply and infrequently. DMI fungicides such as propiconazole (Banner Maxx™) should be applied before symptom development occurs (when soil temperatures reach 65˚ F). Once symptoms have occurred, thiophanate methyl (Cleary's 3336™) and strobilurin (Heritage™) fungicides manage summer patch best although some turf loss will be encountered. Repeat application every four weeks.

Brown Patch caused by Rhizoctonia solani occurs in highly damaging form during high temperatures and high humidity of summer months. Large areas can become blighted very rapidly. Dense, highly fertilized, and highly watered turf is most susceptible. Rhizoctonia is a natural inhabitant of the soil and thatch. When soil temperatures reach 60˚ F, the fungus begins to grow, but does not parasitize the grass foliage until the air temperature rises into the mid 80s with high humidity and warm nights. Disease development begins only after the grass plants have suffered heat stress and high temperature growth stoppage. If the weather stays hot and humid, a considerable amount of grass may be lost and recovery will not occur until cooler weather. Excess nitrogen enhances disease frequency and severity; avoid high nitrogen levels, but maintain adequate potassium and phosphorous to enhance plant resistance. Good surface and subsurface drainage is essential to reduce humidity within the turf canopy; leaf wetness can be minimized by manual removal of dew and pruning of shrubs and trees to increase air flow. Reduce thatch by mechanical means if it is excessive. Fungicides registered for Rhizoctonia include azoxystrobin (Heritage™), chlorothalonil (Daconil™), thiophanate methyl (Cleary’s 3336™), flutonil (ProStar™), vinclozin (Touche™) and propiconazole (Banner Maxx™).

Pythium blight - Pythium is a natural inhabitant of the soil and requires only the proper conditions to become pathogenic. The disease is favored by high temperatures, high soil moisture, and high humidity. If conditions remain conducive, large areas of turf can be lost in as little as 24 hours. Good soil drainage is essential for the effective management of Pythium. Heavy watering will increase the severity of the disease. Light watering at sunrise and syringing at midday will help to manage Pythium blight; nighttime watering should be avoided altogether. Improve air circulation by pruning trees and shrubs. High nitrogen levels make Pythium blight worse. Apply mefenoxam (Subdue Maxx™), propamocarb (Banol™), or fosetyl-aluminum (Signature™) on a 10-14 day schedule when disease pressure is high. To avoid the development of fungicide resistance and maximize fungicide applications, alternate active ingredients or apply different fungicides as a tank mix.

Summer Bentgrass Decline (SBD) is a physiological syndrome evidenced by thinning of the turf canopy, yellowing of leaves, and death of roots. The cause has been attributed to many factors including high soil temperature, excessive or deficit soil moisture, poor soil aeration, and excessive accumulation of organic matter in USGA sand greens. The reductions of soil temperature by as little as 5˚ F improves creeping bentgrass quality as well as shoot and root growth. Various methods including fans, syringing, and subsurface cooling systems can be utilized to reduce soil temperatures. Excessive accumulation of organic matter results in a decrease in larger soil pores (aeration pores) and results in oxygen stress. Low soil oxygen leads to reduced water uptake, stomatal closure, and direct high temperature kill. Under hot, humid weather, root death occurs rapidly and the fresh, dead roots plug macropores and reduce water infiltration. The remaining live, oxygen stressed roots cannot obtain enough water uptake for transpirational cooling. Reduce organic matter accumulation by twice annual core aeration followed by heavy topdressing.

Submitted by: M. Bess Dicklow

DISCLAIMER - As always, it is the responsibility of the applicator to verify the registration status of any pesticide BEFORE applying it. Different states have different regulations as well. The author and the University of Massachusetts are not liable for any consequences of any pesticide "recommendations". Mention of any trade name is not to be considered endorsement of a product.

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