Sclerotinia homoeocarpa
Dollar spot of turfgrass

David J. Lee

North Carolina State University
Plant Pathology 728 Class Project


Kentucky bluegrass cultivar differences to dollar spot. Dollar spot lesions on Kentucky bluegrass leaf.

Introduction

 

Dollar spot is a major pathogen of turfgrass worldwide. The dollar spot pathogen has a wide host range and infects both warm and cool-season grasses. Infections generally result in unsightly lesions and small patches that rarely jeopardize the overall health of the turf area , but can distract greatly from aesthetics of the turf.


Identification

 

Sclerotinia homoeocarpa is the traditional name of the fungus but taxonomists have disagreed on the classification of the organism because members of the Sclerotinia genus typically produce a sclerotium. However, this fungus produces a flat stroma. Correct classification of the dollar spot fungus is not possible without a fertile teleomorph. Biochemical and immunological studies conducted on infertile apothecia from isolates grown in pure culture indicate that this fungus belongs to either the genus Lanzia or Moellerodiscus.

The fungus grows well on potato-dextrose agar at a temperature range of 20 to 30 C. It can be identified by a rapidly growing fluffy white mycelium. Colors of the mycelium range from green, gray, yellow, to brown depending on isolate age and substrate. The fungus will produce a thick black stroma approximently 3 weeks after isolation on nutrient medium.


 

Dollar spot on creeping bentgrass.
(click on image to enlarge)
Dollar spot mycelium in Kentucky bluegrass.
(click on image to enlarge)

Symptoms

Dollar spot symptoms begin as yellow green blotches on the leaves and can appear water-soaked. The lesions then become straw-like tan with reddish-brown margins. The area between the margins on the leaf often becomes constricted giving the infected leaf a hour-glass appearance. Lesions can blight the entire leaf which may result in the desiccation of the infected tissue. Symptom appearance is often dependent on the management practices of the turf area.

On closely mowed turf (<1/2 inch), symptoms begin as small, sunken, circular patches of blighted turf (approximently the size of a silver dollar). These patches can enlarge to 2 to 3 inches in diameter and in heavily infested areas can coalesce into large, irregular patches. During heavy dew periods it is not uncommon to see white, cottony mycelium masses on affected areas.

In turf with higher mowing heights (athletic fields, home lawns, parks, and golf course fairways) blighted areas are irregularly shaped with diameters ranging from six inches to twelve feet. Lesion appearance is consistent regardless of mowing height but because of the size of the blighted areas in tall turf dollar spot damage may be confused with drought injury, fertilizer burn, or other abiotic factors that hinder optimal growth.


Pathogen Life Cycle

The dollar spot fungi overwinter as dormant mycelium and stroma in infected plant tissue and grow in response to favorable environmental conditions. Favorable conditions for disease include warm humid days, cool nights, heavy dews, and dry soils that are low in nitrogen.

Plant to plant spread can occur via hypha extension from one leaf blade to adjacent leaves. This method of dispersal usually occurs in high humidity environments. Primary dispersal of mycelium and stroma occur through movement of infected leaf tissue by equipment, wind, soil, people, and animals. The fungus enters susceptible plants through stomates, leaf tips that have recently been mowed, and by direct penetration of intact leaf tissue.


Control

Cultural

Cultural practices can create environments that do not favor pathogen development but fungicide use is necessary in severe outbreaks or in high value turf areas. Soil moisture should be maintained close to field capacity with deep, infrequent watering. Irrigation should be done in the morning to minimize the duration of leaf wetness and to remove dew and leaf guttation. Turf areas should be mowed at recommended heights, which tends to remove necrotic lesions that the fungus uses for nutrition and leaf-to-leaf spread. Mowing should also be done during the early morning hours to remove dew and guttation. Pruning surrounding trees and shrubs and the installation of fans can increase air movement and reduce periods of leaf wetness. Maintaining adequate plant nutritional levels will help promote plant health and minimize disease damage. Aerification, verticutting, and regular topdressing programs all reduce thatch layers that may reduce dollar spot severity.

Fungicidal control of dollar spot on creeping bentgrass.
Dollar spot resistance to benzimidazole fungicide.

Chemical

There are a wide variety of fungicides that provide adequate dollar spot control. Fungicide resistance has been reported for many of these fungicides. Some of the fungicides that have been used for control are: tridimefon, anilanzine, propiconazole, cyproconazole, thiophanate-methyl, benomyl, iprodione, fenarimol, and chlorothalonil.

Effective fungicide control is dependent on many local variables (environment, turf species, turf use, pathogen resistance). Thus, control decisions need to account for these factors and others when developing effective management strategies.


References

Alexopoulos, C.J., C.W. Mims, and M. Blackwell. 1996. Introductory Mycology. 4th Ed. John Witey & Sons.

Couch, H.B. 1995. Diseases of Turfgrasses. Krieger Publishing Co. Malabar, Fl. pp 65-69.

Smiley, R. 1983. Dollar Spot. Compendium of Turfgrass Diseases. APS Press, St. Paul, Minnesota.

Turgeon, A.J. 1991. ‘Disease Control’ In Turfgrass Management. Prentice Hall, Englewood Cliffs, NJ pp262.


Acknowledgement

All images courtesy of Dr. Lane P. Tredway, Turfgrass Pathologist, North Carolina State University.


David J. Lee

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