Leaf Spot and Melting Out
Leaf Spot and Melting Out
Leaf spot and melting out are two fungal diseases of turfgrass. They are found within the “Helminthosporium” leaf, crown, and root disease complex. The leaf spot pathogen, Bipolaris sorokiniana, attacks bluegrasses, bentgrasses, ryegrasses, and fescues. Melting out, cause by Drechslera poae, is the most serious disease of Kentucky bluegrass and also occurs on ryegrasses and fescues. B. sorokiniana is a warm weather pathogen and D. poae is a cool weather pathogen.
Both diseases are favored by dry periods alternating with prolonged cloudy, wet weather. Leaf spot is most active at temperatures between 70°F and 85°F. Optimum temperatures for melting out are 65°F to 75°F. Under these conditions, both pathogens produce tremendous numbers of spores on plant debris. These spores are spread to new growth by various activities. This includes wind, mowers and other turf equipment, splashing water, foot traffic, dragging hoses, and infected grass clippings. The spores germinate when they contact water droplets on the leaf. The fungus can enter the leaf within hours. When symptoms appear a new crop of spores can be produced within 7 – 10 days.
A variety of leaf spot symptoms accompany the stages of disease development. Early symptoms are small, dark-purple to black spots on the leaf blade. Older symptoms are round to oval spots with buff-colored centers surrounded by a dark-brown to dark-purple margin. Melting out starts out as black to purple spots on the leaf blades and sheaths. Infected leaf sheaths turn a uniform dark chocolate brown, causing leaves to yellow and then drop from the plant. From a distance, affected turf appears yellow and thin.
Symptoms on bentgrasses differ from those on other hosts. Infection of bentgrass golf greens gives a smoky blue cast to the turf. This blue cast progresses to a yellowing and finally complete blighting of the leaves and thinning of the turf. The affected area conforms to a definite pattern with distinct margins. Leaves within the affected area are water-soaked and matted. On bentgrass fairways initial symptoms are yellow flecks on the leaves that develop into small oval lesions and then into irregular water-soaked blotches.
Prevention and Treatment
The most effective preventive for leaf spot and melting out combines the use of improved cultivars with good turfgrass management practices and fungicide sprays. Only improved disease resistant cultivars should be used for new turf establishment or renovation. Good cultural practices include a fertilization program that does not stimulate lush growth, thatch management, and watering in the early morning. It is always best to mow at a frequency adjusted to the growth of grass. When necessary, apply a fungicide beginning in April, followed by two or three additional application spaced three to four weeks apart.
J. E. Watkins. (1997). Diseases of Cool Season Turfgrass. In F. Baxendale, Ph.D., & R. Gaussoin, Ph.D., Integrated Turfgrass Management for the Northern Great Plains (pp. 120-122). Location: Nebraska
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