Southern turfgrass managers are all too familiar with Gray Leaf Spot, a potentially devastating disease of St. Augustine grass and bermudagrass. In recent years, northern turf managers have become familiar with it as well, in particular on their tall fescue and perennial ryegrass turf. Long confused with brown patch in southern climates, northern outbreaks of gray leaf spot (affecting a broad area from Virginia to Ohio to New Jersey) seem to be more easily confused with drought.

Gray Leaf Spot affects a wide range of warm and cool season grasses. Its primary warm season hosts are St. Augustine grass and bermudagrass, while its primary cool season hosts are tall fescue and ryegrass. The pathogen, Pyricularia grisea, is a widely distributed fungus that survives inactive periods as dormant mycelium and conidia on plant litter and infected lower leaves.

Gray Leaf Spot occurs during moderate to warm weather (80–90 F) that is accompanied by periods of prolonged leaf wetness. When environmental conditions are conductive to pathogen growth, P. grisea sporulates profusely and the resulting conidia are easily disseminated by wind, water and machinery. Conidia germinate on host foliage when relative humidity is high and leaf surfaces are moist. Initial infection of newly developing leaves occurs with early summer rains as daily air temperatures pass 80 F. As the temperature increases and there are increasing periods of high humidity, there is a progressive build up of disease. Leaf lesions begin as olive green to brown, water soaked, pinhead spots that enlarge to form larger spots than a general lesion. Development of the pathogen to epidemic proportions, requires a minimal period of 24 hours of continual leaf wetness and air temperatures above 85 F. Under these climatic conditions, spores germinate rapidly in the free water on the leaf surface. Eight hours after germination, germ tube penetration of the epidermal cells has occurred and active parasitism has begun. Rapid movement of the mycelium through the leaf structures is both inter- and intra- cellular, creating visible lesions within five days.

The explosion of disease that is often noted with this pathogen, is associated not only with the ability of conidia to rapidly infect host tissue under optimal conditions, but also the secondary disease cycles that are capable of producing very large numbers of conidia. Initial infection by gray leaf spot likely occurs during the early part of the summer, but the infection levels remain low enough that turf managers do not notice. These initial infections are capable of producing conidia that infect new plants, produce more conidia and infect still more plants. The ‘explosion’ of disease is associated with that point in time when the secondary infection cycles coincide with optimal disease conditions and the potential for this pathogen to reproduce quickly. Gray Leaf Spot has been shown to be particularly severe on newly seeded perennial ryegrass when the combination of juvenile turf and profuse conidial production coincide during late summer seeding.

Prevention In Key Areas
The rapid development and severe damage caused by this pathogen demand that the turf manager take appropriate steps to monitor and prevent disease expression. Once the pathogen’s biology is understood, there are several key cultural and chemical practices that can be used to reduce disease expression.
1. Avoid applications of high rates of nitrogen fertilizer, especially quick-release, water–soluble forms, during wet summer months.
2. Modify irrigation practices to decrease the length of time the leaves remain wet and to avoid drought stress.
3. Correct management practices that create drought, compaction and herbicide related (e.g. spring applications of ethofumesate) turf stress. Grasses stressed by these factors may be at higher risk for severe damage.
4. Plant resistant cultivars when possible. Turfgrass breeding programs have made significant advances in breeding for resistance to this disease. Check with your local extension service for current recommendations.
5. Plan an integrated approach with a sound fungicide program.

Chemical Control Of Gray Leaf Spot
Research into the chemical control of this pathogen is ongoing. Disease control trials indicate that the most effective fungicides for the control of this disease are thiophanate methyl (3336®, 3336 Plus®), azoxystrobin and trifloxystrobin. DMI fungicides (propiconazole and triadimefon) are not as effective in controlling this disease, especially when disease pressure is high. The contact fungicides mancozeb (Protect DF®) and chlorothalonil are also labeled for the control of this disease. Although these contact fungicides will not provide acceptable levels of control on their own, they make excellent tank mix partners with the above-mentioned systemic fungicides.

Recent research conducted at North Carolina State University (Figure 1), determined that the use of 3336, an acropetal systemic fungicide, provided acceptable control of this disease when applied curatively. The initial fungicide application was made during a natural disease outbreak on August 24. Subsequent applications were made at 28 day intervals.

Figure 1

3336 For Prevention of Gray Leaf Spot

Pyricularia grisea is aggressive and highly pathogenic to warm and cool season grasses under specific conditions. Climatic monitoring and preventive treatments with 3336 provide excellent control of Gray Leaf Spot (Figure 2). The superintendent and turf manager now have another reason to use 3336 in the management and protection of their turf.

back to newsletter