Summer Patch

Summer Patch is a destructive, warm weather disease problem of annual bluegrass, Kentucky bluegrass and fine fescues throughout their ranges. The disease was originally identified as Fusarium Blight Syndrome (FBS) in the 1960’s. Reports of inconsistent control from chemical applications and a wide range of environmental conditions associated with FBS led researchers to investigate further. Research in the early 80’s was able to determine that there were at least two distinct diseases that had been considered FBS: Summer Patch caused by Magnaporthe poae and Necrotic Ring Spot caused by Ophiosphaerella korrae.

Disease Cycle And Symptoms
Summer Patch, caused by the soil borne fungus Magnaporthe poae, attacks the roots, crowns and rhizomes of bluegrass and fine fescue. There have also been reports of M. poae parasitizing bentgrass in the southern US. Classified as an ectotrophic root pathogen, M. poae produces dark runner hyphae that colonize the surface of turf roots. Colonization of new turfgrass roots, crowns and rhizomes begins as soil temperatures reach 65-70°F in the spring. Initially, the damage caused by the pathogen is not noticeable because the death and dysfunction of infected roots is equaled by new root growth. However, as soil temperatures increase, the balance between host and pathogen shifts dramatically. Cool season turfgrass roots reach their optimal growth between 55-65°F. M. poae reaches its optimal growth between 82-86°F, i.e. as turfgrass root production slows down, pathogen growth potential increases. Research has shown that the fungus is capable of growing and spreading from plant to plant at a rate of 1-3 cm per week. However, symptoms do not typically appear until periods of hot (86-95°F), rainy weather or when high temperatures follow periods of heavy rainfall. The ectrotrophic growth habit means that as soon as the turfgrass plant becomes stressed (by high temperatures or other factors), the pathogen is able to quickly initiate multiple infections and overwhelm the host defenses. The development of Summer Patch symptoms therefore is twofold: 1) high soil temperatures that favor the pathogen and 2) a stressed host.

The expression of disease is directly related to infection severity and turfgrass vigor. In fairways and landscape turf situations, Summer Patch infections begin as gray green, wilted patches that change to a straw color as the leaf tissue dies. Patches are usually 4-6 inches in diameter and can reach 12 inches. The centers of the patches are often recolonized by either the same turf species or by a weed species producing the typical ‘frog-eye’ patch associated with this disease. Individual patches often coalesce to form large, irregular areas of damaged turf. On golf greens with high populations of annual bluegrass, Summer Patch normally appears as small, reddish brown patches, 2-3 inches in diameter. As stress increases, these patches may also coalesce. There are no distinctive leaf lesions associated with Summer Patch, but an examination of the root and crown tissue of the diseased plants reveals black, necrotic tissue. Because of the visual similarity with symptoms of other diseases such as Fusarium blight, Necrotic Ring Spot, Rhizoctonia blight, and Pythium blight, the only positive method of disease diagnosis is laboratory analysis.

Cultural And Chemical Controls
Summer patch is a perennial disease meaning that once established, patches often reappear in successive years in the same area. Because Summer Patch is a root disease, cultural practices that alleviate plant stress and promote root development will reduce overall disease expression and severity. Common agronomic practices to reduce plant stress and Summer Patch symptom expression are:

1. Avoid mowing below recommended heights, especially during period of heat stress.
2. Fertilize with ammonium sulfate or sulfur coated urea fertilizers at rates adequate to maintain proper growth. Research has shown that these fertilizer sources have an acidifying effect on the soil root zone that is antagonistic to the Summer Patch pathogen.
3. Irrigate deeply enough to wet the entire root zone.
4. Reduce soil compaction and improve drainage through aeration. Aeration can potentially spread the pathogen, so be sure to remove the plugs.
5. In historical areas of expression, maintain soil pH levels below 6.0. Research has shown that lower soil pH values can reduce the growth and spread of the pathogen.
6. Overseed susceptible turfgrass with non-susceptible hosts such as bentgrass, perennial ryegrass or tall fescue to reduce disease potential.

Fungicide efficacy and application timing studies have yielded valuable information. Because infection occurs well in advance of symptom expression, preventive applications should begin in late spring when soil temperatures approach 65°F at a two inch depth. Fungicide treatments should be applied two to three times at 21-28 day intervals to areas with a history of Summer Patch infection. Research has also indicated that increased levels of efficacy can be achieved when the applications are applied with enough water to insure complete root zone coverage. Application volumes or supplemental irrigation should be controlled to match the known drainage capacity of the site. Care should be taken not to move the fungicide treatment below the active root zone.

Conclusions

Research has shown (see below) that 3336 is an effective chemical option for the management and control of Summer Patch. For prevention, two to three treatments should be applied at 21-28 day intervals beginning when the soil temperatures reach 65°F. Cleary’s 3336 or 3336 PLUS can be utilized in a Summer Patch control program in rotation with fungicides with different modes of action to deliver effective control while minimizing the risk of developing fungicide resistance. Fungicides belonging to the chemical classes known as 1) benzimidazoles (3336 & 3336 Plus); 2) demethylation inhibitors (DMI’s); and 3) strobilurins have all been shown to be effective in Summer Patch control programs.

Magnaporthe poae is a soil-borne, ectotrophic, root pathogen of certain cool season grasses. It is capable of overwintering in infected host tissue and causing annual outbreaks of disease. Management of cultural practices that reduce plant stress is essential to any Summer Patch control program. Reducing soil pH, reducing compaction and raising mowing heights during periods of stress are critical. Preventative applications of 3336 or 3336 PLUS are a proven control technique. 3336 also provides the best curative response, effectively stopping the disease and allowing recovery of the damaged turf.