Summer patch is a root disease that affects cool-season turfgrasses, primarily Poa and Festuca species. It was first identified as a separate disease in 1984, previously being a component of the Fusarium blight disease complex. Summer patch has since been recognized as a disease in Agrostis species, particularly in putting greens in transition zones or subtropical climates.

Credit: PACE Turf

Causal agents

Magnaporthiopsis poae

Primary Hosts

Poa and Festuca spp. and to a lesser extent, Agrostis stolonifera.

Disease Cycle

Magnaporthiopsis spp. are believed to survive the winter as mycelium in previously colonized plant debris and in perennial host tissue. Root infection commences in the spring when soil temperatures reach 18 to 21°C (64 to 70°F). The fungus moves from plant to plant by growth of ectotrophic hyphae along the surfaces of roots, rhizomes, and stolons. Symptoms develop during hot, rainy weather or when high temperatures follow periods of heavy rainfall. Patches may continue to expand through the summer and early autumn. As in many diseases caused by ectotrophic root-infecting fungi, infection centers are perennial and may increase by 8 to 20 cm in diameter per year. M. poae may be spread by aerification and dethatching equipment or by the transport of infested sod or other plant material. The role of ascospores or conidia in dissemination is unknown because they are rarely observed in nature.

Epidemiology

Summer patch, caused by Magnaporthiopsis spp., thrives in temperatures of 25 to 30°C (77 to 86°F). Root colonization and infection occur when soil temperatures reach 18 to 21°C (64 to 70°F) in spring and continue throughout summer. Wet soil conditions from poor drainage, rainfall, or excessive irrigation promote infection. Symptoms emerge in mid to late summer due to heat stress, with temperatures of 28 to 35°C (82 to 95°F) for Poa pratensis and 30 to 40°C (86 to 104°F) for Agrostis spp.

Factors like soil compaction, drought, low fertility, and mechanical wear contribute to disease severity. Sunny, exposed, poorly drained, compacted areas are more susceptible during hot, wet years. Low mowing height and frequent light irrigation increase severity. High soil pH, especially from lime application, favors the disease but can be counteracted with acidifying fertilizers. Summer patch is common in turf older than 2 years.

Symptoms

Summer patch in turfgrass manifests with various symptoms that can help identify and diagnose the disease. Typically, this disease is most prevalent in turfgrass stands that are three years old or older. The initial signs of infection are small centers, measuring around 3 to 8 cm (1 to 3 in) in diameter, which gradually expand over multiple seasons, reaching 30 cm (12 in) or larger. During the late spring to early summer, mild symptoms start to emerge as patches or rings of slow-growing, thinned, or wilted turf become noticeable.

As heat and other stresses accumulate, the symptoms of summer patch intensify. The foliage rapidly undergoes a dramatic color change, turning from a healthy green to shades of orange and reddish brown. Eventually, the affected turf collapses and becomes tan or straw-colored. Along the outer edges of the infection centers, the turf retains its orange to reddish-brown appearance, indicating active infection as the pathogen spreads outward in a radial fashion. These distinct patches may coalesce under severe infection, forming large, irregularly shaped areas of turf that resemble damage caused by heat stress, insect infestations, anthracnose, or other diseases.

It is worth noting that creeping bentgrass exhibits tolerance to summer patch in temperate climates. They often colonize the areas where Poa spp. (bluegrasses) have been killed or severely damaged by the disease. In subtropical or tropical climates where Agrostis spp. are subjected to prolonged heat stress, summer patch is now recognized as a  problem that may have been previously misdiagnosed as take-all patch or other diseases. In these regions, circular patches, typically measuring 15 to 30 cm (8 to 12 in) in diameter, initially display signs of wilting and chlorosis followed by orange dieback. Necrosis of roots and stolons can be observed, leading to desiccation and thinning of the affected turf.

Credit: John Kaminski/Penn State University

Signs

The signs of the pathogen associated with summer patch include the presence of dark brown to black ectotrophic hyphae, which form a sparse network on infected roots, rhizomes, or stolons. These hyphae are typically 2 to 5 μm wide. Hyphopodia, darkly pigmented and globose structures, are also observed, ranging from 6 to 12 μm in diameter. These hyphopodia are formed singly or in aggregates known as “growth cessation structures.”

As the infection progresses, extensive vascular discoloration and cortical rot occur, resulting in dark brown to black roots, rhizomes, and crowns. No perithecia (sexual fruiting bodies) have been observed under field conditions.

 

Credit: John Kaminski/Penn State University

Figure 13.7.2: Signs of Magnaporthiopsis poae include (a-c) various shaped growth cessations and (d) dark ectotrophic hyphae on the roots. Although the pathogen does produce perithecia, they are rarely found in nature.

 

Management

In the management of summer patch, it is crucial to adopt an integrated approach that combines cultural practices and preventive fungicide applications. This approach aims to suppress pathogen growth and minimize stress on the turfgrass, promoting healthy and resilient turf. By implementing specific cultural practices, such as maintaining proper soil pH, alleviating stress factors, and promoting root development, you can effectively reduce the severity of summer patch.

 

Cultural

Maintaining soil pH around 6.0 using acidifying fertilizers is recommended, while lime should be applied sparingly based on soil test results and using coarse grades to avoid rapid pH increase. Cultural practices that alleviate stress and promote root development, can help reduce disease severity. Interventions include the following:

• proper mowing heights
• aerification (in the absence of foliar symptoms)
• >improved drainage
• reduced compaction
• syringing to reduce heat stress

 

Chemical

To effectively control summer patch, preventive applications should begin when the soil temperature at a 2-inch depth reaches at least 65°F (18°C) at 2PM for three to four consecutive days.

DMI fungicides at summer patch rates should be applied no later than early-June to minimize the risk of excessive turf growth regulation and phytotoxicity.

Putting greens treated with growth-regulating DMI fungicides may suffer from increased algae infestations in the summer due to reduced turf density. To enhance the effectiveness of fungicides on putting greens, consider implementing a foliar “spoon-feeding” program from June through August.

For preventive treatments, DMIs (e.g., propiconazole) and QoIs (e.g., azoxystrobin) are recommended, along with the combination of azoxystrobin + difenconazole (Briskway). Thiophanate-methyl has shown good control as a curative treatment, while triadimefon has demonstrated good efficacy only as a preventive measure.

When spraying, fungicides (particularly thiophanate-methyl) should be applied in at least 5 gal of water/1,000 sq ft or washed into the root zone with 0.1-0.125 inch of irrigation before drying. If using granular products, apply them to dry turf and then irrigate into the thatch. Avoid repeated use of chlorothalonil during mid- to late-summer, as it may enhance symptom development of summer patch.