Large patch, caused by the fungus Rhizoctonia solani, was previously known as “zoysia patch” due to its common occurrence in Zoysia spp. However, it can also affect other warm-season turfgrass species such as Buchloë dactyloides, Cynodon dactylon and hybrids, Eremochloa ophiuroides, Pennisetum clandestinum, Stenotaphrum secundatum, and Paspalum vaginatum. The disease symptoms can be severe and are characterized by the formation of large, circular patches in the turf.
Causal agents
Rhizoctonia solani anastomosis group (AG) 2-2 L
Primary Hosts
All warm-season turfgrass species are susceptible, but this is considered the primary disease of zoysiagrass.
Disease Cycle
Rhizoctonia solani AG 2-2 LP, the causal agent of large patch disease, survives unfavorable periods by forming sclerotia, monilioid cells, or thick-walled mycelia in plant debris. It can be found in sheath tissues, healthy plants, and rhizosphere soil, suggesting its ubiquity in the phyllosphere and its capability as a soil saprophyte. The primary mode of infection is through direct penetration of leaf sheaths and stem bases, although entry through wounds, lenticels, and stomata is also possible. Hyphae accumulate along these plant parts, forming infection cushions with penetration pegs that penetrate plant cells. During the infection process, R. solani produces various secondary metabolites that contribute to tissue maceration. The specific details of root infection by R. solani AG 2-2 LP on warm-season turfgrasses are not well-documented.
Epidemiology
Large patch disease is most commonly observed during spring and fall when temperatures range from 20-25°C (68-77°F). These conditions are optimal for pathogen infection and sheath blighting in warm-season grasses, particularly Zoysia spp. The disease is less prevalent during the summer when the grasses are actively growing. Factors such as persistent rainfall, high humidity, cloud cover, abundant precipitation, and poor drainage contribute to the development and spread of large patch symptoms. In ideal conditions, the patches can merge and form irregularly shaped areas of blighted turf. Poor air circulation also increases the likelihood of extensive damage.
Symptoms
Symptoms of large patch appear during cool periods when warm-season grasses are transitioning into or out of dormancy in autumn or spring. Patches can expand and reach a diameter of 8 meters (26 feet) or more if left untreated. Affected tillers at the patch margins display yellow, orange, or reddish-brown discoloration, particularly noticeable with dew. In autumn, ring-like or patchy areas with interspersed affected tillers among healthy ones are observed. In spring, reduced tiller density and slower leaf growth in patchy areas become apparent. Leaf lesions are rare, but tip dieback may occur. Discolored roots, water-soaked lesions on sheath tissue, and basal rot with easily detached shoots are common.
Credit: John Kaminski/Penn State University
Signs
Signs of the pathogen generally include dark brown mycelium on infected sheath bases and stolons. Hyphae exhibit the typical right-angled branching and constriction near the septa of other Rhizoctonia spp. No clamp connections are found and sclerotia typically do not develop in nature.
Management
To effectively manage large patch disease, a comprehensive approach that combines cultural practices and fungicide applications is necessary. Cultural management practices focus on creating unfavorable conditions for disease development, such as optimizing nitrogen fertilization, improving water drainage, enhancing air circulation, and implementing proper mowing and thatch management. Chemical control through the use of fungicides containing specific active ingredients can provide effective control of large patch. Multiple applications, particularly in the fall and spring, along with adequate spray volume, enhance control efficacy. Early spring applications, timed with soil temperature averages, can effectively prevent large patch outbreaks.
Cultural
Management of large patch disease includes various cultural practices and fungicide applications. Nitrogen fertilization during spring and summer does not worsen the disease, and a small application of nitrogen in the spring can stimulate grass growth and reduce disease severity. Adequate surface and subsurface water drainage is essential to minimize excessive soil wetness and humidity. Removing dew from leaf surfaces and improving air circulation through selective pruning and removal of barriers can also help manage the disease. Mowing heights should be raised above 1.2-2.5 cm, and thatch should be mechanically removed if it exceeds 1.2 cm in depth. Aerification and verticutting can be performed during the active growth period of warm-season grasses. Currently, commercially available turfgrass cultivars with resistance or improved tolerance to large patch are limited.
Chemical
Large patch control requires at least one fungicide application in autumn on high-pressure sites with a history of the disease. Recent observations show the disease can become active as early as the first week of August. Preventive fungicide use is best for affected areas, with treatments in late-August to early-September, followed by potential additional applications 4-6 weeks later and/or in early spring under high disease pressure. Curative applications in autumn may show benefits by mid-spring, providing some disease control for the following season.
Zoysiagrass fairways may require re-treatment in early spring, especially with wet weather. Spring re-treatment is more valuable in areas with a risk of bermudagrass encroachment. For zoysiagrass, apply fungicide when the first sign of active disease appears. Late-spring nitrogen fertilization on bermudagrass can help turf outgrow damage without the need for spring fungicide applications. Use the highest labeled rate of the selected product and apply in a minimum of 2.5 gal of water/1,000 sq ft. No irrigation is needed after application if clippings are not removed. Limited research suggests a premix of azoxystrobin + propiconazole, flutolanil (Prostar, Pedigree), PCNB, select DMIs can be effective.