IPM Identification and Mangement Of Turfgrass Diseases

Author(s)

Large Patch

Pathogen: Rhizoctonia solani
Hosts: Primarily zoysiagrass, but also buffalograss and bermudagrass

Symptoms and signs

Circular, slightly matted areas of straw-yellow zoysiagrass initially develop in late September to early October during cool, rainy weather (Figure 1). During favorable weather, the fungus progressively blights more shoots, resulting in large, blighted patches of turfgrass with bright orange margins (Figure 2). Examination of individual shoots within the patch reveals reddish brown to black lesions at the base of the leaf sheaths, beyond which the leaf blades exhibit a yellow-orange color (Figure 3). The yellow-orange color is due to an accumulation of anthocyanins caused by girdling of the leaf sheath.

The yellow-orange margin is indicative of active disease, and the large, cinnamon-brown colored hyphae of Rhizoctonia solani are most easily observed in the leaf sheaths during this phase (Figure 4). Lesions normally do not develop on the leaves. Depending on disease severity, unaffected, living shoots may be scattered throughout the patch. Patches of blighted turf often exceed 20 feet in diameter.
Sheath infection can continue into November as long as soil temperatures remain above 50°F and soil moisture is high. Patch enlargement after fall dormancy (October) may not be noticed until zoysiagrass resumes spring growth.
Patches of zoysiagrass as well as buffalo-grass damaged by R. solani reappear in spring as light brown, sunken areas that are slower to recover from dormancy than surrounding, healthy turfgrass (Figure 5). Leaf sheath lesions typically are not present in early spring, but sheath rotting may resume in April and May as soil and air temperatures increase. This can result in patch expansion and symptoms similar to those described for fall infection. New patches also may develop in spring. The activity of R. solani usually continues through May but is suppressed by high summer temperatures.
The fungus does not kill stolons and roots within the patch, and new shoots will form in the damaged areas with time. Because infected zoysiagrass is slow to recover, weed invasion in the thinned areas of the patch is common (Figure 6).

Conditions

Large patch symptoms can occur anytime during the growing season, but they are most common in spring and fall as zoysiagrass enters or breaks winter dormancy. The disease occurs on both residential lawns and golf courses, but it is more severe when the grass is mowed to heights of less than one inch (e.g., golf course fairways) and where soil drainage is poor.
Large patch development is favored by heavy thatch. R. solani oversummers in the thatch as bulbils (resting structures). It does not produce spores, but can easily be moved from one location to another on contaminated sod.
The R. solani that infects zoysiagrass belongs to a different subgroup (AG 2-2 LP) and has a lower temperature range (50 to 86°F) for infection than the brown patch fungus that damages cool-season turfgrasses. Therefore, large patch almost always occurs during relatively cool weather in spring and fall, whereas brown patch is a problem on cool-season turfgrasses in midsummer. Large patch development is favored by frequent rains and excessive soil moisture.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.