“Leaf and sheath spot” refers to symptoms in turfgrass caused by Waitea zeae, W. oryzae, and W. prodiga. Although multiple species have been found in association with these symptoms, W. zeae is more commonly isolated from affected turfgrass.

Leaf and sheath spot occurs in cool-season grasses during extreme heat, but rarely causes widespread damage. However, in warm-season turfgrasses, particularly Cynodon spp. used for putting greens, it can be devastating. In cases on warm-season turfgrasses, the disease is referred to as “mini-ring”.

Causal agents

Waitea zeae, W. oryzae, and W. prodiga are documented causes of leaf and sheath spot in turfgrasses. They reproduce mainly through vegetative hyphae, with rare production of asexual or sexual spores. The hyphae have a diameter of 3 to 10 μm, form right-angle branches, and are multinucleate. The macroscopic characteristics of isolate cultures and sclerotia are often used for species identification. W. zeae and W. oryzae mycelia grow below the agar surface, appearing white to salmon-colored on potato dextrose agar. In contrast, W. prodiga forms yellow to pink aerial mycelium on the same medium. Mature sclerotia of W. zeae are orange to dark orange or dark brown, while those of W. oryzae are somewhat angular and salmon-colored. Mature sclerotia of W. prodiga vary in size, shape, and color, ranging from yellow to salmon to brown.

Primary Hosts

Commonly occurs in cool-season grasses, especially during extreme heat. In warm-season turfgrasses, particularly hybrid Cynodon spp., managed as putting greens, this disease can be devastating.

Disease Cycle

The disease cycle of leaf and sheath spot has not been extensively studied. The causal fungi are believed to survive unfavorable environmental conditions as sclerotia embedded in plant tissue or in the soil, as well as mycelia within plant tissue or organic matter. Germinating sclerotia or surviving mycelia are likely the sources of initial infection.

In growth chamber studies, initial infection in Cynodon spp. appears to occur in the root or near the soil surface close to the crown. This coincides with observations of diseased Cynodon spp. turf, where W. zeae mycelium is typically found in the soil-thatch interface or deep within the plant canopy.

Epidemiology

Leaf and sheath spot in both cool-season and warm-season turfgrasses commonly occurs at temperatures ranging from 28 to 36°C (82 to 97°F), with the optimal range being 30 to 32°C (86 to 90°F). These temperatures are higher than the optimal range for cool-season turf, and symptoms typically become visible within this temperature range. For warm-season turfgrass, the ideal temperature for leaf and sheath spot development aligns with the temperature range favorable for turfgrass growth. Therefore, factors other than high temperatures often trigger the onset of disease symptoms. Mechanical damage from agronomic practices, low-light conditions, and the onset of cool weather can contribute to symptom expression, particularly in Cynodon spp. The role of leaf wetness or humidity in the development of leaf and sheath spot is still uncertain and requires further investigation.

Symptoms

Leaf and sheath spot symptoms in warm-season grasses, such as Cynodon spp., appear as small chlorotic or tan spots, which can develop into frog-eye patches. The outer ring of the patch turns bronze, tan, or bleached, while the center remains unaffected. Eremochloa and Stenotaphrum spp. show foliar lesions and basal sheath rot resembling large patch symptoms. Paspalum spp. exhibit chlorosis and necrosis of lower leaf blades. In cool-season grasses like Agrostis and Festuca spp., leaf and sheath spot symptoms resemble brown patch. W. zeae can cause rings or patches of yellow, orange, brown, or gray in Agrostis, while symptoms in Lolium and Festuca spp. are indistinguishable from R. solani.

Credit: Lane Tredway Syngenta

Credit: John Kaminski/Penn State University

Signs

Visible signs of leaf and sheath spot are rarely observed with the naked eye. However, white to cream-colored mycelium may be visible in cool-season turfgrasses under favorable environmental conditions. Mycelia can be seen deep within the turfgrass canopy or thatch layer using a dissecting microscope. Despite the name “leaf and sheath spot,” most of the pathogen activity in Cynodon spp. occurs on stolons and rhizomes. Sclerotia, a diagnostic feature of this disease, are rarely found in turfgrass tissues grown under field conditions. However, when infected Cynodon spp. is grown under controlled conditions optimal for W. zeae, sclerotia have been observed on leaves, sheaths, roots, and near the plant’s crown. On cool-season grasses, sclerotia are often commonly found in field infections (Figure 14.5.2b).

Management

The management of leaf and sheath spot in turfgrass, particularly in cool-season and warm-season varieties, requires a combination of cultural and chemical control strategies. While cultural practices specific to leaf and sheath spot have not been extensively studied, the application of general cultural controls, such as reducing turfgrass stress, managing thatch, and controlling excess moisture, is presumed to be effective in suppressing the disease. However, the availability of resistant cultivars for leaf and sheath spot is limited due to its minor impact on cool-season turfgrass.

In warm-season turf, such as Cynodon spp., managing leaf and sheath spot can be particularly challenging. Symptoms often arise when the turf is not actively growing, making recovery difficult even with the suppression of the causal fungi. Therefore, preventive strategies are crucial in controlling the disease. Fungicides should be applied proactively, before the appearance of leaf and sheath spot symptoms, especially when environmental conditions favor the growth of the causal fungi.

Cultural

Control of leaf and sheath spot in cool-season turfgrass has not been extensively studied. However, cultural controls similar to those used for managing brown patch, such as reducing turfgrass stress, avoiding excessive nitrogen fertilization, managing thatch, and controlling excess moisture during favorable environmental conditions, are presumed to also help suppress leaf and sheath spot. Resistant cultivars for this disease are not well-known due to its minor impact on cool-season turfgrass. Limited research suggests that endophytic fungi may suppress W. zeae in Festuca arundinacea.

Balanced fertility, including adequate nitrogen to promote active growth, is essential for managing leaf and sheath spot in Cynodon spp. Ammonium sulfate has been found to increase the severity of leaf and sheath spot, although the underlying mechanisms are not well understood. Alternative nitrogen sources like urea should be used when leaf and sheath spot is a problem. Minimizing cultivation practices and abiotic stress when the turfgrass is not actively growing is also recommended.

Chemical

In warm-season turf, particularly Cynodon spp., managing leaf and sheath spot can be challenging. Symptoms often manifest when the turf is not actively growing, making recovery difficult even with the suppression of causal fungi. Therefore, preventive strategies are crucial. Fungicides should be applied before the appearance of leaf and sheath spot symptoms when environmental conditions favor the growth of the causal fungi. Fungicides used to control brown patch are generally effective against leaf and sheath spot, except for thiophanate-methyl, which does not have activity against these pathogens.