Ornamental Plants
Annual Reports and Research Reviews
2001

Special Circular 186-02

Summary of Recommended Turfgrass Cultural Practices, Weed Control, and Disease Problems: 2001

Turf Diseases

Necrotic Ring Spot (Leptospharia korrae)

Necrotic ring spot is a disease that causes dead rings and arcs in Kentucky bluegrass, especially when the thatch layer is dense. Although the symptoms do not show until early summer, when the turfgrass is relying heavily upon its roots for moisture, the fungus Leptospharia korrae becomes active in the spring when the soil is cool and moist. It attacks the crown, roots, and stems and spreads into adjacent plants, causing concentric brown patches of dead turf. Under a microscope, black runner hyphae can be found on the roots and crown area, similar to the fungi causing summer patch or take-all.

After affected turfgrass has been killed, resistant cultivars of turfgrass or weeds will gradually grow in the patch, causing a "frog-eye," a round green patch with a brown ring around it. If necrotic ring spot has plagued a lawn in the past, it would be very beneficial to aerify the soil and fertilize the turf to encourage root growth. It is also important to apply a preventive treatment of the fungicide azoxystrobin (e.g., Heritage) in late April and again in 28 days. Over-seed the area with disease-resistant cultivars of Kentucky bluegrass or a different, less susceptible species of grass, such as perennial ryegrass.

Helminthosporium Leaf Spot (Drechslera spp., Bipolaris sorokiniana, and Others)

In 2001, cool, wet conditions throughout May expedited leaf spot disease symptoms in late May on susceptible cultivars of Kentucky bluegrass. The leaf blades became spotted with small, rounded, brown to black lesions. Two fungal pathogens are usually involved with this leaf spot/melting out disease complex. Drechslera poae is active in the cooler, wet months of spring and fall, while Bipolaris sorokiniana is active from 70F to 98F. Both fungi overwinter in dead material and thatch.

Lesions caused by Drechslera poae appear during cooler temperatures. The fungus spreads to new leaves with splashing water. Melting out occurs at warmer temperatures as stems, crowns, and roots become necrotic. Once temperatures rise above 68F, Drechslera poae becomes dormant until cool, wet conditions return. The thinned turf may recover unless it is infected with Bipolaris sorokiniana, which causes spots at temperatures between 70F to 85F. As temperatures rise above 85F, the disease progresses to its melting out stage in which the leaf sheaths, crowns, rhizomes, and roots become blighted and leaves turn straw colored and die. The best control for these diseases is to maintain healthy turfgrass by fertilizing with fertilizer at recommended rates and using improved cultivars of grass seeds that have disease resistance.

Red Thread (Laetisaria fuciformis)

This disease is caused by the fungus, Laetisaria fuciformis, which occurs on susceptible varieties of perennial ryegrass, fine fescue, and some cultivars of Kentucky bluegrass. Under cool (50F to 75F), moist conditions, this disease produces red or pinkish "threads" of mycelium along the cut ends of the leaf blades which, when dried, stick the blades together, forming a brownish mat. As long as surface moisture is present, the disease spreads to adjacent turfgrass, causing brown, withered patches in the lawn. To reduce the severity of the disease, apply regular applications of fertilizer which contain 30% slow release nitrogen and maintain less than 1/2" of thatch. Irrigate only when the turfgrass appears drought stressed and allow leaves to dry before dusk.

Powdery Mildew (Erysiphe graminis)

Powdery mildew, caused by the fungus Erysiphe graminis, was reported on Kentucky bluegrass starting in mid May in 2001. An obligate parasite, this fungus lives on the surface of the leaf blades and inserts haustoria or suction-like mycelium into the leaf to obtain nourishment. As long as temperatures remain between 60F to 70F and humidity is high, the fungus produces copious amounts of spores, causing a white powdery look on infected blades. Although some grass blades may turn yellow and wilt, powdery mildew does not seriously damage turfgrass. Turfgrasses can be managed by pruning trees and shrubs to reduce shade and improve air circulation, and by seeding with improved cultivars resistant to powdery mildew. Fungicides are available for powdery mildew, however, they must be used preventively.

Gray Leaf Spot (Pyricularia grisea) Identified in Ohio

A case of gray leaf spot (Pyricularia grisea) was confirmed in southwestern Ohio on a perennial ryegrass fairway. Symptoms appeared as spreading patches of dark, withered leaf blades, or apparent drought stress that actually worsens when irrigated. The fungal disease is also called "blast" since it scorches the turf, making it appear as if it has been blasted with a torch.

Gray leaf spot occurs most frequently on stands of newly established perennial ryegrass during warm temperatures, when the leaves stay moist for extended periods of time. Excessive rates of fertilizer, compacted soil, and other stresses tend to exacerbate the disease.

Rhizoctonia Brown Patch (Rhizoctonia solani and R. zeae)

The sudden increase in temperatures in mid June, combined with high humidity and heavy dew, created the perfect environment for brown patch. This foliar fungal disease causes rounded, brown, or smoke-colored patches in short-cut turfgrass, but more irregular patches in high-cut Kentucky bluegrass and tall fescue. Closer examination reveals irregular dark brown lesions along a leaf margin and dry, withered blades.

Two pathogens are linked to this disease, Rhizoctonia solani and R. zeae. Symptoms appear when temperatures reach 73F to 85F and can completely blight a stand of susceptible turf in 24 hours. If R. zeae is present, it can continue to infect at temperatures above 90F. To manage this disease, water early in the day if the soil is dry, and syringe mycelium seen in the morning.

For turfgrass areas with a history of contracting brown patch, follow a preventive fungicide program.

Crown Rot Anthracnose (Colletotrichum graminicola)

When conditions of warmer temperatures and higher humidity occur during already stressful conditions in midsummer, crown rot anthracnose often develops on susceptible cultivars of annual bluegrass and bentgrass on golf courses. This fungal disease often begins with reddish- brown patches of turf which quickly turn to a pale yellow or tan. Turf blades may exhibit reddish brown lesions, and in later stages, black spinelike fruiting bodies.

Unfortunately, symptoms may not be noticed until infection has reached the stems, crowns, and roots. The plants may shrivel. At this point, a gentle pull on the plant may dislodge the entire plant, exposing dead roots. Although curative fungicides are available, best control occurs when treated preventively, or at least early in the infection. It is important to keep infected turf watered to avoid further stress.

Slime Molds

Slime molds became visible with the cooler, wet temperatures in late May. Tiny, gray, ball-shaped spore masses develop along the surfaces of turfgrass leaves. Although more abundant in shaded or moist areas, slime molds often appear on slow-growing turfgrass with dense thatch. These primitive fungi live on decayed or dying organic material but depend upon turfgrass and other weedy grasses for structural support.

As the spore masses dry and release the spores, the blades are covered with a dusting of purplish, gray, or white spores, causing the blades to look sooty. If the infection continues, the turf will become chlorotic and wilt, due to reduced photosynthesis.

Although slime molds are not a serious threat to turfgrass, infestations can be reduced by improving air circulation and drainage and mowing the turfgrass to knock the spores back into the thatch.

Fairy Rings

Fairy rings were quite evident in some lawns in early August in 2001, particularly in the southern part of the state, where rainfall was frequent and turf was thriving. Fairy rings may be produced by any of about 50 species of soil-inhabiting fungi. Occasionally, mushrooms (fruiting bodies of the fungus involved) sprout in a ringlike pattern. The curious circular pattern of mushrooms is responsible for the common name of this disease.

Generally, the rings are lush, dark-green circles or arcs of turf growth in the lawn. Rings vary in size from 3' to 4' in diameter up to 200'. Rings can enlarge each season from a few inches to several feet in diameter, though they sometimes disappear for a year or more and then reappear.

At times, the rings may turn straw colored to brown. This "collapse" of the fairy ring may occur if conditions that favor rapid fungal growth (e.g., moist weather) are followed by a sudden stretch of dry weather. Once they dry out, the dense masses of fungal mycelial growth (fungal threads) produce hydrophobic conditions in the soil which inhibit water penetration. In fact, mycelial growth may become so dense, it resembles "cotton candy."

Control of this disease is difficult. Management strategies include improving water penetration of the area by intensive aerification or by using a hose-end root feeder, increasing spring applications of nitrogen fertilizer to mask symptoms, and removal and replacement of the soil in the infected area. Although several turf fungicides are labeled for control of fairy rings, applications may produce erratic results, and suppression of this disease is often only temporary.