July brings gray leaf spot to perennial ryegrass

July brings gray leaf spot to perennial ryegrass Protection from fast-acting disease is possible, but pricey. Peter H. Dernoeden, Ph.D. On perennial ryegrass, gray leaf spot commonly appears during the rainless heat and humidity of July, but may not subside until November frost, putting fall seedings at risk..nitrogen fertilization can intensify summer outbreaks, but may reduce autumn epidemics. Where the disease is chronic, preventive applications of fungicides appear to be the best approach to control. Gray leaf spot, which is caused by the fungus Pyricularia grisea, can be particularly damaging to newly sprigged turf and inflicts significant damage to St. Augustinegrass, annual and perennial ryegrass and tall fescue (4,5,6,8,9). Epidemics of the disease have been associated with unusually dry and warm conditions, but the cause of its rapid spread is unknown. There are no reports of the disease in the Pacific Northwest where most perennial ryegrass seed is grown, and there is no evidence that the pathogen is being spread by seed. The most likely means of dissemination are by wind dispersal of spores or through golf shoes or clubs contaminated with the pathogen. In winter, the pathogen probably survives as dormant mycelia in dead tissues. Recent mild winters may have enhanced pathogen populations in the United States. Environmental factors Most information about the environmental conditions that trigger the disease comes from studies on St. Augustinegrass, rice and tall fescue. In St. Augustinegrass, the primary source of infection is splash or wind dispersal of spores following spring rain (4). To reach epidemic levels, rainy weather or prolonged periods of high humidity and temperatures above 70 F are required. One study (9) reported that gray leaf spot in tall fescue was favored most by leaf wetness durations of 36-40 hours at 80 F, or of 16-40 hours at 83 F. The disease, however, could develop in tall fescue after eight hours of leaf wetness at temperatures above 76 F. Disease severity intensifies with increasing temperature (9). A growing-degree-day model, which is a method used to quantify how organisms respond to heat accumulation during a season, may help to predict gray leaf spot outbreaks. Ryegrass golf turf At golf courses in Maryland, the first infected perennial ryegrass plants usually appear by the third week of July. Most samples arrive at diagnostic labs in August and September. Once blight- 62 Golf Course Management July 1999

Three characteristics of gray leaf spot are evident around this green. The higher-cut perennial ryegrass in the surrounds is more blighted than the lower-cut collar, and the bentgrass green appears completely unaffected. ing develops, the disease can remain active until the first hard frosts in early November, although it generally subsides in October. The fungus initially attacks leaves, and leaf tips may appear water-soaked and chlorotic within hours. Thereafter, leaf spot lesions and leaf twisting occur. Below the twisted areas, a few leaf lesions are sometimes evident. Frequently, the youngest leaf emerging is twisted in the shape of a fish hook. Plants with the fish hook symptom may not have lesions. Spores can be disseminated rapidly by mowing, resulting in a streaking pattern similar to that associated with Pythium blight. Evidently, within a very few hours, spores can germinate, penetrate cut leaf tips and begin to blight before leaves dry in the morning. The lesions are circular to oblong, about ~ inch to ~ inch long and grayish-brown with a dark-brown border. Gray or brown lesions with or without a dark-brown border frequently develop along the edges or margins of leaf blades. A yellow halo occasionally can be observed bordering lesions. Some lesions may be the size of a pinhead and very dark brown, resembling those caused by the net blotch pathogen, Dreehslera dietyo ides. Net blotch, however, is generally associated with extended periods of overcast and rainy spring weather. In the early morning, the twisted leaf tips or lesions on the margins of leaves may appear felted, and infected tissues may be gray, dark brown, purple or yellow. The felted appearance is the result of the production of huge numbers of spores and their spore-bearing stalks, known as conidiophores. From a standing position, the first observable symptom is the appearance of reddish-brown spots, 1-2 inches in diameter, which could easily be confused with Pythium blight. However, foliar mycelia are not associated with gray leaf spot. During prolonged hot and humid periods without rain, the dead spots of turf enlarge, ranging from 3 inches to 18 inches in diameter. At this point, disease symptoms mimic brown patch (Rhizoetonia solani), and large areas of turf may collapse in three to five days. continued Golf Course Management July 1999 63

continued from p. 63 Under less-favorable environmental conditions, large pockets of dead turf may develop over a three- to four-week period. Infected plants sometimes develop a bluish-gray hue, typical of drought stress symptoms. The disease is most severe in hot areas of the course. The disease is generally more destructive in roughs and less severe in low-cut, perennial ryegrass approaches and collars (1). Field research (10) has shown that gray leaf spot incidence was less severe when mowing height was reduced from 4 inches to 1.5 inches. Cause of death Although previous research indicates that P. grisea does not infect stems (5,8), a new biotype may have developed that attacks meristematic tissues (shoot tips). It is unknown whether roots are attacked. High temperature stress and dry soils predispose plants to leaf infection by P. grisea. Drought may demand night watering, producing long periods of leaf wetness, which allows huge numbers of P. grisea spores to be produced overnight. The spores are efficiently spread by mowers and irrigation. The rapid loss of all leaves and sheaths, combined with The effects of herbicides and plant growth regulators on the incidence and severity of gray leaf spot are unknown. Some management practices that may help reduce gray leaf spot severity include: Reduce mowing height Avoid mowing when leaves are wet Collect clippings Apply small amounts of nitrogen fertilizer (0.1-0.125 pound per 1,000 square feet) in spoon-feeding programs during the summer Maintain adequate soil moisture levels by irrigating during daytime hours extended periods of high temperature stress (which probably caused root dysfunction), is a possible mechanism of plant death. Cultural management Research at Rutgers University (10) determined that nitrogen fertilization reduced disease severity when epidemics developed in autumn. However, when the epidemic began earlier in the summer, applications at rates exceeding 1.5 pounds of nitrogen per 1)000 square feet intensified the disease. Removal of clippings was also beneficial when disease pressure was low. Earlier research on St. Augustinegrass also showed that clipping removal reduced disease severity (7). In extreme cases, renovation to a resistant species may become necessary. During renovation, seed must make contact with soil. This is best achieved using a slicer seeder. Broadcasted seed do not establish well either because the seed is attacked directly or seedlings are killed by disease pathogens as they emerge. Hence, tank mixes of a broad-spectrum fungicide with a Pythium-targeted material should be applied to seedlings. Low-rate, weekly spray applications of a water-soluble nitrogen source such as urea (0.1-0.125 pound of nitrogen per 1,000 square feet) and syringing two to three times daily also help improve seedling vigor. Although creeping bentgrass, bermudagrass and Kentucky bluegrass are reported hosts, these grasses have exhibited outstanding resistance to gray leaf spot on golf courses. Creeping bentgrass, bermudagrass and zoysiagrass are all good fairway species. Choice of species depends on region (i.e., bermudagrass and zoysiagrass are better adapted to areas south of St. Louis, Mo.; Cincinnati, Ohio; and Baltimore, Md.) and budget (2). Creeping bentgrass is more expensive to maintain than bermudagrass. Zoysiagrass requires less maintenance, but is usually most successful if sodded. In roughs, mixtures of tall fescue and Kentucky bluegrass (90:10 percent) or 64 Golf Course Management July 1999

Gray leaf spot is best controlled with fungicide applications. There is tremendous variation in the effectiveness of selected fungicides and rates. Kentucky bluegrass and hard or blue sheep fescue (90:10 percent) are being used in the Mid-Atlantic region. Tall fescue has been observed to be a host for gray leaf spot only in Georgia and North Carolina, but the disease may eventually spread to tall fescue in more northern regions. Tall fescue grows rapidly during cool and wet periods and is Jvery susceptible to brown patch. Furthermore, many golfers may find it difficult to hit golf balls out of highercut tall fescue roughs. Kentucky bluegrass and fine-leaf fescues also have drawbacks (2,3). Kentucky bluegrass is slow to germinate and become established. It develops thatch and may chronically suffer from summer patch (Magnaporthe poae), which requires expensive fungicide rates for control. Kentucky bluegrass also requires more water and fertilizer than tall fescue or perennial ryegrass. Hard and blue sheep fescues are better adapted to the Mid-Atlantic region (3) and may persist in intermediate roughs if they are well irrigated and mowed above 1.75 inches. Other fine-leaf fescues perform best in deep roughs or out -ofthe-way natural areas that are not mowed in the summer (3). All fine-leaf fescues are severely damaged by mowers and golf car traffic when soils are dry during the summer. Many are also damaged by cworothalonil (Daconil), a primary means of controlling gray leaf spot (3). Chemical control Where the disease is chronic, preventive application of fungicides appears to be the best approach to control. Once the disease has developed, higher rates and more frequent fungicide applications generally are required. Although Daconil is one of the most effective fungicides, even high rates provide only five to 10 days of control. Because Daconil is a contact fungicide, its effectiveness is diminished by routine mowing and probably by UV light and other factors. Penetrants provide a longer residual effect, but may provide only 10-21 days of commercially acceptable levels of colltinued Golf Course Management July 1999 65

continued from p. 65 control. Heritage (azoxystrobin) is effective, particularly at the high label use rate (0.4 ounce per 1,000 square feet). Spectro (a pre-packaged mixture of chlorothalonil and thiophanatemethyl), CL 3336 (thiophanate-methyl), Banner (propiconazol) and Bayleton (triadimefon) also control the pathogen. Compass (trifloxystrobin), which is expected to become available this summer, and Lynx (terbuconazole), which is not registered yet, are also effective. Banner and Bayleton alone are not effective, but when tank-mixed with Daconil, they provide 10-14 days of control during high-pressure epidemics. CL 3336 and Spectro also provide 10-14 days of control at higher label use rates. We evaluated Compass in only one year and generally found that the high label use rate (0.2 ounce) provided a level of control equivalent to the low rate (0.2 ounce per 1,000 square feet) of Heritage. In curative programs, however, all of these fungicides should be tank-mixed with Daconil. Where gray leaf spot is known to occur, fungicide application should begin in early to mid-july. However, you need to know when the disease develops in your area to be able to time the first spray. The disease is best managed before any blighting becomes evident. Once blighting appears, a high rate of Daconil should be tank-mixed with a penetrant. Affected areas should be re- treated in five to seven days with another application of Daconil. Thereafter, tank-mix combinations will probably be required at 10- to 21-day intervals, depending on the fungicide, application rate and environmental conditions. Vigilant scouting for gray leaf spot requires almost daily attention from July through October. The disease can be very active in September and October, and is especially destructive to new seedlings in overseeded areas. References 1. Dernoeden, P.H. 1996. Perennial ryegrass and gray leaf spot. Golf Course Management 64(1):49-52. 2. Dernoeden, P.H. 1997. The transition from perennial ryegrass to creeping bentgrass fairways for the Mid-Atlantic Region. USGA Green Section Record 35(5):12-15. 3. Dernoeden, P.H. 1998. Fine fescues on golf courses. Golf Course Management 66( 4):56-60. 4. Freeman, T.E. 1967. Diseases of southern turfgrasses. Florida Agricultural Experimental Station Technical Bulletin 731. 5. Landschoot, P.]., and B.P. Hoyland. 1992. Gray leaf spot of perennial ryegrass turf in Pennsylvania. Plant Disease 76:1280-1282. 6. Malca, M.L, and ].H. Owens. 1957. The gray leaf spot disease of St. Augustinegrass. Plant Disease Reporter 41:871-875. 7. Parris, G.K. 1971. Practical control of two lawn grass diseases without fungicides by removal of mower clippings. Plant Disease Reporter 55:775-779. 8. Trevathan, L.E., M.A. Moss and D. Blasingame. 1994. Ryegrass blast. Plant Disease 78: 113-117. 9. Uddin, W., 1.1. Burpee and K.L. Stevenson. 1997. Influence of temperature and leaf wetness duration on development of gray leaf spot (blast) of tall fescue. Agronomy Abstracts, p.137-138. 10. Vaiciunas, S., and B.B. Clarke. 1998. Impact of cultural management practices and genotype on development of gray leaf spot in cool-season turfgrasses. Agronomy Abstracts, p. 140. This photo shows three types of leaf symptoms of gray leaf spot (from left): gray lesion on leaf margin, yellow halo around a leaf lesion, twisted and blighted leaf tip. Peter H. Dernoeden, Ph.D., is a professor of agronomy in the department of natural resource sciences and landscape architecture at the University of Maryland at CollegePark. 66 Golf Course Management July 1999