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Issue link: http://read.dmtmag.com/i/605094
Q&A with Pamela Sherratt T his fall I was asked to visit a university with a large soccer complex in south- east area Ohio. There has been a history of gray leaf spot (caused by the patho- gen Pyricularia oryzea) in that area for many years with no sign of it letting up. The fields are unfortunately located at the bottom of a hill, in a bowl, and so hot and humid condi- tions prevail. The purpose of the visit was to come up with a plan for 2016, to prevent the utter destruction of fields observed in August 2015. Gray leaf spot is a serious disease of perennial ryegrass, with outbreaks typically occurring in the late summer and early fall. The assumption is that the spores are car- ried north by the jet stream each year during hurricane season. This theory is being tested by Ohio State pathologists Francesca Hand and Joe Rimelspach, who have set spore traps to see if they are indeed carried by the wind, whether they survive the winter, and if the spore traps might be a viable way to predict an infection. Gray leaf spot epidemiology has shown that the optimal temperature for infection is between 82-90°F, especially when leaf tissue is wet for prolonged periods of time. Leaf wetness/continuous leaf moisture parameters required for infection at 68-75°F is 36 hours, and at 82-90°F is 9 hours. New seedlings are also more at risk from infection than mature turf. Since gray leaf spot on cool-season turf was confirmed a decade ago, it has become one of the most important turfgrass diseases. This unforgiving disease kills the plant from severe leaf blight and can destroy an entire athletic field in days. So while initial leaf-spot symptoms appear not too bad, the disease quickly progresses to look like drought, then dead turf. In later stages of disease develop- ment, the sward may take on a gray color as a result of the mass production of conidia by the pathogen. Water drop-shaped conidia can be found on leaves infected by gray leaf spot and this feature is a key to accurate diagnosis of the disease. Once environmental condi- tions are favorable for gray leaf spot to occur, turfgrass should be scouted regularly, and appropriate and immediate actions should be implemented. This is critical for gray leaf spot because a rapid outbreak can occur in a very short period of time. There are many ways to both prevent and control gray leaf spot disease. First and foremost, if the disease is causing utter destruction to perennial ryegrass fields each year it's a good idea to think about switch- ing to Kentucky bluegrass, either through an aggressive overseeding program or by complete renovation with seed or sod. Even if perennial ryegrass continues to be used as an overseeding tool during the playing sea- son, a sound base/foundation of Kentucky bluegrass should be established whenever possible. Any perennial ryegrass used for overseeding should have genetic resistance to gray leaf spot. A list of those cultivars can be garnished from the National Turfgrass Evaluation Program website (NTEP.org) or Extension materials produced by University turfgrass breeding programs. If a pre- packaged GLS-resistant blend of ryegrasses is being used, make sure ALL varieties are resis- tant and not just a couple. The second best defense against gray leaf spot is to grow healthy turf. Healthy turf is accomplished by growing it in free- draining & fertile soils, by mowing at the correct height and by applying fertilizer and irrigation judiciously. Over-applying water and fertilizer can result in lush, weak plants susceptible to the disease. Growing healthy turf also means limiting any kind of practices that cause plant stress during the hot summer period, like scalping, scarifying/verti-cutting and herbicide/PGR applications. Since gray leaf spot infection is so reliant on leaf mois- ture, it's also a good idea to adopt practices that minimize leaf wetness, such as well- timed irrigation (avoid prolonged, over-night leaf wetness), dew removal by mowers or drag brushes, and cautious use of rain tarps. Rain tarps in particular provide the perfect growth chamber for gray leaf spot. Fields that have a history of gray leaf spot, or are covered with rain tarps frequently dur- ing periods of high disease pressure, should be protected with preventative applications of fungicides. Applications of fungicide should begin before the onset of disease and continue every 1 to 3 weeks as needed when the muggy weather prevails. The extent of disease pressure will greatly influence fungi- cide performance. If gray leaf spot outbreaks are in an advanced state by the time fungicide sprays are initiated, it is likely that levels of control will be unacceptable. Since fungicide- resistant strains of the pathogen have been identified it's critical that a fungicide pro- gram is put together that addresses resistance and maximizes efficacy. This includes: (1) minimizing consecutive applications of fungicides having a similar mode of action, especially during the period of highest disease pressure; (2) tank-mixing products with dif- ferent modes of action during periods of high pressure; and (3) switching from strobilurin fungicides and thiophanate methyl to other products during periods of low disease pres- sure. Current fungicide efficacy ratings for grey leaf spot control can be found online via Dr. Paul Vincelli at the University of Kentucky. In summary, the best defense against gray leaf spot is to switch to Kentucky bluegrass and/or use GLS resistant ryegrasses. A sound cultural program coupled with cautious use of tarps, fertilizer and irrigation should be implemented, and finally a budget and plan in place for a fungicide program leading up to, and during the period of highest disease pressure. ■ ST Gray leaf spot woes—have a plan for 2016 Questions? Send them to 202 Kottman Hall, 2001 Coffey Road, Columbus, OH 43210 or sherratt.1@osu.edu Or, send them to Grady Miller at North Carolina State University, Box 7620, Raleigh, NC 27695-7620, or email grady_miller@ncsu.edu 50 SportsTurf | December 2015 www.sportsturfonline.com