Gray leaf spot, caused by the pathogen Magnaporthe oryzae, is a serious disease of both cool and warm-season turfgrasses. It has been a chronic disease in St. Augustinegrass (Stenotaphrum secundatum) for many years. More recently, gray leaf spot has caused serious problems in common cool-season grasses, especially annual ryegrass (Lolium multiflorum) and perennial ryegrass (Lolium perenne). Tall fescue (Festuca arundinacea) has been damaged by this disease in the southeastern U.S. The same fungus causes blast on rice.
Gray leaf spot.
PHOTOS COURTESY OF PAM SHERRATT.
On cool-season grasses, gray leaf spot is most active when temperatures are between 68 and 75 degrees Fahrenheit (20 to 23 degrees Celsius) and leaf tissue is wet for prolonged periods of time. Outbreaks of this disease vary from year to year, with the worst cases occurring in hot summers followed by warm, humid days of late summer and fall, especially after experiencing prolonged rainfall.
On warm-season grasses, warm, rainy spells from May to September commonly produce extended periods (12 hours and greater) of leaf wetness and relative humidity greater than 95 percent. During these periods, turfgrass leaf blades can remain wet and air temperatures often hover between 80 and 90 degrees Fahrenheit (27 to 32 degrees Celsius).
Environmental conditions such as these are ideal for the pathogen growth, infection and colonization of St. Augustinegrass. If these favorable conditions persist, leaf spots can expand to blight aboveground plant tissue.
The sexual stage producing ascospores was found in the fungus infecting rice, so the rice blast fungus is generally called Magnaporthe grisea.
In turfgrasses, ascospores have not been observed, and the gray leaf spot fungus is referred to as Magnaporthe oryzae. Host range of the fungus is more than 50 grass species, including wheat, crabgrass and foxtail. However, individual isolates of the fungus are limited to infecting one or a few host species. For example, Magnaporthe oryzae isolated from perennial ryegrass cross-infects tall fescue, but it is less likely to infect rice. In contrast, the fungus isolated from rice rarely infects turfgrass. Molecular analyses indicate that the fungus infecting turfgrass is related to the fungus infecting rice.
This field was wet for several days during a hurricane event. The conditions were favorable for gray leaf spot and the predominant grass on the field was perennial ryegrass. All of these factors (think disease triangle) added up to a devastating disease outbreak that killed most of the field.
Magnaporthe oryzae is distributed over a wide geographic region. Although regarded as a major disease on St. Augustinegrass, recent epidemics on perennial ryegrass in the mid-Atlantic states and the Midwest have expanded the scope of this deadly disease. It was not until 1996 that the disease was confirmed in Ohio.
The disease has become one of the important emerging turfgrass diseases. The pathogen kills the plant from severe leaf blight. Part of the blighting process involves the production of phytotoxic chemicals that disrupt the normal biochemical and physiological balance within the turfgrass.
On St. Augustinegrass, gray spot appears as tiny lesions that enlarge rapidly into spots that are first round, then round to oval, and later elongate across the entire leaf blade. The leaf spots are tan to gray, often depressed at the center, with irregular purple to brown margins.
On cool-season grasses, gray spot may not be noticeable. Initial symptoms often appear as tiny pinprick lesions, which often go unnoticed or are mistaken as another less aggressive disease. Under optimal environmental conditions and on susceptible hosts, these small spots quickly turn into water-soaked lesions, which rapidly coalesce together and progress to twisted necrotic leaf tips. If left unchecked, the disease moves rapidly to other susceptible plants. The overall appearance often resembles severe drought stress, but if soil moisture is checked, the test will show that the soil has adequate water. Severe outbreaks look as if the turfgrass was scorched with a flamethrower.
The onset of gray leaf spot has been described as a general chlorosis of the sward, followed by a rapid blighting of the entire plant. This is an unforgiving disease. In later stages of disease development, 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. This feature is a key to accurate diagnosis of this disease. Left untreated, gray leaf spot can cause severe turf dieback, exposing the underlying soil.
Genetic host resistance
Turfgrass cultivars vary in their susceptibility to gray leaf spot, although none can be considered completely resistant. The level of disease resistance depends upon the severity of the disease and the maturity of the turf, with seedling turf more susceptible than established turf. Plant breeders are working on new material that shows better disease tolerance.
When it comes to selecting grass seed, most sports turf managers will buy a blend or mix that has been selected by their seed supplier. It is possible to handpick cultivars, but there is usually a minimum amount of seed, for example 500 pounds, that must be bought in order to have a seed distributor make a custom blend.
Water drop-shaped conidia can be found on leaves infected by gray leaf spot.
PHOTOS COURTESY OF JOE RIMELSPACH UNLESS OTHERWISE NOTED.
Whether the seed mix has been chosen by the customer or the seed supplier, both entities can gain valuable information about each cultivar from the National Turfgrass Evaluation Program website (www.ntep.org). NTEP evaluates both warm and cool-season turfgrasses for traffic tolerance, seasonal quality, and biotic and abiotic stresses. Results are posted on the website and can be accessed free of charge. An important point to make is that selection of a seed blend should not be made on the one disease alone. For example, perennial ryegrass cultivars that show improved resistance to gray leaf spot disease may not be resistant to other destructive diseases like brown patch (Rhizoctonia solani) or Pythium blight (Pythium aphanidermatum). It is important to select a blend of three to five cultivars that show resistance to at least one or more turf diseases (See Table 1).
* Based on NTEP perennial ryegrass final reports 2005-2009 and New Jersey progress reports 2011. Top statistical grouping of cultivars are listed in alphabetical order.
If the pressure from gray leaf spot is so extreme each year that games are canceled and fields are ruined, then sports field managers should consider switching to alternative turf species. In northern states, sodding or overseeding with Kentucky bluegrass is recommended for repairing perennial ryegrass stands consistently killed by gray leaf spot.
Once environmental conditions are favorable for gray leaf spot to occur, turfgrass should be scouted on a regular basis, and appropriate and immediate actions should be implemented. This is critical for gray leaf spot because a rapid outbreak can occur in a short period of time.
Balanced maintenance practices that reduce stress should be used for gray leaf spot management.
Adequate nitrogen fertility is recommended, but high nitrogen application rates during summer months should be avoided. An important practice to prevent the risk of gray leaf spot disease includes prudent use of quick-release nitrogen during warm, humid and rainy weather, because these conditions cause excessively lush (soft) turfgrass that is highly susceptible to gray leaf spot infection.
Close-up of the early symptoms of gray leaf spot.
Irrigation timing and frequency is also important. Leaf tissue should remain as dry as possible during warm, humid weather. Leaf tissue that remains wet for prolonged periods of time is more susceptible to the disease. The photo on page 20 shows a soccer field that was wet for several days during a hurricane event. The conditions were favorable for gray leaf spot and the predominant grass on the field was perennial ryegrass. All of these factors (think disease triangle) added up to a devastating disease outbreak that killed most of the field. Keeping the field dry can be a challenge for turf managers that use rain covers or tarps during the fall playing season. The physical removal of dew through mowing or brushing will help, but ideally, and especially on game fields, there needs to be some form of added protection with a preventative fungicide if the field has a history of gray leaf spot problems.
Fungicides are important for gray leaf spot control on sports fields, particularly high-profile game fields with a lot of perennial ryegrass on them. Applications of fungicides should begin prior to the onset of disease, (around August 1 in Ohio. Check with your local extension office for timing in your state), and continue every one to three weeks as needed when muggy weather prevails.
The extent of disease pressure will greatly influence fungicide 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. Fungicide-resistant strains of the pathogen have been identified. Fungicide selection should be guided by a sound resistance-management strategy. For specific fungicide recommendations/resources, check with your local university turfgrass extension program. Members of the Sports Turf Managers Association can also access resources such as Michigan State's Turfgrass Library (www.tic.msu.edu) and Plant Management Network (www.plantmanagementnetwork.org).
In summary, an effective gray leaf spot management program includes:
- Prompt and correct disease identification
- Using more tolerant turfgrass cultivars
- Cautious use of fertilizer and water during times of high disease pressure
- Informed fungicide selection and effective timing
Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011. Joe Rimelspach is a program specialist at Ohio State University.