Gray leaf spot comes on strong
Blast disease-perhaps better known as gray leaf spot-is an emerging problem. Serious epidemics on perennial ryegrass fairways have become more frequent in recent years in several regions of the United States. Gray leaf spot is not a new disease. It has been long recognized on St. Augustinegrass. However, its severity on St. Augustinegrass rarely reaches the level that occurs on ryegrass fairways.
Disease severity varies by year, location and time of disease development during the growing season. Ryegrass fairways on numerous golf courses in the Northeast and Midwest were devastated in 1998. In Pennsylvania, turf loss exceeded 90 percent in some cases. In 1999, by contrast, gray leaf spot was not a problem in the Mid-Atlantic region and only moderate levels of the disease occurred in Pennsylvania. However, it was reported in the New England states for the first time, and some severe cases were confirmed in Indiana.
The catastrophic potential of this disease, plus its peculiar cyclic pattern of outbreaks (every 2 to 3 years) in ryegrass fairways has precipitated research into the nature of this disease. Obviously, we need to know more about gray leaf spot. In the meantime, however, turf managers have some options for control.
Historical background Since the initial descriptions of the gray leaf spot pathogen early this century (it was the causal agent of blast disease on rice), some confusion has existed regarding its nomenclature and the biological relationships of the various strains. Currently, the pathogen is recognized as Pyricularia grisea for rice, wheat and grass, and the disease is referred to as either blast disease or gray leaf spot. It is important to recognize that "blast" and "gray leaf spot," which are both used in technical and trade publications, are synonymous. Most fungicide labels use "gray leaf spot," but also frequently specify "Pyricularia grisea" to avoid confusion with other leaf spot diseases.
Reports of gray leaf spot on forage ryegrass in the southern United States first occurred in the early 1970s in Louisiana and Mississippi. In 1991, it was reported on ryegrass turf on fairways in Pennsylvania, primarily in the southeastern part of the state. In 1998, researchers at The Pennsylvania State University diagnosed the disease in western Pennsylvania for the first time. Perennial-ryegrass samples received at Penn State from golf courses in New York, Virginia and West Virginia also tested positively for the disease. That same year, reports began accumulating from the Midwest, and the disease also was noticed on tall-fescue turf in Georgia. In 1999, it was reported from Connecticut and Rhode Island.
Gray leaf spot lifecycle The gray leaf spot pathogen, Pyricularia grisea, has a wide host range that includes more than 30 grassy and broadleaf species. It is recognized as the pathogen for blast disease of turfgrasses, rice and all other grassy hosts. Our research on the biology of P. grisea isolated from rice, wheat, finger millet and several grass species indicates that several strains of P. grisea cause the disease in each of these species, and that P. grisea from perennial ryegrass on golf courses in the United States is distinct from the strain that occurs on rice. They cannot cross-infect each other's host. However, the pathogen of ryegrass can cause blast disease in some varieties of soft white winter wheat and triticale.
Although the worldwide genetic diversity of P. grisea appears to be tremendous, our studies suggest that P. grisea in ryegrass fairways in the United States may not be so genetically diverse. In fact, it appears that all U.S. populations we studied are likely to have descended from a common ancestor.
The sexual stage of the fungus is extremely rare and only the asexual stage has been found in the United States. In each infection cycle, reproduction occurs through production of millions of conidia (spores) within a short period (1 or 2 days) when conditions are conducive. Several consecutive infection cycles may follow during a single season, resulting in large sections of dead turf in affected fairways. The fungus appears to overwinter as mycelia in the infected live blades or dead plant debris in the soil. Research that should bring more understanding to this currently is underway at Dr. Rick Latkin's laboratory at Purdue University.
Curiously, despite the genetic uniformity, we have found that significant variations exist in virulence among the P. grisea populations from various regions of the United States. Additionally, greenhouse and laboratory studies have shown that creeping bentgrass, Kentucky bluegrass and annual bluegrass are not immune to this fungus. However, disease incidence in these grasses was relatively low-only a small percentage of the plant population developed the disease when inoculated. I also should make clear that this disease has not been reported in the field on these turfgrass species.
Diagnostic features During warm, humid periods in summer, gray leaf spot develops on the blades of perennial ryegrass and tall fescue as small water-soaked lesions that become necrotic spots within a few hours. The spots expand rapidly and develop into gray, grayish-brown or light-brown circular spots with purple to dark-brown borders, often surrounded by a yellow halo. As the disease progresses, the necrotic spots coalesce, become irregular in shape and cause partial blight (tip blight) or complete blighting of the leaf blades. Blighted blades often exhibit a conspicuous twisting or "flagging" appearance. Development of tip blight or complete necrosis of the blades initiates t he blighting phase that quickly kills the entire plant. This can occur within a short time, often within 48 hours when conditions are favorable. Partial or complete clearing of the turf can occur within 3 to 5 days.
Symptoms in the early stages of the disease can resemble those of several other diseases, such as Helminthosporium leaf spot/melting out, Rhizoctonia light (brown patch) and Pythium blight. In the advanced stage of the disease, the blighting symptoms may resemble stress-related disorders caused by heat and drought. At this stage, it takes an experienced turf manager, pathologist or diagnostician to accurately identify the problem.
Visual evaluation of the symptoms on individual blades or the patterns in larger turf areas may not always be a reliable means of distinguishing gray leaf spot from similar diseases. It may appear as an off-color, diffused blight, followed by development of sunken or pocketed areas, or irregular large patches. In fairways and roughs, the disease may follow patterns of distribution along low-lying areas or drainages that may be associated with high humidity and prolonged leaf wetness. Because Pythium blight is commonly diagnosed in such areas, diagnosticians must exercise caution when analyzing problems in such areas.
Additionally, the development of this disease in roughs, prior to its encroachment on adjacent collars and fairways, may appear similar to that of Rhizoctonia blight (brown patch) symptoms on tall-cut turf. During the time of high spore production, the necrotic or blighted blades appear dusty and grayish-white and often have a velvety texture.
It is possible that spores of Pyricularia grisea may not be present on the necrotic lesions of the blades at the time of examination. Further, saprophytes (fungi that consume non-living organic matter) or secondary fungal pathogens such as Rhizoctonia spp., Drechslera (formerly known as Helminthosporium) or Pythium may be present on the blades that were previously killed by P. grisea.
As this discussion suggests, gray leaf spot is not always easy to diagnose. It is crucial that you contact the nearest turfgrass diagnostic laboratory in your state if you observe the early symptoms of gray leaf spot or if you are unsure of the identity of the disease.
Factors that favor disease development At Penn State, aided by funding from the Pennsylvania Turfgrass Council and the Golf Course Superintendent's Association of America, we are conducting research to determine, among other things, the environmental factors critical to disease development. During the past 2 years, our field observations have shown that the disease develops in ryegrass fairways and roughs during periods of warm days with high humidity and prolonged leaf wetness in late summer (mid-August to early October). Data from our environmental monitoring systems confirm that high temperature, relative humidity and leaf wetness are the critical environmental factors that influence disease development.
During September and October, 1999, all three of these factors were present at our study sites and the disease began to show up on turf. At that early stage, disease severity was low with scattered spotting of leaves and, rarely, blighting in the slightly off-color turf areas. Temperatures subsequently dropped from the low 80s to the mid-60s and stayed there for more than 48 hours. Despite high humidity and prolonged leaf wetness, disease severity did not increase because the temperature was not high enough to promote disease to the point of a serious epidemic with complete blighting and turf death.
This corresponds well to the results of our controlled-environment chamber experiments. Models from both field and chamber studies indicate that the fungus can initiate infection even under moderately cool temperature (mid-60s) when high relative humidity (more than 80 percent) and prolonged leaf wetness (3 hours or more) are present, but it requires warm temperatures (mid-80s to low 90s) for the disease to further increase and finally become a severe epidemic. Our research indicates that relatively shorter periods of leaf wetness are needed as temperature increases (up to the 90s). For example, just 1 hour of leaf wetness is adequate for development of gray leaf spot at 90oF and 70 percent relative humidity.
Production of large numbers of spores is necessary for perpetuation of an epidemic. In another of our studies, we found that high relative humidity is critical to this process of consecutive spore production (secondary cycles) on infected blades that perpetuates the disease and spreads it to previously uninfected turf areas. Spores may spread via wind, rain, irrigation water or human activities (maintenance crews and golfers).
Cultural factors such as fertility, soil moisture and compaction, pesticides, turf height and density in fairways, and management of the rough may influence disease development. We are conducting research to discern how certain cultural factors interact with the critical environmental conditions before and during epidemics.
Disease management Gray leaf spot is manageable, but you must have some understanding of it and stay informed of new developments. Because this is an emerging problem, information on the disease in reference books and manuals is scanty. As new information becomes available, turf managers can stay informed about this disease by following industry publications and attending regional or national turf-related meetings.
* Fungicides. In the short term, you can effectively manage gray leaf spot in ryegrass fairways and roughs with fungicides. However, several fungicides have performed inconsistently in efficacy trials. This may be due to factors such as virulence of the pathogen, amount of inoculum present, different turf-management practices and specific field conditions at the test sites. More evaluations are necessary to clarify the inconsistent results we've seen.
One important limitation in disease management with fungicides is cost. Untreated roughs can be significant sources of inoculum, so treatment in these areas may be necessary for good control. However, fungicide treatments for roughs may not fall within a typical disease-management budget. Naturally, each course is unique.
In managing the disease with fungicides, chlorothalonil alone or in tank-mixes has proven to be effective. The problem with preventive applications such as this is duration of effectiveness. The effectiveness of contact fungicides can diminish relatively quickly due to mowing, wash-off, degradation or other factors. Nevertheless, preventive applications appear to be the most effective strategy where the problem is chronic.
Systemic compounds such as azoxystrobin, thiophanate-methyl and triadimefon also are effective against gray leaf spot. (Note: triadimefon is not labeled for gray leaf spot at this time.) Our studies in 1999 under severe disease pressure in inoculated plots showed that azoxystrobin at 0.2 ounce (14-day intervals) and 0.4 ounce (14- and 28-day intervals) provided similar control.
A new compound, trifloxystrobin (Novartis' Compass), applied at 0.25 ounce in 14-day intervals, also provided good control. (Trifloxystrobin is "mesostemic:" it has high affinity for the waxy cuticle of leaves and "redistributes at the plant surface by superficial vapor movement and redeposition," according to Novartis. However, while trifloxystrobin does penetrate plant tissue, "there is little or no transport in the vascular system of the plant," distinguishing it from systemic compounds.)
Tebuconazole (a new registration-pending compound) applied at 1.1 ounce every 14 days also was effective.
* Resistant turfgrasses. A traditional disease-management strategy for many diseases is the use of resistant turfgrass varieties. Unfortunately, although extensive research is underway in Dr. Andrew Hamblin's research unit at University of Illinois on selection of resistant turfgrass germplasm, no resistant cultivar or line of ryegrass is available at this time.
* Fertility. The effects of fertility on the disease are unclear. Current research may eventually provide a better understanding of this topic.
* Clippings. Collecting clippings may help because it reduces the amount of inoculum.
* Irrigation. Maintain adequate soil moisture during hot, dry periods in summer, but use caution irrigating the turf, especially during periods of heat and drought in summer. Watering early in the morning could be as damaging as watering in the evening or at night when temperatures remain warm (upper 60s to 70s). Also take care when syringing disease-prone areas, especially when temperatures are in the upper 80s to mid-90s in the afternoon. The bottom line is leaf wetness and canopy humidity during hot periods. Water deeply and less frequently whenever possible. Time your watering so that drying of the canopy will be rapid. Good air circulation, which aids drying in the turf canopy, is important.
We have much to learn about gray leaf spot. Even now, though, an integrated disease-management system based on knowledge of environmental and cultural conditions that favor the disease, proper turf-management practices and a prudent fungicide program will go far in managing this destructive disease.
Dr. Wakar Uddin is assistant professor of plant pathology at The Pennsylvania State University (University Park, Pa.).
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