New diseases continue to challenge even the most skilled turfgrass managers. The most recent threat, a summertime patch disease of creeping bentgrass, was reported in North Carolina in 2003. Although rapid blight and bentgrass dead spot first appeared a few years ago, reports of new outbreaks occur each season. These diseases have more than their newness in common. As described below, they share the fact that environmental issues play an extraordinarily large role in disease development and symptom expression.
Rapid blight is still a new disease to many parts of the country. Since initial reports of the disease in the late 1990s, incidence of rapid blight appears to be spreading in the southeastern and southwestern United States. The disease primarily affects rough bluegrass (Poa trivialis) and perennial ryegrass in over-seeded bermudagrass greens and fairways. Annual bluegrass (Poa annua) and creeping bentgrass also are susceptible. Initial symptoms include water soaking of affected leaves. As the disease progresses, leaves die and irregular patches of olive green-brown colored turf appear. Infection results in dead plants, significantly disturbing the appearance and playability of the turf surface. The consequences of rapid blight are most severe on putting greens, but the disease has been identified on fairway turf, and in a few cases, residential lawns.
The disease is caused by an obscure pathogen (genus Labyrinthula) that, until a few years ago, was only recognized as a very destructive parasite on seagrass and eelgrass growing in marine environments and salt-water estuaries along both coasts and the Gulf of Mexico. Reports describe severe effects on marine plants where water salinity was unusually high. Initial outbreaks of rapid blight on golf courses were reported along the coastal areas of South Carolina and Southern California. The greatest threat of rapid blight establishment appears to be associated with high salinity in irrigation water. Golf courses where dormant bermudagrass is over-seeded with rough bluegrass and perennial ryegrass in areas with high salt irrigation water are especially vulnerable. Because of frequent leaching rains and little history of water salinity problems, it is not likely that the susceptible cool-season grasses will be affected in the Pacific Northwest and in midwestern and northeastern states.
The pathogen appears to be related to the slime molds and primitive fungi. Once it is introduced, the pathogen most likely survives within the turf canopy where it is sustained by a high salt environment. It produces motile spores that contribute to localized disease spread. For that reason, it is recommended that you schedule mowing of infected greens and fairways last, and clean mowers before moving between areas of infected and healthy turf.
Because the over-seeded grasses are the prime targets for rapid blight, it appears that moderate temperatures during long periods of leaf wetness and high relative humidity favor disease development and spread. Monitoring the seasonal activity of the disease on your course will help you decide when to initiate fungicide sprays. Superintendents manage the problem with repeated applications of pyraclostrobin (Insignia), trifloxystrobin (Compass) and mancozeb (Fore and other trade names). Not all strobilurin fungicides were effective against rapid blight in recent fungicide trials. Fungicides often are applied to greens at 14-day intervals throughout the fall and winter months. Anticipating outbreaks and applying preventative sprays with effective fungicides usually succeeds in suppressing rapid blight infection. The preventive approach is especially important for turf stands with a history of the disease. Fungicides applied after symptoms are apparent will help protect uninfected plants, but recovery of damaged areas will be slow.
BENTGRASS DEAD SPOT
Bentgrass dead spot was first reported in 1998. Despite the fact that it is not a new disease, new outbreaks occur each year, especially on recently constructed or renovated golf courses. It appears to be limited to sand-based putting greens. Since the naming of the disease, outbreaks have been identified on hybrid bermudagrass greens. The disease is caused by a soil-inhabiting fungus that is closely related to fungal pathogens that cause necrotic ring spot and spring dead spot of bermudagrass.
Symptoms of bentgrass dead spot resemble injury due to ball marks (un-repaired) because spots (often about an inch in diameter) create depressions in the putting surface. The dead spots also can appear very similar to a severe case of dollar spot. The spots tend to have tan colored centers with reddish-brown margins and can expand to 2 to 3 inches in diameter.
The bentgrass dead spot pathogen infects and impairs the functions of roots, stolons and crowns. As a result, symptoms tend to appear during periods of summer stress including heat, drought and compaction. However, symptoms also have been observed in spring and fall. The dead spots also are more prevalent on ridges or south-facing slopes. In that regard, its behavior is comparable to that of other root diseases such as summer patch. Recovery can be very slow; in some cases damage will persist through the fall and winter.
It seems that intensively managed sand-based greens are at the greatest risk of infection. Tactics used to suppress other root diseases should help prevent or limit the development of bentgrass dead spot. These practices involve promoting root growth in spring and fall, and relieving stress during summer months by syringing, raising mowing heights and redirecting traffic. Where outbreaks have already occurred, you can apply fungicides to limit disease progress and hasten turf recovery. A variety of fungicides may be effective, especially if you can anticipate outbreaks and apply preventive sprays. You can expect a natural decline in symptom expression as affected greens become more mature (5 or 6 years after construction or fumigation). Like some other root diseases, it appears that as a balance is achieved in soil microbial populations, the pathogen population is reduced to levels that result in negligible damage.
SUMMER CROWN ROT
The newest disease to emerge is the summer crown rot of creeping bentgrass. Numerous outbreaks were reported in 2003 in southeastern and mid-Atlantic states. The disease appears to be limited to relatively new varieties of creeping bentgrass. Crenshaw, L-93 and the new Penn series (A1, A4 and G2) have been classified as susceptible to summer crown rot; no resistant varieties have been identified. Initial surveys indicate that creeping bentgrass maintained on sand-based greens constructed to USGA specifications are most vulnerable to infection and disease development. As the name implies, symptoms appear during the heat of the summer. Symptoms include 6- to 12-inch-diameter (sometimes even larger) patches of wilted (gray-green) and dead (tan-orange) turf. The patches resemble those associated with take-all patch, an acknowledged disease of bentgrass raised on sand-based greens. In fact, the causal fungus of summer crown rot (Gaeumannomyces sp.) is very closely related to the species of the take all patch pathogen.
The pathogen infects roots and crowns, interfering with the plant's capacity to conduct water to shoots and leaves. Wilting occurs as a result of severe infection, especially during the heat of the summer. Management options begin with practices that promote deep and extensive root systems in the spring and fall. Aeration and topdressing are at the top of the list. During summer months, you must avoid aggravating normal summer stresses. Deep and infrequent irrigation and raising the mowing height can contribute towards relieving heat and drought stress. It is too early to interpret the results of fungicide trials targeted towards summer crown rot control. However, given the nature of the disease and its close relation to take-all patch, fungicides with systemic properties (acropetal penetrants) should help suppress disease development. Spray timing will influence fungicide performance. As with other root diseases, fungicides applied after symptoms appear may not be effective.
THE BOTTOM LINE
For each disease, the turf is predisposed to infection, symptom expression and subsequent damage by extraordinary environmental stress. Although rapid blight is caused by a new pathogen, infection and disease development are instigated by extremes in soil and water salinity. The likelihood of the disease appearing in northern areas where salt problems are not common is remote. However, southern tier states with water salinity issues remain at risk to rapid blight development, especially juvenile over-seeded grasses. Bentgrass dead spot and summer crown rot are caused by pathogens that almost certainly are common soil inhabitants. They become problems when turf is managed too intensively for the growing conditions. Plants suffer environmental stress to the point that they become unusually susceptible to fungi that ordinarily are weak and inconsequential pathogens.
As we learn more about these diseases, it is likely that effective remedial treatment programs will be described. However, the best way to manage these new diseases is to try to avoid or relieve the environmental stresses that promote their development.
Richard Latin is professor of plant pathology at Purdue University (West Lafayette, Ind.).
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