New & Emerging Turf Diseases
New diseases are constantly cropping up in the turfgrass industry. Pathologists are often asked to explain this phenomenon, and we have heard just about every theory in the book. Are the pathogens mutating? Did the pathogen come in on the seed? Is it because of that new fungicide that came out last year? Is global warming to blame?
The answer is probably much more simple. Consider the major changes that have occurred in turfgrass management over the past 20 years, or even over the past 10. Golf course putting greens are mowed at 0.125 inch or less, are irrigated only when on the brink of death, and are fertilized with miniscule amounts of nitrogen and other nutrients. New putting greens are constructed with root zone mixes that can percolate 20 inches of water per hour or more. Many new varieties of cool- and warm-season grasses have also been released.
All of these changes have an impact on fungal pathogens and the turf's susceptibility to attack by these microorganisms. For example, brown patch used to be a severe problem in cool-season putting green turf in many parts of the country. Brown patch is now a rare occurrence because of the dry and low-fertility condition of greens and the availability of highly effective fungicides such as Compass, Heritage, Insignia and ProStar. Other diseases, such as anthracnose, dead spot, rapid blight, Pythium root dysfunction and fairy ring are becoming more common.
Anthracnose diseases, caused by Colletotrichum cereale, are not new to the turfgrass industry, but they are increasing in their frequency and distribution. Annual bluegrass is particularly susceptible to anthracnose, which, next to seedhead production, is one of the main drawbacks of this turf species. However, some varieties of creeping bentgrass can become susceptible to anthracnose in hot and humid climates. Anthracnose is now a persistent problem on susceptible bentgrasses, such as Penncross, Pennlinks, Crenshaw and Dominant, in the southern United States.
Anthracnose can develop as a foliar blight, in which only the leaves are attacked (see top photo, page 14), or as a basal rot, which infects the crowns, stolons and leaf sheaths (see bottom photo, page 17). Both phases of the disease are encouraged by stress on the turfgrass plant, including low mowing, drought stress, nutrient deficiency and inadequate soil aeration. In the quest for putting speeds, many golf course greens are purposefully managed in this condition, so it is no surprise that anthracnose is becoming more of an issue. Correcting cultural programs to relieve turf stress will help to minimize anthracnose problems (see photos on bottom of page 14 and top of page 17).
Fungicides can provide good anthracnose control, but they should be applied on a preventative basis for best results. The anthracnose pathogen can develop resistance to fungicides very quickly. Resistance to thiophanatemethyl (3336, Systec and T-Storm) and the QoI fungicides (Compass, Heritage and Insignia) has already developed in many locations, and the pathogen is also becoming less sensitive to the DMI fungicides. Take care to rotate and tank-mix fungicides from different chemical classes to slow resistance development. Recent studies have demonstrated that fosetyl-Al (Signature and Prodigy) and phosphite salts (Alude, Magellan, Resyst and Vital) provide excellent control of anthracnose.
Dead spot, caused by Ophiosphaerella agrostis, was first observed in 1998 on creeping bentgrass putting greens in Maryland, but quickly became a problem across the Southeast and Midwest over subsequent years (see photo, left, page 18). Dead spot specifically attacks newly constructed putting greens or those that have been recently fumigated, with the damage typically most severe in the first year after establishment. After this initial epidemic, the disease declines in its severity over a one- to four-year period, eventually disappearing in most cases.
Dead spot has been most common on creeping bentgrass greens, but has also been observed in bermudagrass greens overseeded with Poa trivialis. The disease appears in small reddish-brown spots, ranging from 0.5 to 4 inches in diameter (see top photo, page 17). The turf is slow to recover from dead spot damage, leaving severe depressions in the putting surface for extended periods of time.
Dead spot is another disease that is stress-induced, particularly by heat, drought and low fertility. The disease can appear at any time during the growing season, but severe outbreaks are most common in June, July and August during periods of heat and drought stress.
While dead spot was a widespread problem between 1998 and 2004, very few cases of the disease have been reported in recent years. Nevertheless, superintendents managing new greens should monitor for dead spot symptoms frequently so that control measures can be implemented quickly. The use of nitrogen sources containing ammonium sulfate can help to reduce disease development, while use of urea, nitrates or lime can encourage disease activity. Fungicides containing chlorothalonil, fludioxonil, iprodione, propiconazole, pyraclostrobin and thiophanate-methyl provide good dead spot suppression, but recovery from existing damage is typically slow.
Rapid blight is the newest turfgrass disease, and one of the few that is not caused by a fungus. The pathogen, Labyrinthula sp., is a single-celled protist, which is more closely related to algae. Rapid blight occurs in arid climates or during extended periods of dry weather. The disease has been observed in the southeastern and western United States on annual bluegrass, rough bluegrass and perennial ryegrass. Creeping bentgrass is occasionally affected in the western United States, as well.
Symptoms of rapid blight appear in yellow, orange or reddish brown patches. Leaves on the affected plants will appear mottled and greasy, but the roots are typically unaffected. As the name implies, rapid blight can spread very quickly to encompass large areas of turf, especially in immature stands of susceptible turfgrasses.
Relatively little is known about the cultural factors that encourage rapid blight, but elevated soil salinity levels are a major predisposing factor. Dry weather conditions, which lead to salt accumulation, or irrigation water with elevated salt levels have been associated with most rapid blight outbreaks. Regular aerification, gypsum applications, acid-treatment of irrigation water and periodic leaching of the root zone will help to minimize soil salinity levels and reduce rapid blight development.
Slender creeping red fescues, creeping bentgrass and alkaligrass have shown good resistance to rapid blight compared to other cool-season grasses, so use these grasses when possible where rapid blight may occur. Preventative applications of mancozeb, pyraclostrobin and trifloxystrobin provide good control of rapid blight. Curative applications will slow or stop disease spread, but severely blighted turf is not likely to recover.
Pythium root dysfunction
Pythium root dysfunction of creeping bentgrass was initially described in 1985 by C.F. Hodges at Iowa State University, who identified P. arrhenomanes and P. aristosporum as the causal agents. Outbreaks of similar symptoms were reported in the Mid-Atlantic and Southeast United States in recent years. Current results in North Carolina indicate that Pythium root dysfunction is caused by a different species: Pythium volutum.
Pythium root dysfunction is completely different from any other Pythium disease you may have experienced. Unlike Pythium blight and Pythium root rot, this disease is most damaging to creeping bentgrass growing in sandy soils, particularly newly constructed putting greens. Symptoms appear in distinct circular patches, from 4 inches to 2 feet in diameter, that initially show signs of wilt or nutrient deficiency, but continue to decline and turn yellow to orange in color (see photo, left, page 18). Pythium root dysfunction is most commonly seen in the summer months, and symptoms are most severe on slopes, ridges, exposed greens or other areas subjected to increased heat and drought.
Current research and observations indicate that P. volutum infects and damages creeping bentgrass roots during the fall and spring. This disease activity often goes unnoticed, but reduces the roots' ability to absorb water and nutrients and therefore increases the turf's susceptibility to summer stresses. Maintaining adequate fertility and soil moisture levels, regular hollow- and solid-tine aeration, and relieving stress by any means possible will help to suppress the summer symptoms of Pythium root dysfunction.
Fungicides can provide effective control of Pythium root dysfunction, but which products are most effective depends on the Pythium species involved. In North Carolina, our research has shown that Insignia (0.9 ounces) or Signature + Banol (4 + 2 ounces) provide good control of P. volutum on a curative basis. Turf managers should rotate among these treatments to prevent the development of fungicide resistance.
Fairy ring is another old disease that seems to be an increasing problem in the turfgrass industry. Many different species of mushroom- and puffball-producing fungi can cause fairy rings in turfgrasses. These fungi do not infect the turfgrass plant, but rather their growth changes the physical or chemical properties of the soil and thatch, which in turn impacts the health and appearance of the turf. The fairy ring fungi can rapidly invade sandy soils and cause severe damage to newly constructed putting greens or other turf areas.
Fairy rings produce three distinct types of symptoms in turfgrasses. A Type I fairy ring causes the soil and thatch to turn hydrophobic (water-repellent) and kills the turf or severely stunts its growth (see photo, above). A Type II fairy ring causes a ring of dark green, rapidly growing turf through the release of nitrogen and other nutrients into the soil (see photo, page 21). A Type III fairy ring produces a ring of mushrooms or puffballs in the turf. The symptoms produced by an individual ring may change during the course of the year, depending on the weather conditions. Type II and III symptoms are more common in the spring during periods of cool and wet weather, whereas Type I symptoms are most common in the hot and dry conditions of summer.
A Type III fairy ring is, of course, easy to manage by removing the mushrooms or mowing them off. However, Type I and II rings are not so easy. Remember that the fairy ring fungus is not infecting the plant, but rather is changing the soil properties. To control these fairy rings, you must correct the soil properties so that the turf can resume normal growth. Fungicides alone will not provide acceptable control in most cases.
To control a Type I fairy ring, use aerification, spiking or other cultivation practices to break-up the hydrophobic layer of soil. Application of soil wetting agents will also help to re-wet the profile and suppress Type I symptoms. Type II fairy are most prevalent in under-fertilized or under-irrigated turf. Applying nitrogen or iron will help to mask Type II fairy ring symptoms. Turf managers should also prevent Type II fairy rings from drying out, as the infested soil can turn hydrophobic and trigger the appearance of Type I symptoms.
Several fungicides are available for fairy ring control, but are most effective when applied preventatively in the spring and early summer. Tank-mixing fungicides with soil wetting agents has been shown to increase fairy ring control by moving the fungicide deeper into the profile and helping to prevent the formation of hydrophobic layers. Because there are so many species that cause fairy ring, fungicide efficacy varies considerably from location to location. Experiment with several products to determine which are most effective in your location.
In the future
There is no telling what the future holds, but as the turfgrass industry continues to advance, new diseases will continue to appear, old ones will become more damaging and others may go away entirely. As we strive to keep up with our customers' demands, it is important to remember that turfgrasses need sunlight, air, water and nutrients to function properly, and that a healthy turfgrass plant is the best protection against disease attack.
Lane P. Tredway, Ph.D., is assistant professor and Extension specialist, Department of Plant Pathology, North Carolina State University (Raleigh, N.C.).
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