New Diseases: hatching near you
Fortunately, diseases that affect turfgrass are limited.
However, their effects can be severe.
It is hard to say what a new disease is when we don’t know what happened 100 or 1,000 years ago. What we do know is that new, emerging and re-emerging diseases result from a multitude of influences, including cultural practices, pesticides and plant genetics. These diseases keep turfgrass managers on their toes and make management challenging. Following is a discussion of three such diseases that researchers have discovered or extensively studied over the past several years. Control tips are also included if you encounter them in your turfgrass.
Gray leaf spot
Gray leaf spot (GLS) on perennial ryegrass is one of the newest diseases in the cool-season regions of the United States. So far, no one has found it anywhere else in the world. Since its discovery, researchers have intensively studied this disease, but we still don’t completely understand it.
In the past, we thought gray leaf spot occurred only during hot, humid weather. However, the Midwest experienced relatively mild weather last year, and the disease appeared during cooler, wet periods that followed a few hot, dry days.
Golf course superintendents often see this disease following aerification and overseeding, even during cooler fall temperatures. Researchers have seen this disease infect untreated immediate roughs and spread directly into treated perennial ryegrass fairways.
In addition to the confounding factors of environment and pathogen, turfgrasses show little or no resistance to this disease. Fortunately, even if GLS eventually moves in, perennial ryegrass can be re-established quickly, and it performs well throughout most of the growing season.
So far, I have not seen extensive damage on low-maintenance sites. Sparsely irrigated, low-fertility sites such as most public parks, home lawns and private establishments are not targets for this pathogen. We have seen it more in highly maintained golf course settings.
Look for GLS from mid-summer through fall in cool-season growing regions. You will first notice a smoky appearance similar to brown patch. In the early stages of development, look for gray to black masses of spores on the leaf surface. Also, look for twisted, straw-colored grass tips.
In later developmental stages, look for yellowing, water-soaked leaf blades that resemble symptoms of Pythium. However, look on the outer fringe of the leaf-spot patches for black spores. These spores indicate that it is gray leaf spot. Use a microscope to definitively diagnose GLS. Look for flame-shaped conidia (asexual spores)—they are easy to spot.
The disease can be spread by wind, splashing rain, mowing and traffic. Plant death can occur within 48 to 72 hours. Seedlings are especially susceptible when planted in infected areas. Take necessary precautions to limit the spread of this disease and to successfully reseed damaged areas.
It isn’t clear whether cultural methods can help control this disease. In general, we expect to see less damage under low-intensity management, but this has not been scientifically proven. Apply fungicides to achieve the most effective control. Strobilurin chemistries and thiophanate-methyl are effective for gray leaf spot control. (The Fungicide Update on page 38 lists more controls.) Apply fungicides with enough water to penetrate the leaf canopy unless label directions dictate otherwise. Breeders are developing resistant varieties. They will likely be the best way to control this disease in the future.
Localized dry spot
Localized dry spot is not a new problem, but researchers have extensively studied it in recent years. Turfgrass managers are commonly plagued by turf that won’t recover when given modest amounts of water. Because many factors may be involved, the problem is difficult to pinpoint.
Often, obvious, physical, non-pathogenic factors account for this problem. Tree roots can out-compete turf in certain locations, and mounds and ridges are prone to dry spots, particularly on southern and southwestern exposures. Also, in low areas, algal crusting, poor soil structure, shallow root zones and excess thatch create conditions that inhibit efficient use of water.
Basidiomycete fungi that cause fairy rings are also associated with localized dry spot. These fungi decompose organic matter in the soil and are beneficial to the ecology of the turfgrass rhizosphere. However, they can also create hydrophobic (water-repelling) conditions in the soil. The fungi release organic substances that coat soil particles, causing them to bind together. Water does not adhere to the soil particles, and it rapidly passes through the soil.
In the past, managers completely excavated these sites. Now, we feel it’s best to use the cultural methods mentioned above combined with fungicides. You can also apply wetting agents. They act like mild detergents to dissolve hydrophobic substances. Use a 5-gallon bucket with small holes drilled through the bottom to apply wetting agents overnight. By using this method you will not disrupt daytime activities.
Ophiosphaerella dead spot
In the fall of 1998, researchers at the University of Maryland identified this new disease on creeping bentgrass. It appeared in Maryland, Virginia and Ohio with symptoms similar to dollar spot. Researchers subsequently called it "ball-mark disease." The causal organism is in the genus Ophiosphaerella.
You will first see this disease appear as small spots, 1- to 2-centimeters in diameter. As the symptoms progress, the spots may eventually reach 8 centimeters in diameter. You will recognize symptoms similar to spring dead spot. However, you will not see the characteristic dark strands of spring dead spot on the root surface. When diagnosing for Ophiosphaerella, look for reddish-brown spots around the periphery of the infected turf patch. You may also see pinkish mycelia on newly infected leaves. At this point, few reports on the epidemiology, symptomatology and control of this disease are available. However, the next time you see symptoms of dollar spot, don’t automatically assume that it is. It may be ball-mark disease.
Dr. Andy Hamblin is an assistant professor at the University of Illinois, (Urbana-Champaign, Ill.).
Tree andornamental pathogens
The number ofdiseases threatening
tree and ornamental plantings is indicative
of the diversity within this group of plants.
By James Houx,technical editor
Sudden oak death
Researchers believe that the primary pathogen of this new disease is a new species of Phytophthora. First discovered in California in 1995, this disease affects tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia) and black oak (Q. kelloggii).
Until recently, the disease was positively identified only in these trees. However, in January 2001, researchers confirmed that the new Phytophthora has been found on rhododendrons in California. They also confirmed that the same fungus is infecting rhododendrons in Germany and the Netherlands. Researchers don’t know where the pathogen found in the rhododendrons originated. However, it is similar to a Phytophthora species that infected rhododendrons in Germany and the Netherlands in the 1990s.
Other pests, such as the fungus Hypoxylon thouarsianum and Ambrosia beetles then attack the weakened trees. Also, another fungus species, Diplodia, may contribute to the symptomatology.
Plum pox potyvirus
Plum pox was discovered for the first time in the United States in the fall of 1999 in an orchard in Pennsylvania. The virus causes stone-fruit tree species to produce worthless, blotched or misshapen fruit. It is spread through seed and via insect vectors, but it does not kill trees. Infected plants may not show symptoms for several years, so determining when infections occur is nearly impossible. However, researchers believe that the virus may have existed in Canada before it was discovered in the United States.
"Fortunately, we have the D-strain which is less virulent than some of the strains seen in Europe and Egypt," says Gary Clement with the USDA, Animal and Plant Health Inspection Service (APHIS) in Carlisle, Pennsylvania. Further, Clement states that although this virus has infected ornamental plant species such as purple-leaf plum in Europe, researchers have not discovered the virus in wild or residential settings or in ornamental nurseries.
This disease was first identified in the United States in August 2000 in Georgia. At this time, the exact identity is unknown, but samples indicate that it is a Puccinia species and perhaps Puccinia hemerocallids. It has also been positively identified in three other states: Florida, Alabama and South Carolina. However, a survey is being conducted this spring and summer by state Department of Agriculture personnel to determine the presence of the disease elsewhere. It could be more widespread than currently known.
For more information and to keep current with this disease, go to www.ces.uga.edu/agriculture/plantpath/daylilyrust.html. This website is maintained by Dr. Jean Williams-Woodward, a University of Georgia extension plant pathologist.
An outbreak of elm yellows has occurred in the Chicago area, according to Karel Jacobs, a research pathologist with the Morton Arboretum (Lisle, Ill.). The disease, usually not found as far north as Chicago, is caused by a bacteria-like organism called a phytoplasma. A phytoplasma is a plant-infecting, wall-less cluster of organelles that behaves like an organism. It is more closely related to bacteria than fungi and needs a living host to survive.
Elm yellows, also called elm phloem necrosis, differs from Dutch elm disease (DED). Once infected, the leaves turn yellow and may eventually fall off. Unlike DED, this disease infects the entire crown and does not discolor the sapwood. As the alternate name suggests, this pathogen infects phloem tissue and can be spread by phloem-feeding insects.
Other diseases reported
Researchers indicate that some well-known diseases are re-emerging in locations where they are not typically seen or have not been a problem. Mary Small from the Colorado State University cooperative extension has seen higher incidence of apple scab, Venturia inaequalis, the past few years. Although common in humid climates east of the Rocky Mountains, this disease usually is not seen in the drier elevations of Colorado. Small sees the disease most often in mature, overcrowded landscapes that need pruning. Small attributes the increased incidence of apple scab to moist weather patterns and says that the disease was a particular problem 2 years ago during an excessively moist period.
Melodie Putnam with the Oregon State University plant clinic has recently seen a strain of Phytophthora syringae emerge that causes stem cankers on some species of Malus. Researchers originally saw the stem canker symptoms during 1997 when the region experienced flooding and left many trees in standing water.
Putnam, like Small, sees weather being the major influence in the incidence of these diseases. "We had some wet weather in 1997 and 1998. However, last year was relatively dry, and we experienced no serious disease problems," states Putnam.
Another phytoplasma, lethal yellowing (LY) of palm trees, continues to threaten non-resistant palm populations in Florida. The disease was first noticed in the Florida Keys in the 1950s and soon decimated palm populations in Key West. By the early 1970s, the disease was seriously affecting palms in the Miami area, and in 1979, it was found in Texas. The disease is slowed by chemical treatments, but eradication has not been possible. Most agencies stress the planting of resistant palm species as the most important control strategy.
The factors affecting the emergence and re-emergence of diseases are complex and not fully understood. For instance, weather conditions greatly affect disease development, but quantifying temperature and moisture in relation to disease cycles is at best difficult. Besides, weather is uncontrollable and can only be used abstractly to predict pathogen development.
Most of the researchers I spoke with, feel that you should be aware of cultural practices that promote healthy, vigorous, but not luxuriant, plant growth. These practices can provide a front-line defense against the unseen presence of pathogens.