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Myth busters
Think you know everything there is to know about fungicides? Think again. We clear up the confusion about one of the most important tools in a superintendent’s arsenal.
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Many superintendents throughout the country labor under misconceptions about turf disease control. Whereas weed control and insect control are fairly obvious — you either see the weed or insect or you don’t — diseases are more insidious. You can’t really see fungi unless they appear above ground as mycelia or mushrooms, and by then, it may be too late.
Several widely believed myths about disease management appear below. The realities may surprise you.
Myth No. 1: Bermudagrass doesn’t get diseases
For a long time, bermudagrass was considered nearly bulletproof when it came to diseases, probably because of its aggressive, indestructible nature. It is true that common bermudagrass appears to have few disease issues, except for the occasional occurrence of spring dead spot.
In newer bermudagrass cultivars grown on putting greens, however, diseases are definitely becoming more of an issue, perhaps because methods of identifying pathogens have improved dramatically in recent years. The increase in bermudagrass diseases also could stem from our quest for the perfect golf course. The widespread use of ultradwarf bermudagrasses on putting greens and the intense management of these grasses have made them more like creeping bentgrass in their susceptibility to diseases and requirement for fungicides.
Curvularia blight is caused by pathogens of the genus Curvularia and is thought to be stress-related. Curvularia blight has become common in midsummer on ultradwarf bermudagrass putting greens and does respond to fungicide treatment. Another disorder called bermudagrass decline, caused by Gaeumannomyces graminis graminis, a root pathogen, also has become more of an issue. Rhizoctonia leaf, sheath blight and even dollar spot have been observed more frequently on bermudagrass. Fairy ring, caused by one of many basidiomycete fungi, has also become a perennial problem on sand-based putting greens.
Preventive fungicide programs to manage these diseases have become increasingly common.
Myth No. 2: Nitrogen fertilization causes disease
Probably one of the most misunderstood phenomena of managing turfgrass disease is the influence of nitrogen fertility. A long-held belief is that nitrogen fertilization will cause or increase turfgrass disease pressure. Although this may be true of some diseases, we are finding that the disease problems on golf courses often are caused by just the opposite — low nitrogen fertility.
A good example of this is anthracnose, which has devoured Poa annua on many putting greens throughout the country. We have learned that the low fertility rates used to keep greens lightning fast have partially contributed to the severity of this disease. Several other “new” diseases discovered on the new generation of creeping bentgrasses appear to be more severe on putting greens that are kept “lean and mean.”
In reality, spoon-feeding greens at 0.10 to 0.125 pound of nitrogen per week will keep diseases manageable and speeds fast. Plant growth regulators also help keep green speeds fast without encouraging anthracnose and other disease problems.
Myth No. 3: Contact fungicides are necessary for curative disease control
When superintendents encounter a disease breakout, their first reaction is to reach for a quick solution. Many believe this quick solution is a contact fungicide. Often, they can see active mycelia on the plant, and they believe a contact fungicide will kill it. However, when a disease is active, it resides within the plant. Contact fungicides stay on the outside of the plant, protecting the plant from infection.
It is true that most contact fungicides will stop the growth of mycelia and cause their collapse. But a contact fungicide will not stop disease already within infected plants. An effective systemic fungicide is needed to “cure” infected plants. It also will stop the growth of the mycelia on the outside of the plant.
A good solution for most turf disease problems is to use a fungicide with both contact and systemic properties. An alternative is to combine a contact fungicide with a systemic fungicide.
Myth No. 4: Fairy ring symptoms always include mushrooms and rings
Stereotypical fairy ring symptoms consist of a ring of mushrooms and a green ring of lush growing turf. However, fairy ring symptoms are as diverse as the 60 or so fungi that can cause this disease. Not all of these signs or symptoms involve mushrooms and rings. For example, fairy ring symptoms might be localized dry spots in the shape of a ring or many partial rings. You might see mushrooms alone or green rings by themselves. Other symptoms include wilting in a circular pattern or coalescing rings — sometimes in the shape of a Mickey Mouse head!
The reason for these diverse symptoms is that fairy ring fungi do not attack the turf; they inhabit the soil, growing on organic matter. Many of the surface symptoms on the turf are caused by the pathogen modifying the soil and releasing nitrogen, toxins or a byproduct that can lead to hydrophobic areas of soil.
These symptoms often can be prevented with proactive treatments of preventive fungicides on greens that have a history of fairy ring.
Curative fungicide applications are not as reliable and will not control localized dry spot. Be aware that other diseases can cause symptoms similar to fairy ring.
Myth No. 5: If I see mycelia, it must be pythium blight
Pythium blight is infamous for its cottony, white mycelia. This white fuzz, clearly visible on turf plants in the early morning hours, can develop rapidly overnight during hot and wet conditions. However, a diagnosis of pythium blight based on this single sign may not be accurate.
Several other diseases can develop white mycelia under the right conditions, and an innocent spider web may be misdiagnosed as pythium blight. Active dollar spot can produce mycelia visible in the early morning hours in the presence of dew. Even brown patch can develop whitish mycelia in the early morning hours. Correct diagnosis of these diseases may only be possible with microscopic identification. An accurate diagnosis should determine the proper selection of fungicide.
Treating brown patch or dollar spot with a fungicide developed to combat pythium blight may not be a good return on investment. Worse yet, treating pythium blight with a dollar spot fungicide can lead to disastrous results and plenty of dead grass.
Myth No. 6: Fungicides kill diseases and sterilize the soil
Pesticides are a group of compounds used to control pests, whether they are insects, weeds or diseases. Fungicides are a specific type of pesticide used to manage plant diseases. The true meaning of the word fungicide is “to kill fungus.” Though the suffix “-cide” refers to killing, the meaning is not completely accurate when it comes to fungicides.
Most fungicides currently available for managing turfgrass diseases are actually considered fungistats. The suffix “-stat” means to slow growth or to prevent from growing. Most plant pathogens that cause disease continually reside in the soil, plant or organic matter. They can be in the form of a saprophyte, which feeds on thatch, or in a dormant, resting stage of the fungi, such as spores or hyphae, which serve as innoculum. When conditions are right, the pathogen will infect the turf and cause disease.
Fungicides must be on or in the plant to prevent the fungus from attacking or colonizing plant tissues. Once they are applied, fungicides may prevent fungi from growing in the soil and may reduce the population of fungi, but they will not completely eradicate the pathogen.
Thus, fungicides do not kill fungi outright. This is why fungicides may need to be applied repeatedly when conditions are favorable for disease activity.
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