You can beat back this common turf disease with cultural practices and chemicals that are money in the bank.
While not a major threat to turf survival, dollar spot is certainly capable of causing serious insult and temporary thinning of the playing surface. As dollar spot primarily affects the plant leaf, long-term damage to the root system rarely occurs, with the blades regrowing once conditions that promote healthy growth have changed or effective treatments have been implemented. In severe cases, when infection and reinfection occur on multiple occasions during the growing season, the individual plants may die because of depletion of nutrients.
When turf is mildly to moderately infected with dollar spot, golf turf is usually able to function reasonably well if managed according to integrated pest management principles.
Overall, disease appearance on closely mowed greens is round, quarter to silver dollar-sized spots that are light tan-bleached. On higher-cut turf, symptoms appear in a mottled, somewhat random pattern of round spots containing bleached turf blades. Within the spots, individual leaf blades develop a characteristic lesion that may be up to 1 inch long, is light tan to white with reddish-brown margins, and spans the entire width of the blade. Thinking outside the box a bit, it’s easy to imagine that a vandal placed small droplets of bleach in the center of the grass blades in an effort to raise the ire of golfers and superintendents alike.
When morning dew is present on the turf, soft, cottony mycelial growth commonly extends from the blades at various locations. This is a good identification aid for new golf course workers. However, soon after sunlight and/or warm temperatures become influential, the mycelia shrivel and retreat to the blade.
Dollar spot is sometimes confused with brown patch, another fungal disease. It’s also mostly considered to be a leaf-affecting disease and produces similar lesions on the blades. Upon closer inspection, the spots appear irregular, with wavy or angled margins that either begin at the leaf edge and extend to the center of the leaf or cover the entire leaf surface, as opposed to dollar spot lesions, which most often span the complete width of the leaf blade. The center of the lesions is light gray to tan rather than light tan to bleached in color for dollar spot. Additionally, the blade tips as well as the middle of the blades are often covered with lesions in the case of brown patch.
Warm days and cool nights that produce dew and high humidity are ideal for the growth of dollar spot. Within the turf stand, dense canopies tend to favor development more than thinner ones. And days with low wind speed are better for infection than hot, windy weather. Under these conditions, the dollar spot fungi produce mycelia on the surface of blades, which is often spread by clippings, mowers, golf cars and foot traffic.
Dollar spot can occur just about anytime from late spring to late autumn, depending on changes in the weather. Periods of cool weather in the midst of a hot summer can often cause a resurgence of dollar spot symptoms. The disease usually shuts down once temperatures are above 90 degrees Fahrenheit. Turf that is under low-fertility regimes is usually at a higher risk of infection. Nitrogen-starved turf has been shown to be more susceptible, largely because a greater amount of dead or dying tissue is present as compared to those maintained under higher fertility levels. Senescent foliage provides a good food source for the fungus, and also serves as a disease-spreading agent from infected to healthy plant tissues.
Soil moisture can also be a factor in disease development and severity. Generally, dry soils enhance the severity of the disease, most likely because of greater drought and nutrient stress on the turf plants, which produces a weaker stand overall.
- Nitrogen starved turf is more susceptible to dollar spot.
- Drought-stressed turfs are more likely to be infected. Irrigate where there’s sufficient water to wet the entire root zone.
- Thatch removal is beneficial in that it removes sources of dollar spot inoculum.
- Several classes of fungicides are available to control dollar spot.
Several cultural practices are helpful in reducing the incidence of dollar spot. As mentioned above, drought-stressed turfs are more likely to be infected. Irrigate in a manner where there’s sufficient water to wet the entire root zone. Keeping the roots moist, not soggy or dry, is recommended.
Early-morning applications assist in the avoidance of prolonged periods of leaf wetness, which is conducive to disease development.
Thatch removal is beneficial in that it removes sources of dollar spot inoculum, improves drainage and percolation of rainfall and applied water, and reduces drought and nutrient stress. Thatch layers in the 0.5-to 0.75-inch range should be considered for removal and possible subsequent topdressing. In a similar way, compacted soils stress the turf plants and slow the expansion of the roots and lower the recuperative potential of the stand.
Since turfs that are maintained under low-nitrogen fertility are prone to infection, light and frequent applications are helpful in terms of disease management and encouragement of even turf growth. On the other hand, close monitoring should be conducted, as overfertilization can also promote infection. Additionally, high fertility facilitates other diseases including brown patch, Pythium blight and bipolaris leaf spot.
Reduced or insufficient air circulation over a green, tee or fairway can also facilitate dollar spot incidence. Steps taken to increase airflow will assist with management of the disease. In some cases, the use of fans to circulate air is a possibility. In others, removal of woody plants or stems, limbs, codominant trunks (which may need to be removed anyway to reduce the possibility of tree failure) can be helpful. It’s wise to consult with university extension faculty or a certified arborist for suggestions on reducing the risk of damage to people and property, as well as logical choices for removal that will reduce the incidence of dollar spot.
Many fungicides are registered for the control of dollar spot, but as with most fungal diseases chemicals aren’t a panacea. Be careful not to cause the development of resistance by using the same fungicide over and over. To limit the possibility of fungicide resistance, it’s best to alternate the usage of products from several different chemical classes or modes of action.
Several classes of fungicides are available including benzimidazoles, demethylation inhibitors, dicarboximides and nitriles. Boscalid, chlorothalonil, fenarimol, iprodione, metconazole, myclobutanil, penthiopyrad, propiconazole, tebuconazole, thiophanate-methyl, triadimefon, triticonazole and vinclozolin are among the products shown to be most effective. There are some new fungicides on the market that provide excellent dollar spot control.
Close monitoring of rainfall, wind speed, humidity, temperature and other weather conditions is helpful in late spring. Turfs with a history of dollar spot should be treated when weather is favorable for disease development. Depending on severity, fungicides should be applied at either a 7- to 10-day or 14- to 21-day interval depending on the product choice and residual activity for the active ingredients.