The components of the disease triangle (host, pathogen and environment) set the stage for disease development. One environmental component that is commonly accepted as an important factor in the onset of disease is nitrogen fertility. It is an either-or situation. Too high or too low in nitrogen fertility and you can set yourself up for a disease problem. Turfgrass becomes more susceptible to certain diseases such as brown patch or Pythium under high nitrogen fertility; conversely, turfgrasses also become more susceptible to other diseases, such as dollar spot or red thread, under low fertility. You're damned if you do and damned if you don't fertilize.
By exploring the relationship between low nitrogen fertility and disease — specifically dollar spot — you can learn what fertilizers work best for helping your turf avoid dollar spot.
Dollar spot, a disease of turfgrasses caused by the fungus Sclerotinia homeocarpa F.T. Bennett, attacks most turfgrasses grown in the South. Traditionally, the name given to the fungus that caused the disease was Sclerotinia homeocarpa F.T. Bennett, but more recently there has been a name change and the dollar spot fungi in the United States are considered species of the genera Lanzia and Moellerodiscus. Hybrid bermudagrasses and zoysia are highly susceptible to dollar spot; while St. Augustine and centipede are less frequently attacked. The disease occurs from spring through fall and is most active during moist periods of warm days (70° to 85° F) and cool nights (60° F) in the spring, early summer and fall. The disease may be spread from one area to the next by water movement across turfgrass surface, turfgrass management equipment, birds and other wildlife and by humans walking from one turfgrass area to another.
WHAT DOES IT LOOK LIKE?
Dollar spot symptoms vary depending primarily on turfgrass species, mowing height and nutritional level. Regardless of mowing height or turfgrass species, dollar spot is more severe when turfgrasses are maintained at less than optimum nutritional levels, particularly N. On fine-textured and closely mowed bermudagrass the disease appears as round, brown to straw-colored and somewhat sunken spots approximately the size of a silver dollar; thus, the common name “dollar spot.” In coarse-textured grasses maintained at taller cutting heights, the dead spots are larger and more diffuse. Under these conditions, dollar spot can be confused with brownpatch, R. solani. Dollar spot is distinguished by characteristic lesions on the leaf blades of live plants near the border of the affected area. Lesions are light tan with a reddish-brown border, and usually radiate from the margins of the leaf blade.
If the turfgrass is examined when the disease is active early in the day before the dew dries, you can see a grayish-white, cobweb-type growth of fungal mycelium. You can distinguish this mycelium from a cobweb because it is three-dimensional, whereas a cobweb is on a single plane.
INTERACTION OF DOLLAR SPOT AND N NUTRITION
Low nitrogen levels have been reported to increase the severity of dollar spot. Some rather severe outbreaks of dollar spot have been brought under control by the application of nitrogen fertilizer. Some researchers have suggested that the beneficial effects of the nitrogen are due to rapid recovery of the turfgrass during periods of reduced disease activity and recommend the application of a fungicide labeled for the disease to prevent it.
Recently, a study initially designed to evaluate the influence of nitrogen sources under different application regimes on turfgrass growth and quality was conducted. Plots were established at the G.C. Horn Turfgrass Field Laboratory on FloraDwarf bermudagrass, which was maintained at 0.1-inch mowing height. In general, the turfgrass was low in N status when the study was initiated and within 30 days, notable symptoms of dollar spot were observed on some of the plots. One of the original objectives of the study was to evaluate a number of different N sources, (soluble, slow-release and liquid) under application management practices, which are typically used by turfgrass managers on a golf course. A number of turfgrass managers use a liquid N program almost exclusively, and some have difficulty maintaining quality when only solutions of soluble N are applied. We wanted to determine which application regime and N source combination would give the best overall response. Unexpectedly, we obtained varying degrees of dollar spot infestation depending on the N source and application frequency. The observed findings are reported here.
GETTING THE BEST RESPONSE
Ammonium Nitrate was applied at 0.5 pounds N per 1,000 square feet every 15 days in granular form. Severe dollar spot symptoms developed on most of the plots during the first 30 days after the initial application. In previous studies with FloraDwarf bermudagrass, 0.5 pounds of N per 1,000 square feet applied on a biweekly basis was sufficient to sustain the turfgrass at acceptable quality. However, in this study, application of ammonium nitrate at 0.5 pounds N per 1,000 square feet every 15 days resulted in less than acceptable turfgrass quality with dollar spot infesting more than 70 percent of the entire plot area (see photo 1, below).
Ammonium Nitrate and IBDU were applied at 0.5 and 0.25 pounds of N per 1,000 square feet, respectively, every 30 days. In an attempt to reduce the number of applications and labor costs, a once-monthly application regime was attempted. IBDU was the slow-release N source selected because it is one of the few slow-release sources that is marketed in small enough particle size to use on a fine-textured ultra-dwarf bermudagrass, such as FloraDwarf. The incidence of dollar spot was not quite as severe as it was with the ammonium nitrate alone, but was still highly evident and covered about 50 percent of the plot area (see photo 2 on page 12). This is still considered to be an unacceptable turfgrass quality response.
Ammonium Sulfate and IBDU
When ammonium sulfate and IBDU were applied in combination at 0.5 and 0.25 pounds of N per 1,000 square feet every 30 days, the incidence of dollar spot was reduced to about 30 percent of the plot area and was not as severe. (See photo 3 on page 12.) In previous studies, ammonium sulfate has given a longer lasting response than ammonium nitrate and the less severe dollar spot infestation observed here might be a result of ammonium sulfate maintaining a higher level of soil N for a longer period of time.
Being a slow-release N source, IBDU is reported to be more efficient in supplying N to turfgrass under high rainfall conditions. This study was conducted during the months of May, June, July, August and September in Gainesville, Fla. During these summer months more than 50 percent of our annual average rainfall of 56 inches normally occurs. Thus, most turfgrass managers use slow-release N sources in their N management program. Considering the higher potential N efficiency of IBDU, a slightly lower rate of N was applied, 0.75 pounds N per 1,000 square feet every 30 days. The response and overall incidence of dollar spot was striking. More than 80 percent of the experimental plot area was covered with a severe infestation of dollar spot. (See photo 4 on page 13.) Needless to say, application of only 0.75 pounds of N per 1,000 square feet every 30 days as IBDU was not adequate to maintain acceptable turfgrass quality.
Urea-Ammonium Nitrate (UAN) Spray
Many turfgrass managers use solution N sources as the primary means of supplying N to their turfgrasses. These N solutions are generally composed of mixtures of ammonium nitrate and urea and are formulated as 28-, 30- or 32-percent N solutions. The common trade name for these solutions is UAN, which stands for urea-ammonium nitrate solution. In most cases it is easier to apply a spray N solution than it is to apply a granular fertilizer application once you get your spray rig calibrated and your N solution mixed in the spray tank. Some turfgrass managers may apply very small amounts of N solutions frequently through their irrigation system by use of an injection pump and a tank containing their UAN solution. UAN was sprayed on FloraDwarf at 0.5 pounds N per 1,000 square feet every 15 days and by mid-June a severe infestation of dollar spot was observed. This infestation covered more than 60 percent of the plot (see photo 5 on page 13). Thus, under the conditions of this study a spray application of 0.5 pounds of N as UAN every 15 days was not sufficient to maintain turfgrass quality and limit the infestation of dollar spot.
Ammonium Nitrate and UAN
To overcome the difficulties of maintaining putting greens with solution only N applications, many turfgrass managers use a periodic granular N application to augment their solution N fertilizer management program. To one set of plots in this study, an application of 0.5 pounds N per 1,000 square feet as granular ammonium nitrate was made every 30 days to augment the 0.25 pounds of N application every 15 days using UAN. With this regime only a very slight incidence of dollar spot was observed. Less than 10 percent of the plot was covered with dollar spot (see photo 6, above left).
Ammonium Sulfate and UAN
Over a number of years in studies involving bermudagrasses and different N sources, ammonium sulfate has almost always produced a darker green and longer lasting growth response than other soluble N sources. Based on tissue analysis it is thought that the acidification properties of ammonium sulfate results in more Fe and Mn being available to the turfgrass for uptake. In most of our high pH turfgrass soils, a response to an application of Fe and/or Mn can almost always be obtained. Ammonium sulfate was applied in granular form at 0.5 pounds N per 1,000 square feet every 30 days and UAN was applied as a foliar spray at 0.25 pounds of N every 15 days. A very high quality turfgrass was maintained with this application regime on plots, which were adjacent to plots having 80-percent coverage with dollar spot (see photo 7 on page 14). Application of ammonium sulfate appeared to reduce the incidence of the disease.
Ammonium Sulfate alone
Application of ammonium sulfate alone at 0.5 pounds of N per 1,000 square feet every 15 days resulted in superior quality turfgrass (see photo 8 on page 14). As mentioned above, we have been successful maintaining FloraDwarf bermudagrass in previous studies with the application of 0.5 pounds N per 1,000 every 15 days. We were able to maintain a superior quality rating (9 in a 1-to-9 rating scale) with the application of only 0.5 pounds of N per 1,000 square feet every 15 days. As shown in photo 1 on page 12, a less than acceptable quality turfgrass was observed with the same 0.5 pounds of N per 1,000 square feet every 15 days on turfgrass which received ammonium nitrate. The exact cause of this observed response is not known, but it is speculated that the response has to do with the acidification properties of the ammonium sulfate and/or the increased availability of Fe and Mn, or the addition of the sulfur. Needless to say, however, the response was highly visible and desirable.
In summary, quality turfgrass could not be maintained with a spray application of a liquid N solution alone. The incidence of dollar spot was reduced by the inclusion of a granular application of N along with the solution regime. When ammonium sulfate was used in granular form on a biweekly basis or was included on a monthly basis with a biweekly application of UAN, dollar spot incidence was completely eliminated.
Jerry Sartain is a professor of Turfgrass Nutrition and Environmental Nutrient Management in the Soil and Water Science Department at the University of Florida (Gainesville, Fla.).
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