Dollar spot disease of turfgrass is caused by that pesky fungus Clarireedia jacksonii (formerly named Sclerotinia homoeocarpa). Turfgrass science textbooks traditionally described it as a disease of poorly nourished, under-fertilized turf. Dollar spot is one of the most common and troublesome turfgrass diseases, but it may not have been as severe in those “good ol’ days” due to higher nitrogen fertilizer inputs, higher mowing heights and maybe those heavy-metal-based products used back then.
In the 1980s, creeping bentgrass putting greens typically received 5 to 7 pounds nitrogen per 1,000 square feet (~245 to 345 kilograms nitrogen per hectare) per year. Since the early 2000s, putting greens may get 1 to 3 pounds nitrogen per 1,000 square feet (~50 to 150 kilograms nitrogen per hectare) per year. With less nitrogen applied today and dollar spot often a chronic problem from spring into early fall, can more nitrogen fertilizer provide some help to control dollar spot? Paul Koch, Ph.D., at the University of Wisconsin led a team to explore this relationship between dollar spot and nitrogen.
Two identical field trials were conducted — one at the O.J. Noer Turfgrass Research and Education Facility in Madison, Wis., and the other at North Shore Country Club in Glenview, Ill., under the supervision of Dan Dinelli, CGCS — for three consecutive years during 2015, 2016 and 2017. Both sites consisted of a creeping bentgrass (Penncross) sand-based putting green, mowed at 0.125 inch (3.175 millimeters) with clippings removed each time, and no additional nitrogen other than the urea (46-0-0) fertilizer treatments. Urea represents a water-soluble, quickly available nitrogen source.
Four nitrogen rates of 0.1, 0.2, 0.4 or 0.6 pounds nitrogen from urea per 1,000 square feet (4.9, 9.8, 19.5 or 29.3 kilograms nitrogen per hectare) were applied every two weeks from spring through summer, for a total of 10 applications per year. This resulted in a total of 1, 2, 4 or 6 pounds nitrogen per 1,000 square feet per year (49, 98, 195 or 293 kilograms nitrogen per hectare per year).
Non-fertilized control plots and plots with a fungicide program only also were included for comparison. The fungicide program consisted of several products and was applied at labeled rates on a rotation schedule starting in late May through late September. All nitrogen and fungicide treatments were applied as a liquid in 2-gallon water carrier per 1,000 square feet (814 liters per hectare). Individual test plots measured 4 feet × 6 feet (1.2 meters × 1.8 meters), and all treatments were replicated four times. Dollar spot occurred naturally at both sites.
Did nitrogen affect dollar spot? Combining all data, only the highest nitrogen rate of 6 pounds nitrogen per 1,000 square feet per year consistently reduced dollar spot severity compared with the non-fertilized control. No nitrogen rate suppressed dollar spot equal to the fungicide program’s ability to effectively control dollar spot. In the absence of fungicide use, however, the only chance at reducing dollar spot severity is at least 0.6 pounds water-soluble nitrogen per 1,000 square feet every two weeks from spring through the summer.
Despite that much nitrogen applied, the researchers didn’t think the turf simply “outgrew” infected foliar symptoms, and therefore infected leaf tissues were simply mowed off. They suggest other factors at work here, such as beneficial soil microbial populations, plant defense mechanisms or pH of the leaf surface influenced by higher nitrogen inputs.
In addition, turfgrass clippings were collected and analyzed for leaf nitrogen. A decline in dollar spot activity was observed at ≥4.5% leaf nitrogen content. Would it be possible to maintain foliar nitrogen content at ≥4.5% without “pumping” the plants full of nitrogen? More work is needed to explore the use of plant growth regulators and higher fertilizer nitrogen rates to reduce dollar spot severity.
This investigation of nitrogen rate and dollar spot is a great example of a thorough field study conducted concurrently on a university turf research facility and a golf course under real world conditions. But what about nitrogen source and dollar spot? This study also looked at different nitrogen sources for their possible influence on dollar spot severity. In next month’s Verdure, we’ll examine that more closely.
Source: Townsend, R., M.D. Millican, D. Smith, E. Nangle, K. Hockemeyer, D. Soldat and P. Koch. 2021. Dollar spot suppression on creeping bentgrass in response to repeated foliar nitrogen applications. Plant Disease 105(2):276-283 (https://doi.org/10.1094/PDIS-05-20-1031-RE).
Mike Fidanza is a professor of plant and soil science in the Division of Science at the Penn State University Berks Campus in Reading, Pa. He is a 20-year member of GCSAA.