In last month’s Verdure, we reviewed research that looked at nitrogen rate and dollar spot (Clarireedia jacksonii, formerly Sclerotinia homoeocarpa) on creeping bentgrass putting greens. Can you simply apply more nitrogen and control dollar spot as effectively as fungicides? The answer is no, although you can reduce dollar spot severity by applying ≥0.6 pounds water soluble nitrogen per 1,000 square feet (29.3 kilograms nitrogen per hectare) every two weeks from spring through the summer. Of course, this high nitrogen application rate and frequency may not be practical.
But what about nitrogen source? Paul Koch, Ph.D., from the University of Wisconsin, also led his team to examine the relationship between dollar spot and nitrogen source.
Again, two identical field trials were conducted — one at the O.J. Noer Turf Research Facility (Madison, Wis.), one at North Shore Country Club (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 daily at 0.125 inch (3.175 millimeters) with clippings removed each time and no additional nitrogen other than the fertilizer treatments.
Three nitrogen sources were evaluated: calcium nitrate (15% nitrogen-0% phosphorous pentoxide-0% potassium oxide), which has an alkaline effect on soil pH; ammonium sulfate (21N-0P2O5-0K2O), which has an acidic effect on soil pH; and ammonium nitrate (33N-0P2O5-0K2O), which has no or a neutral effect on soil pH. In 2015 and 2016, all fertilizers were applied at 0.2 pounds nitrogen per 1,000 square feet (9.8 kilograms nitrogen per hectare) every two weeks from spring through summer, for a total of 10 applications per year, or 2 pounds nitrogen per 1,000 square feet (98 kilograms nitrogen per hectare) per year.
In 2017, based on the nitrogen rate study, the amount was changed to 0.6 pounds nitrogen per 1,000 square feet applied every two weeks, or 6 pounds nitrogen per 1,000 square feet (293 kilograms nitrogen per hectare) per year. Nonfertilized control plots, and nonfertilized plots with a fungicide program only, also were included for comparison. The fungicide program consisted of various products applied at labeled rates on a rotation schedule from May through September.
All nitrogen and fungicide treatments were applied as a liquid in 2 gallons 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 source have an effect on dollar spot? Not really. Overall, none of the nitrogen sources consistently reduced dollar spot severity compared to the nonfertilized control, which had an unacceptable amount of dollar spot activity, and of course acceptable dollar spot control was seen on all fungicide-treated plots. Although none of the nitrogen sources applied alone effectively controlled dollar spot, foliar disease symptoms were lowest in plots treated with calcium nitrate. Creeping bentgrass leaf tissue pH was highest in calcium nitrate plots; however, irrigation water pH at both test sites ranged from 7.8 to 8.5, and it’s unclear what role that may have played. No nitrogen source had any effect on soil pH, which ranged from 7.0 to 7.4 at both test sites.
Current research suggests three possible theories for dollar spot’s response to nitrogen fertilizers as: (1) a buildup of beneficial microbial populations in leaf tissues, thatch and soil that outcompete or “crowd-out” the dollar spot pathogen; (2) increased turf growth enables leaf blades to “escape” as infected tissues and be mowed off; or (3) foliar or soil pH influences the pathogen’s ability to infect leaf tissues or influences the turf’s ability to resist infection. Point No. 2 wasn’t observed with the nitrogen rate study, and point No. 3 wasn’t observed with the nitrogen source study. Current research is focusing on the turfgrass rhizosphere, particularly with fertilizer practices that might influence the structure and function of soil microbial communities and their effect on dollar spot.
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.