Verdure: That’s a sappy relationship

Can different in-field plant measurements — a clipping sap test or handheld reflectance meters — be used assess turf’s foliar nitrogen status to minimize anthracnose?


For years and years, a basic tenet of nitrogen (N) fertilization of turfgrass is that we base it on crop response. You watch it grow, you watch the color, and you apply nitrogen as the turf shows it’s ready. This is not to say that people have not spent entire careers trying to calibrate various soil and tissue tests for nitrogen that would allow us to make nitrogen fertilizer recommendations.

With the advent of various remote sensing technologies, there has been a renewed interest in nitrogen fertilizer recommendations based on sensor technologies, and this month’s paper is a great example of that. At the University of Connecticut, John Inguagiato, Ph.D., and Karl Guillard, Ph.D., examined three different types of in-field plant nitrogen measurements, and examined how those related to nitrogen fertilization, foliar nitrogen and anthracnose in an annual bluegrass putting green.

For two years, an annual bluegrass putting green was fertilized with eight rates of nitrogen (as urea), applied every 14 days from May to August at 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.75 pound N/1,000 square feet/14 days (0, 0.24, 0.49, 0.98, 1.46, 1.95, 2.44 and 3.66 grams/square meter/14 days).

The severity of anthracnose was determined by counting damaged areas in each plot using a line-intercept method. This was done approximately weekly in June and July of each year.

At the same time that data was being collected, researchers also collected clippings from each plot, and these were dried and analyzed for total nitrogen. A sap extract was collected from fresh clippings and extracted using a plant press, and was used to determine sap nitrate-nitrogen concentration. Last, commercially available handheld meters were used to collect normalized difference vegetative index (NDVI) and chlorophyll meter readings from each plot.

The severity of the anthracnose decreased as the nitrogen rate increased, up to an estimated nitrogen rate of 0.26 pound N/1,000 square feet/14 days. This nitrogen rate was similar to those observed in previous studies. Additional nitrogen did not further reduce anthracnose.

With an identified nitrogen rate for minimal anthracnose, the next question was: How well did any of the meters or the sap test predict this nitrogen status, and could the meters or the sap test be used to guide nitrogen fertilization or assess leaf nitrogen status to minimize anthracnose?

Well, the use of clipping sap was a complete fail. No relationship was found between clipping sap and the turf area infested with anthracnose.

The NDVI and chlorophyll meters worked better, and there was a linear relationship between foliar nitrogen and the NDVI/chlorophyll readings, up to a point, after which the relationship plateaued. This plateau occurred at a foliar nitrogen content of 3.7% to 4.0% nitrogen in the annual bluegrass.

So, if foliar nitrogen exceeded 4% nitrogen, the NDVI or chlorophyll meter did not produce a value that correlated well. Below 4% nitrogen, they did.

The good part was that this foliar nitrogen content (3.7% to 4.0% N) was well beyond the critical foliar nitrogen needed to minimize anthracnose (which is ~3.4% N), so the meters could be used to help manage foliar nitrogen to reduce anthracnose.

To use this research on your golf course, fertilize a small area of your green or nursery to maximize the greening response (reference plot). Use handheld reflectance meters to measure the reference plot and monitor remaining greens. Fertilize remaining greens so they are at 94% to 98% of the reading from the reference plot for NDVI and at 73% to 93% for chlorophyll index. This should maintain sufficient foliar nitrogen (3.4% N) to minimize anthracnose severity in annual bluegrass putting green turf.

Source: Inguagiato, J.C., and K. Guillard. 2016. Foliar N concentration and reflectance meters to guide N fertilization for anthracnose management of annual bluegrass putting green turf. Crop Science 56:3328-3337.

Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the 2019 president of the Crop Science Society of America. She is a 21-year member of GCSAA.