Verdure: Out, damned (dollar) spot!

Recent social media chatter prompted a fresh look at a study that put various nitrogen sources’ ability to reduce dollar spot to the test.


Lately, there have been some really interesting discussions in a Facebook turfgrass group that are related to the effects various nitrogen fertilizers may have on dollar spot (caused by Sclerotinia homoeocarpa F.T. Bennett).

The topic created enough discussion that I thought I’d see what was in the scientific literature that addressed the topic. I found a 2002 paper published by Dr. Pete Dernoeden of the University of Maryland, in which he and his graduate student examined a range of nitrogen sources for their effects on dollar spot, organic matter and soil microbial activity.

Specifically, for the study, they used an established plot of Southshore creeping bentgrass that was managed as a fairway. There were three replications of each treatment. The specific treatments were commercial nitrogen sources from a variety of waste sources, including composted sewage sludge (biosolid), activated sewage sludge and poultry waste materials (both composted or activated). Trade names of the various products included Milorganite, Earthgro, Sustane, Ringer, Compro and Scotts (the company’s organic product).

The synthetic organic nitrogen fertilizers urea and sulfur-coated urea (SCU) were also included, as were plots that received no nitrogen. All materials were applied to the plots in October, November, December and May of each year at a rate of 1 pound nitrogen/1,000 square feet (50 kg/hectare). These nitrogen sources were applied for a total of seven continuous years, with the data discussed here collected in the last three years of the applications.

Collected data included turfgrass quality, thatch depth and organic matter content, leaf tissue nitrogen, and populations of plant parasitic nematodes. In the summer months, total soil microbial activity was measured, and that data was collected every two weeks.

Finally, dollar spot infection was measured by taking a weekly count of the number of infection centers in each plot from 1998 to 2000. The number of infection centers was a guide to determining the level at which the turf was considered unacceptable by the standards of a turfgrass manager.

Turfgrass quality varied depending on the nitrogen source, with highest turf quality found in plots fertilized with the slow-release source SCU or with a commercial product made from poultry waste (Ringer Lawn Restore). None of the nitrogen sources reduced thatch (as compared with the unfertilized control plot), and application of one source (Compro, a composted sewage waste that contained wood chips) increased thatch depth.

The only treatment that significantly and consistently increased organic matter — only in the upper 1 inch (2.5 cm), and not deeper — was a nitrogen source from poultry waste (Sustane). None of the nitrogen sources reduced any populations of the plant parasitic nematodes that were measured.

None of the applied nitrogen sources consistently reduced dollar spot when averaged over the growing season. However, application of the poultry waste material Ringer Lawn Restore or the synthetic organic-source urea did delay the time until the dollar spot reached an unacceptable threshold.

When these two products were applied, dollar spot did not exceed the unacceptable threshold until mid-June, instead of reaching it in May. This was not an effect of any microbial stimulation, but was rather a result of better nitrogen availability from those products. In fact, the composted sewage waste product that contained wood chips (Compro) decreased turf quality and increased dollar spot as compared with the unfertilized plots, a result of decreased nitrogen availability from that product.

In this study, any ability of organic nitrogen products to affect dollar spot was more a result of nitrogen availability from the products and not related to any stimulation of the microbial pool or increased soil organic matter content.

Source: J. Graham Davis and Peter H. Dernoeden. 2002. Dollar spot severity, tissue nitrogen, and soil microbial activity in bentgrass as influenced by nitrogen source. Crop Science 42:480-488.

Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.