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Verdure: Just dew it


Leaf wetness plays a well-documented role in the development of dollar spot (Sclerotinia homoeocarpa F.T. Bennett). It’s also well known that removing dew in the early morning can lessen dollar spot symptoms, as it reduces prolonged leaf wetness. Of course, dew removal alone may not be enough for commercially acceptable dollar spot control, and thus fungicides are often applied throughout the year to obtain optimal control. These fungicides are often applied early in the morning, when dew may be present. We know that dew removal lessens dollar spot, and we know that fungicides are a valuable tool for dollar spot control, but what about the relationship between those two dollar spot control tools? Does spraying fungicides when dew is present affect the quality of dollar spot control?

This is the question that graduate student Yu Huang — led by Drs. Kaminski and Landschoot of Penn State — set out to investigate. Additionally, because so many superintendents combine plant growth regulators (PGRs) as a part of their spray program, a specific PGR, trinexapac-ethyl (TE) (Primo Maxx, Syngenta), was also included in the study. Using a 9-year-old stand of Penneagle creeping bentgrass (Agrostis palustris L.)/annual bluegrass (Poa annua L.) maintained as a fairway, treatments were combinations of TE, dew removal and three fungicides.

Specifically, the study had the following treatments: (1) dew management treatments: dew not removed; dew removed, mowed; and dew removed, not mowed; (2) fungicides chlorothalonil (Daconil Ultrex 82.5 WDG, Syngenta), propiconazole (Banner Maxx, Syngenta) and iprodione (Chipco 26GT, Bayer); and (3) the PGR TE. Dew was removed by mowing each respective area, with the cutting reels not engaged for the “dew removed, no mow” treatment. All treatments were applied over two years in late summer and spring for 10-week treatment periods (conducted as three separate experiments). Collected data included quantification of dew present, dollar spot severity, and progress of the disease over time.

In each 10-week experiment, the level of dollar spot was considered moderate to severe. Application of any fungicide significantly reduced dollar spot, and in two of the experiments, the application of iprodione produced better dollar spot control than the other two fungicide treatments. 

Benefits from the application of TE often became apparent as each experiment progressed. At two to three weeks after the start of each experiment, plots treated with TE had less disease (typically around 20% to 40% less) compared with plots that did not receive the PGR. However, the use of TE alone (no fungicide applied) was not enough to produce commercially acceptable dollar spot control. This effect of TE on dollar spot control was usually observed within a TE-by-fungicide interaction. An interaction occurred because the effect of TE on dollar spot was evident when fungicides were not sprayed, but not when they were. In the absence of fungicides, TE sometimes (but not always) reduced dollar spot. When fungicides were applied, applying TE did not change the level of dollar spot control. Basically, for dollar spot control, application of the fungicide trumped application of the PGR.

But what about the presence of dew? Over the course of the three experiments, the dew treatments significantly affected dollar spot only once. This occurred in fall (Sept. 7), when bentgrass in which the dew had been removed but no mowing had occurred had less dollar spot than bentgrass plots where dew had not been removed. Basically, the presence or absence of dew had little influence on fungicide performance or on residual effectiveness of the fungicides. It was noted that this work focused solely on the effectiveness of TE and fungicides as affected by dew. 

Source: Huang, Y., J.E. Kaminski and P.J. Landschoot. 2015. Regulation with trinexapac-ethyl and dew removal at the time of fungicide application did not influence dollar spot control. HortScience 50:496-500.

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