Influence of Summer Spoonfeeding Six Nitrogen Sources On Dollar Spot Severity and Chlorothalonil Efficacy in Creeping Bentgrass

Abstract

Dollar spot (Sclerotinia homoeocarpa F.T. Bennett) injury can be suppressed by applications of nitrogen fertilizer to creeping bentgrass (Agrostis stolonifera L.). Previous studies evaluated applications of relatively high rates of nitrogen (≥ 24 kg N/ha) for dollar spot suppression. In this study, ammonium nitrate, ammonium sulfate, urea, potassium nitrate, calcium nitrate, and 20-20-20 were applied to fairway height creeping bentgrass on two week intervals at 7.3 kg N/ha in 2008 and 2009. The N sources were applied alone or tank-mixed with a low rate of the contact fungicide chlorothalonil (1.6 kg a.i./ha in 2008 and 3.2 kg a.i./ha in 2009). It was hypothesized that N + chlorothalonil would improve the level or longevity of dollar spot control compared to chlorothalonil alone. Treatments were initiated prior to the onset of dollar spot symptoms. Area under the disease progress curve data revealed that only ammonium sulfate had reduced dollar spot in both years, whereas ammonium nitrate and 20-20-20 did not reduce dollar spot over the season in any year. There was a trend for less dollar spot control in plots treated with N + chlorothalonil versus the low chlorothalonil rate alone in 2008, but in 2009 the deleterious effect was significant. It is likely that N stimulated plant growth and thus the contact fungicide was more rapidly removed from plants by mowing or there was more unprotected tissue for a longer period compared to plots treated with chlorothalonil alone. Ammonium nitrate, urea and 20-20-20 were not injurious. Potassium nitrate and calcium nitrate caused a short-lived tip burn, but ammonium sulfate elicited a severe burn since the N sources were not watered-in immediately.