Influence of Early and Late Season Fungicide Applications on Dollar Spot Severity.

Dollar spot (Sclerotinia homoeocarpa F.T. Bennett) is becoming increasingly difficult to control on golf course fairways. The study objectives were to: 1) determine the ability of non-traditional fungicide application timings to control dollar spot; 2) assess the influence of droplet size on disease severity; and 3) evaluate the influence of post-application irrigation on fungicide performance. This study was conducted on a mixed stand of annual bluegrass (Poa annua L.) and creeping bentgrass (Agrostis stolonifera L.) at Stanley Golf Course in New Britain, CT. Four fungicides (boscalid, triadimefon, vinclozolin, and chlorothalonil + propiconazole) were applied on one of four dates (18 or 24 October 2006; 24 April 2007; or 23 May 2007). Fungicides were applied using one of two nozzles designed to produce either a very coarse or medium droplet size. Treatments were either allowed to dry on the foliage or subjected to post-application irrigation (2.5 mm). Treatments applied at two of the non-traditional timings (18 October 2006 or 24 April 2007) reduced dollar spot (44% to 97%) when compared to the untreated control on all dates in the rating period (23 May to 4 July 2007). On 4 July, dollar spot severity was reduced between 36 and 64% in plots treated in October or April, when compared to control plots. Only plots treated on 24 April, however, had levels of dollar spot similar to plots treated on 23 May. In most instances, disease severity increased in plots receiving post-application irrigation. Nozzle type generally had no impact on dollar spot, but the efficacy of chlorothalonil + propiconazole was improved when applied using a nozzle that produced a medium droplet size. Data suggest that early season applications may delay the onset of dollar spot symptoms and that nozzle type and post-application irrigation had negligible and reduced influence on fungicide performance, respectively.