Evaluation of goosegrass response to combinations of topramezone and chlorothalonil

Abstract Few herbicides are registered for goosegrass control in creeping bentgrass turfgrass. Topramezone controls goosegrass and is labelled for use in creeping bentgrass, but potential injury risks lead many turf managers to utilize a low-dose, frequent application program. This type of program increases the likelihood that topramezone treatments will be mixed with fungicide treatments. Previous research found that fungicides can reduce activity of some herbicides, but effects on topramezone efficacy are unknown. Four studies were established between Blacksburg, VA and North Brunswick, NJ in 2021 to determine if chlorothalonil reduces goosegrass control from topramezone. In controlled-environment-dose-response studies, the amount of topramezone needed to reduce goosegrass biomass by 50% increased from 3.04 g ha-1 to 5.27 g ha-1 when chlorothalonil (7400 g ha-1) was added to the mixture. In field experiments in Virginia and New Jersey, topramezone at 3.7 and 6.1 g ha-1 controlled goosegrass 50 and 63%, respectively at 42 days after treatment when averaged across herbicide admixtures. The addition of chlorothalonil alone and chlorothalonil plus acibenzolar-S-methyl to topramezone reduced goosegrass control from 73% to 52 and 45%, respectively, when averaged across topramezone rate. From these studies we can conclude that chlorothalonil has the potential to reduce goosegrass control with topramezone when topramezone is applied at the maximum allowable topramezone rate (6 g ae ha-1) or less. This is the first report of fungicides reducing herbicidal weed control efficacy in turfgrass systems.