The 2012 ASHS Annual Conference

Clomazone herbicide (Command 3ME) is registered for cabbage in the U.S., but not for other cultivar groups within Brassica oleracea.  Cabbage cultivars vary in clomazone tolerance, and recommended use rates can cause severe foliar chlorosis and yield reduction to susceptible cultivars.  The objective of this research was to compare the tolerance of broccoli and cabbage cultivars to clomazone in order to assess the potential for using clomazone for weed management in broccoli production.  Broccoli (Captain, Green Magic, Legacy, and Patron) and cabbage (Bravo, SC 100, Stone Head, and Vantage Point) cultivars with varying tolerance to clomazone were selected from previous reports or based on observations from preliminary screening experiments.  In a greenhouse concentration-response experiment, seedlings were transplanted into potting medium containing clomazone at 0, 0.25, 0.5, 1.0, 2.0, and 4.0 mg ai/kg soil.  Bravo cabbage was most susceptible, and its injury ratings at 0.5 mg/kg were similar to ratings for the other cabbage cultivars at 2.0 mg/kg.   Bravo shoot weight reduction at 0.25 mg/kg was similar to the other cabbage cultivars at 1.0 mg/kg.  The broccoli cultivars also varied in clomazone tolerance.  Patron was highly susceptible, exhibiting injury and shoot weight reduction similar to Bravo.  Green Magic was most tolerant, and it exhibited injury and growth reduction levels similar to the tolerant cabbage cultivars.   A field experiment was conducted to assess the tolerance of the eight cultivars to clomazone applied pre-transplanting.  Application rates were 0.28 (the recommended rate for broccoli on sandy soils) and 0.56 kg ai/ha for an experiment planted in March, 2011 and 0.28, 0.56, and 1.12 kg/ha for an experiment planted in September, 2011.  Clomazone at 0.28 kg/ha cause only minor chlorosis to the susceptible cultivars, Bravo and Patron.  Injured plants recovered, and no chlorosis was visible at maturity.  At 1.12 kg/ha all cultivars exhibited chlorosis early; however, the tolerant cultivars recovered rapidly and no injury was visible at mid season.  At 1.12 kg/ha, Bravo and Patron exhibited severe chlorosis early, and moderate chlorosis persisted until harvest.  Clomazone did not reduce average head weight or percentage of plants producing marketable heads.  This study indicates that the range of clomazone tolerance in broccoli cultivars is similar to that in cabbage cultivars, and that the herbicide can be used safely on tolerant broccoli cultivars at rates that are labeled for cabbage.