Tolerance of broccoli cultivars to pre-transplanting clomazone

Clomazone has been used for weed management in cabbage (Brassica oleracea L., Capitata group) production in the U.S.A. for over 20 years; however, the herbicide is not currently registered for other crop groups within B. oleracea. The specialty crops pesticide registration program (The IR-4 Project) is pursuing clomazone registration for broccoli (B. oleracea, Italica group) and cauliflower (B. oleracea, Botrytis group). The objective of this study was to assess the tolerance of a genetically diverse collection of 44 hybrid broccoli cultivars to pre-transplanting clomazone application and identify susceptible cultivars not suitable for clomazone use. A greenhouse experiment demonstrated differences in tolerance among broccoli cultivars when injury ratings and shoot weights were compared. The most tolerant cultivar, Chief, was only slightly injured and its shoot weight was not reduced in comparison to the untreated controls by clomazone incorporated into potting medium at 3.0 mg/kg; whereas, the most susceptible cultivar, Patron, was moderately injured and its shoot weight was reduced by clomazone at 1.5 mg/kg. Injury ratings from field experiments also demonstrated differences between cultivars in response to pre-transplant clomazone application. At two weeks after transplanting into treated soil, the most tolerant cultivars exhibited minor injury at 1.12 kg/ha, and the most susceptible cultivars were moderately injured by 0.28 kg/ha. Injury ratings were much lower at 6 weeks after transplanting than at two weeks. Broccoli head weights were reduced by 0.28 kg/ha for 4 cultivars in 2012 and 1 cultivar in 2013, and stem diameters of 2 cultivars were reduced by this treatment in 2012. The low rate did not affect harvest date for any cultivar, but at 1.12 kg/ha, clomazone delayed head maturation for twelve cultivars in 2012 and 4 cultivars in 2013. These responses were inconsistent between years, and the greater injury observed in 2012 than in 2013 may have been due to the transplanting method which moved treated soil into the root zone. Overall, the results indicate that pre-transplanting application of clomazone at the rate that is approved for cabbage, 0.28 kg/ha on coarse soils, is relatively safe for use on most of the 44 broccoli cultivars evaluated in this study. The risk of crop injury that may occur under unfavorable environmental conditions can be reduced by choosing the most tolerant cultivars. Published by Elsevier Ltd.