|Harrison Jr, Howard
Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2014
Publication Date: 1/1/2015
Citation: Harrison Jr, H.F., Farnham, M.W., Jackson, D.M. 2015. Tolerance of broccoli cultivars to pre-transplanting clomazone. Crop Protection. 69:28-33.
Interpretive Summary: Clomazone (Command 3ME herbicide) is used extensively for weed control in cabbage, but is not yet registered for use in other cole crops, including broccoli that are the same species as cabbage. The IR-4 Project has completed a research project to obtain registration of clomazone for broccoli and cauliflower which are included with cabbage in the heading brassica crop grouping use for developing pesticide regulations. The purpose of this study was to determine the response of a genetically diverse collection of 44 hybrid broccoli cultivars to clomazone applied before transplanting and to identify varieties that are too susceptible to allow safe use of the herbicide. Greenhouse and field studies demonstrated that all varieties were adequately tolerant to allow safe use of the herbicide at the maximum application rate recommended for cabbage (0.25 lb/acre on sandy soils). Varietal differences in tolerance were observed, and the most tolerant varieties were only slightly injured and their yields were not affected by 1.0 lb/acre clomazone. Clomazone will probably become an important component for integrated weed management in broccoli, because it controls weeds that are not controlled by the herbicides currently available for broccoli. This investigation led to the identification of highly tolerant varieties that could be used to reduce the risk of broccoli injury by clomazone that may occur under unfavorable conditions, and it demonstrated that developing new varieties with improved clomazone tolerance may be possible.
Technical Abstract: Clomazone has been used for weed management in cabbage (Brassica oleracea L., capitata group) production in the U.S. for over 20 years; however, the herbicide is not currently registered for other crop groups within B. oleracea. The U.S. specialty crop 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 weight reductions were compared. Tolerant cultivars were slightly injured and growth was not affected by clomazone incorporated into potting medium at 3.0 mg/kg; whereas, susceptible cultivars were severely injured and their shoot weights were reduced by 1.5 mg/kg. Injury ratings from field experiments also demonstrated differences between cultivars in response a pre-transplant clomazone application. The most tolerant cultivars were injured less by 1.12 kg/ha than were the most susceptible cultivars by 0.28 kg/ha. Injury ratings were much lower at 6 weeks after transplanting than at two weeks. Broccoli head weights and stem diameters were not consistently affected by clomazone treatment, but the 1.12 kg/ha treatment delayed head maturation for several cultivars. Overall, the results indicate that clomazone at the rate that is recommended for cabbage, 0.28 kg/ha on sandy soils, is relatively safe for use on the 44 broccoli cultivars. Safety can be improved by choosing the most tolerant cultivars.