Identification of Citrullus lanatus Germplasm Accessions Tolerant to Clomazone Herbicide

Clomazone herbicide is registered for use in watermelon; however, crop tolerance is marginal, and the recommended use rates (0.17 to 0.28 kg a.i./ha) are lower for watermelon than for other crops. In a greenhouse germplasm evaluation experiment including 56 germplasm accessions and watermelon cultivars, three Citrullus lanatus var. citroides PI accessions (PI 482324, PI 5003540, and PI 532624) were not injured by clomazone, whereas most of the other accessions and cultivars were moderately or severely injured. A greenhouse concentration response experiment demonstrated that the clomazone concentration required to cause moderate injury to the tolerant ‘PI 500354’ was approximately three times the concentration required to cause similar injury to the susceptible citroides accession ‘PI 244017’, and the concentration required to cause 50% shoot biomass reduction was approximately five times greater for ‘PI 500354’ than for ‘Charleston Gray’ watermelon. Subsequent field experiments demonstrated that two tolerant accessions (‘PI 500354’ and ‘PI 482324’) were injured less initially by clomazone and recovered more rapidly from clomazone injury than two susceptible accessions (‘PI 244017’ and ‘PI 271773’) and two watermelon cultivars (‘Charleston Gray’ and ‘Crimson Sweet’). Tolerant germplasm accessions like ‘PI 482324’ and ‘PI 500354’ may be useful as sources of clomazone tolerance in watermelon breeding. Chemical name used: clomazone {2-[(2-chlorophenyl)methyl]-4,4-dimethyl-3-isoxazolidinone}.