Influence of Terbacil, Halosulfuron, Clomazone, and S-Metolachlor On Grafted Seedless Watermelon

Herbicide trials were conducted in 2009 to examine the effects of four herbicides on grafted watermelon.  Field trials were located in Quincy, FL, Charleston, SC, and Kinston, NC.  Watermelon plants utilized in the study include non-grafted Tri-X-313 watermelon and Tri-X-313 watermelon grafted onto gourd, inter-specific hybrid squash, wild watermelon, and seedless watermelon (“self-grafted” to Tri-X-313) rootstocks.  Two rates of terbacil (0.11 and 0.22 kg ai/ha), halosulfuron (0.35 and 0.53 kg ai/ha), clomazone (0.28 and 0.42 kg ai/ha), and S-metolachlor (1.07 and 1.42 kg ai/ha) were applied pre-transplant.  Additional treatments included the same two rates of halosulfuron applied post-transplant and an untreated control.  Phytotoxicity ratings were conducted at 19 days after transplanting and at one week following post-transplant halosulfuron application (27 days after transplanting). Yield was recorded at each of four harvests.  Establishment of watermelons grafted to certain rootstocks was difficult at Quincy and Kinston.  Due to low stand count, only data from Tri-X-313/gourd, Tri-X-313/inter-specific hybrid squash, and non-grafted Tri-X-313 was included from Quincy.  Likewise, only data from Tri-X-313/gourd and non-grafted Tri-X-313 was included from Kinston.  Each location was analyzed separately.  At Quincy, all plant types (grafted and non-grafted) exhibited bleaching when clomazone was applied pre-transplant.  Watermelons grafted onto gourd and inter-specific hybrid squash rootstocks appeared to be more tolerant of clomazone compared to non-grafted plants.  All plant types also exhibited bleaching due to clomazone at Charleston.  However, at Charleston, plants grafted onto gourd and inter-specific hybrid squash appeared to be less tolerant of clomazone compared to non-grafted plants.  Considerable visible injury due to pre-transplant applications of S-metolachlor and halosulfuron was only observed at Kinston.  No terbacil injury was reported at any location for any plant types.  Pre-transplant applications of clomazone and S-metolachlor at Quincy resulted in significantly shorter vines compared to the untreated control for non-grafted plants and those grafted onto inter-specific hybrid squash.   Injury was observed on all plant types at Quincy subjected to post-transplant applications of halosulfuron.  Symptoms of injury included stem splitting, shorter vines, and an increase in the number of nodes at the distal end of the vines.  At Charleston, post-transplant halosulfuron application did not cause such injury.  At all locations, none of the herbicide treatments resulted in a significantly lower total marketable yield when comparing the same scion/rootstock combination from treated and untreated plots.  At Quincy, only non-grafted plants had significantly lower early marketable yield following clomazone, S-metolachlor, and halosulfuron (high rate post-transplant) herbicide treatments.