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Title: Herbicide study in direct-seeded watermelon, 2004

Author
item SHREFLER, J - OSU, LANE,OK
item BRANDENBERGER, L - OSU, STILLWATER,OK
item Webber Iii, Charles
item GOODSON, T - OSU, LANE,OK

Submitted to: Oklahoma Agriculture Experiment Station Departmental Publication
Publication Type: Experiment Station
Publication Acceptance Date: 1/27/2005
Publication Date: 1/31/2005
Citation: Shrefler, J.W., Brandenberger, L.P., Webber III, C.L., Goodson, T.L. 2005. Herbicide study in direct-seeded watermelon, 2004. In: Brandenberger, L., Wells, L. editors. 2004 Vegetable Trial Report. Oklahoma State University, Division of Agricultural Sciences and Natural Resources, Department of Horticulture and Landscape Architecture, Stillwater, Oklahoma. MP-162. p. 7-9.

Interpretive Summary: Halosulfuron-methyl (Sandea) is a new preemergence herbicide for use in watermelon. Although halosulfuron-methyl has postemergence weed control activity, it is unclear whether there is a sufficient margin of crop safety for its use as a posteemergence herbicide in watermelon. If it can be determined that halosulfuron-methyl is an affective postemergence herbicide with sufficient crop safety for watermelon, once labeled, it would be an important asset to the watermelon producers. The objective of this study was to evaluate postemergence application of halosulfuron-methyl for efficacy in controlling weeds and for crop safety. The study was conducted in 2004 at Lane, OK on a Bernow fine sandy loam soil. 'Jubilee' watermelon was seeded July 12. Ethalfluralin (Curbit) was applied at 0.75 lb a.i./acre (July 13) as a preemergence to all plots except the weed-free (hoed) and weedy-check (untreated) plots. Halosulfuron-methyl was then applied at 1, 3, 5, or 7 weeks after watermelon emergence (WAE) with 0.024 lb a.i./acre or at 5 WAE with 0.016, 0.032, or 0.048 lb a.i./acre. Visual evaluations of weed control and crop phytotoxicity and stunting were conducted on August 12 and September 1 and 20. Overall, all applications of halosulfuron-methyl impacted the crop through stunting, phyto or a combination of both. Some phytotoxicity was evident for all applications of halosulfuron-methyl except the one made at 1 WAE. Although phytotoxicity was not observed for the 1 WAE application of halosulfuron-methyl these plants were stunted, as were those of the 2 and 3 WAE applications. Halosulfuron-methyl provided varying degrees of control of the broadleaf weed species that were evaluated in this study. Control of groundcherry (Physalis angulata) was marginal in all except the earliest two applications. Tumble pigweed (Amaranthus albus) was controlled to a great extent by the preemergence application of ethalfluralin. Halosulfuron-methyl applications at 1, 2 or 3 WAE eliminated most of the surviving tumble pigweed. Eclipta (Eclipta prostrate) was the most susceptible weed species to the postemergence halosulfuron-methyl applications. In summary, postemergence application of halosulfuron-methyl herbicide has the potential to impact watermelon through phytotoxic effects and stunting. In addition to controlling weeds, herbicides often have an impact on crop growth. The ability of the crop to recover from this and produce satisfactory yield should also be considered in order to make a complete assessment of the impact of an herbicide on a crop.

Technical Abstract: Watermelon producers currently have inadequate chemical weed control options for direct-seeded culture. Broadleaf weeds are particularly troublesome because there are no approved herbicides that provide broad-spectrum postemergence weed control. There are several preemergence herbicides currently available for use in watermelon. However, these do not give adequate control of all of the weeds that commonly occur in watermelon crops. Halosulfuron-methyl (Sandea) is a newly available herbicide for preemergence use in watermelon that has been found to be useful in watermelon production. Although it has postemergence weed control activity, it is not yet approved for postemergence use in watermelon. Its potential usefulness when applied postemergence in watermelon has not been thoroughly explored. The objective of this study was to evaluate postemergence application of halosulfuron-methyl for efficacy in controlling weeds and for crop safety. The study was conducted in 2004 at Lane, OK on a Bernow fine sandy loam soil. 'Jubilee' watermelon was seeded July 12. Ethalfluralin (Curbit) was applied at 0.75 lb a.i./acre (July 13) as a preemergence to all plots except the weed-free (hoed) and weedy-check (untreated) plots. Halosulfuron-methyl was then applied at 1, 3, 5, or 7 weeks after watermelon emergence (WAE) with 0.024 lb a.i./acre or at 5 WAE with 0.016, 0.032, or 0.048 lb a.i./acre. Visual evaluations of weed control and crop phytotoxicity and stunting were conducted on August 12 and September 1 and 20. Overall, all applications of halosulfuron-methyl impacted the crop through stunting, phyto or a combination of both. Some phytotoxicity was evident for all applications of halosulfuron-methyl except the one made at 1 WAE. Although phytotoxicity was not observed for the 1 WAE application of halosulfuron-methyl these plants were stunted, as were those of the 2 and 3 WAE applications. Halosulfuron-methyl provided varying degrees of control of the broadleaf weed species that were evaluated in this study. Control of groundcherry (Physalis angulata) was marginal in all except the earliest two applications. Tumble pigweed (Amaranthus albus) was controlled to a great extent by the preemergence application of ethalfluralin. Halosulfuron-methyl applications at 1, 2 or 3 WAE eliminated most of the surviving tumble pigweed. Eclipta (Eclipta prostrate) was the most susceptible weed species to the postemergence halosulfuron-methyl applications. In summary, postemergence application of halosulfuron-methyl herbicide has the potential to impact watermelon through phytotoxic effects and stunting. In addition to controlling weeds, herbicides often have an impact on crop growth. The ability of the crop to recover from this and produce satisfactory yield should also be considered in order to make a complete assessment of the impact of an herbicide on a crop.