Weed control and crop tolerance with S-metolachlor in seeded summer squash and cucumber

Residual herbicides remain the primary tool for efficient weed control in cucurbit crops because of the lack of crop tolerance to many POST herbicide options. Field experiments were conducted in New Jersey in 2018 and 2019 to determine weed control efficacy and tolerance of direct-seeded cucumber ‘Python' and summer squash ‘Gold Prize’ to S-metolachlor applied at 0.7 or 1.4 kg ai ha–1 at planting (PRE) or when crops reached the second- to third-leaf stage (EPOST). Regardless of applied rate, S-metolachlor PRE or EPOST provided 96% to 100% control 3 wk after planting (WAP) of smooth pigweed, large crabgrass, and giant foxtail. S-metolachlor PRE significantly improved American black nightshade and carpetweed control 3 WAP with respect to bensulide, and smooth pigweed with respect to clomazone + ethalfluralin. Summer squash showed excellent tolerance, regardless of S-metolachlor rate or timing of application, with stunting not exceeding 17% 4 WAP and 3% 7 WAP at the 1.4 kg ha–1 rate. Marketable yield decreased by 15% with S-metolachlor PRE or POST at 1.4 kg ha–1 with respect to clomazone + ethalfluralin, a reduction not noted when comparing with bensulide or the handweeded control. Marketable fruit number plant–1 and individual fruit weight were not affected by S-metolachlor applications. Conversely, cucumber was more sensitive to S-metolachlor than summer squash was with 30% seedling emergence reduction and 36% to 43% stunting 4 WAP when S-metolachlor was applied PRE at 1.4 kg ha–1. EPOST application resulted in 15% to 26% cucumber injury 1 wk after treatment. Marketable yield declined by 21% and 39% with the 0.7 and 1.4 kg ha–1 rates of S-metolachlor, respectively, compared with clomazone + ethalfluralin. Therefore, S-metolachlor may be a novel alternative to already labeled residual herbicides for summer squash, but unacceptable injury and yield reduction do not support its registration on cucumber. Nomenclature: Bensulide; clomazone; ethalfluralin; S-metolachlor; American black nightshade, Solanum americanum Mill.; carpetweed, Mollugo verticillata L.; giant foxtail, Setaria faberi Herrm.; large crabgrass, Digitaria sanguinalis (L.) Scop.; smooth pigweed, Amaranthus hybridus L.; cucumber, Cucumis sativus L.; summer squash, Cucurbita pepo L.