Response of Pumpkin to Over-the-Top Applications of S-Metolachlor

Expanding preemergence herbicide registrations to include over-the-top (OTT) applications of S -metolachlor in pumpkins could extend the duration of residual weed control, which would provide the crop with a competitive advantage over weeds. In 2021, field trials were conducted at two locations in Indiana (IN) and a single location each in Delaware (DE), Maryland (MD), Michigan (MI), North Carolina (NC), North Dakota (ND), New Jersey (NJ), New York (NY), Ohio (OH), and Pennsylvania (PA) to evaluate the safety of S -metolachlor OTT in jack-o-lantern varieties across multiple production regions. All locations included treatments consisting of S -metolachlor applied at 1.4 and 2.8 kg·ha −1 at one of two timings (early or late). At some locations, additional treatments included halosulfuron-methyl at 26 g·ha −1 a.i., combined with 0.25% (v/v) nonionic surfactant tank-mixed with S- metolachlor and applied as previously described. With the exception of MI [20 d after planting (DAP)], early applications were made 12 to 15 DAP. Late applications were made between 25 and 31 DAP at all locations. S -metolachlor applied at 1.4 or 2.8 kg·ha −1 caused less than 10% crop injury within a week of application, which declined over time. The highest injury levels were observed with early applications and on loamy sand soils containing ≤2% organic matter. The addition of halosulfuron-methyl significantly increased crop injury to 15% at 1 week after treatment (WAT), with early applications causing more severe damage (25%) than late applications (11%), although pumpkins recovered completely by 6 WAT. Although total and marketable fruit yields were unaffected by herbicide treatments, adding halosulfuron-methyl to OTT application reduced marketable fruit numbers to 84% of the standard control and decreased the proportion of large fruits from 97% to 65%. S -metolachlor applied at 2.8 kg·ha −1 tended to reduce the percentage of commercial-grade fruit and, depending on application timing, increased the proportion of small fruit. Results indicate that applications of S- metolachlor at 1.4 kg·ha −1 a.i. applied 12 to 31 DAP causes minimal crop injury and yield loss and could be a valuable tool for extending residual weed control in pumpkins.