Pumpkin Injury and Yield Response to Low Rates of 2,4-D Choline and Dicamba

Abstract

The recent release of 2,4-D- and dicamba-tolerant soybean traits has increased the risk of off-target herbicide injury and yield loss for specialty crop growers in the midwestern United States. Most dicotyledonous plants, including many specialty crops like pumpkin (Cucurbita pepo), are susceptible to synthetic auxin herbicides; however, the relationship between off-target herbicide rate, visible crop injury, and eventual yield loss is not well documented. The objective of this 2-year field study in 2019 and 2020 was to determine the effect of sublethal herbicide rates of 2,4-D and dicamba on visible injury and crop yield loss in pumpkin when applied at the vegetative and flowering growth stages. Herbicides included 2,4-D choline salt (1066 g ae·ha−1 labeled rate) and dicamba diglycolamine salt (560 g ae·ha−1 labeled rate) ranging from 1/500 to 1/4 of the labeled rate. Visible injury ratings were recorded every 7 d after application and pumpkins were harvested and weighed when ripe. Injury and yield data were fit to a four-parameter log-logistic regression model to estimate effective doses (ED) required for 5% to 50% visible injury or yield loss. Pumpkin treated with the 1/10 and 1/4 rates of 2,4-D at both growth stages had visible injury (± 1 SE) ranging from 8% (± 3%) to 55% (± 3%), but injury did not always result in yield loss. Maximum yield loss from 2,4-D was 32% (± 2%), observed following the 1/4 rate at the vegetative growth stage in 2020 (estimated ED for 20% yield loss was ∼1/50). Pumpkin treated at the vegetative growth stage with the 1/10 and 1/4 rates of dicamba resulted in 65% (± 6%) to 82% (± 1%) visible injury and 33% (± 2%) to 86% (± 14%) yield loss (estimated ED for 20% yield loss was ∼1/10 in 2019 and ∼1/15 in 2020). At the flowering stage, dicamba rates of 1/10 and 1/4 caused visible injury of 31% (± 2%) to 74% (± 5%) and yield loss of 26% (± 10%) to 60% (± 14%) (estimated ED for 20% yield loss was ∼1/20 in 2019 and ∼1/5 in 2020). Susceptibility of pumpkin to 2,4-D and dicamba suggests herbicide applicators and pumpkin growers should consider strategies that mitigate off-target movement, including using nozzles that increase droplet size, shielded sprayers, thorough tank cleanout, buffer zones, and programs that facilitate communication between applicators and growers.