Multi-Year Impact of Dicamba and Glyphosate Herbicides on 'Granny Smith' Apple Trees

Off-target movement of low-volatile dicamba products has resulted in injury to sensitive plants. An experiment was conducted to determine the effect of dicamba alone or dicamba + glyphosate on tree growth and fruiting of 'Granny Smith'/`Budagovsky 9' (B.9) apple (Malus domestica Borkh.) trees over a four-year period. On 8 June 2017, three rates corresponding to 1/2, 1/20, or 1/200 of the manufacturer's labeled rate of dicamba (0.56 kg ae/ha) or dicamba + glyphosate (0.56 kg ae/ha + 1.10 kg ae/ha) were applied to potted one year-old apple trees. On 1 Nov. 2017, trees were transplanted in the field for further evaluation through 2020. Herbicides caused foliar injury on apple trees by three weeks after treatment. Also, trunk diameter growth of trees was reduced by 22 to 45% by herbicide treatments compared with the nontreated control at five months after herbicide exposure. The following growing season, trees treated with dicamba alone or dicamba + glyphosate had 7 to 36% fewer flower clusters, as well as 29 to 63% less cumulative fruit yield in 2020 than control trees. When herbicide treatments were compared, the dicamba + glyphosate treatment resulted in a higher injury rating, a lower number of flower clusters in 2018, and less fruit yield in 2019 than dicamba alone. While the adverse effect of dicamba was generally less than that of dicamba + glyphosate on apple trees, a single dose of these herbicide treatments at driftable rates caused a successive three-year reduction of reproductive organs compared with nontreated controls.