Impact of Herbicides Applied Annually for Twenty-three Years in a Deciduous Orchard

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Abstract
A semi- and high-density apple and peach orchard was established at Blacksburg, VA. Starting 1 yr after transplanting, tree rows were treated with paraquat, 2,4-D, or amitrole plus simazine for 4 yr. Thereafter, diuron, simazine, or terbacil was applied in conjunction with paraquat or glyphosate. During the 12th growing season, young trees planted between existing trees failed to make proper growth. Oat bioassay of soils collected from treated rows revealed that the tree row topsoil (0 to 7.5 cm) produced less oat biomass than did deeper row soils (to 30 cm) or soils of corresponding depths from adjacent non-treated tall fescue sod alleyways. Poor growth of trees may have been related to other factors (competition from older trees, drought conditions, etc.) since herbicide residues in the upper 7.5 cm would have little effect on tree roots below this depth. After 3 yr the trees developed normally. Tree rows became heavily infested with weeds 1 yr following cessation of 23 yr of consecutive herbicide treatments.