Shrubs and trees can be damaged by herbicides in two ways. Herbicide drift results in damage to the leaves of contacted plants downwind of the spray. Herbicides applied to weeds around the base of a tree or shrub may also end up poisoning those plants.
Typically, curled leaves and distorted stems, possibly followed by death reveals herbicide damage. This damage can look superficially like aphid damage, but lack of aphids or honeydew suggests herbicide drift. Damage limited to the upwind side of shrubs as well as indiscriminate damage across many different types of trees or shrubs all point to herbicide damage rather than a pest or disease.