Herbicides applied improperly can damage or kill trees. Many herbicides used to control broadleaf weeds in lawns contain ingredients that are essentially synthetic growth hormones, such as 2,4-D, triclopyr, picloram, and dicamba. These cause the weeds to grow abnormally and die. When trees are affected, strange growth distortions can occur, including curling, thickening, and fasciation (flattening of cylindrical structures).
At right is Pinus edulis with swollen, defoliated shoots. Also shown is a P. ponderosa seedling in which the bark at the bases of the needles grew abnormally due to herbicide exposure.
Among such herbicides, picloram and dicamba are much more persistent in the soil and more readily taken up by tree roots. These herbicides tend to be used in utility rights-of-way and fencelines to control broadleaf plants including woody vegetation. They can severely injure or kill trees over time.
Other herbicides, such as atrazine, may cause chlorosis of foliage, while glyphosate (in Roundup) may cause an unusual coloration of foliage followed by necrosis. Both can then lead to necrosis of foliage, branch dieback, and death if exposure is high enough. But these herbicides are not readily absorbed by roots, so exposure must be by drift or direct spraying.
If you suspect herbicide damage, look for damage to other kinds of plants, especially weeds in the area. However, symptoms in trees may persist for years, whereas the weeds will soon be gone.
More general information is available on abiotic diseases and injury.