Herbicide damage to a white fir. Courtesty of USDA Forest Service Ogden Archive, Bugwood.org (#1467206)
Chemical damage can result from pesticide applications made during the growing season. There are varied reasons for this type of damage, such as mixing incompatible materials, using the wrong concentration, applying to tender foliage, overlooking drift or overspray, using equipment that has previously contained an herbicide, or even using a material that is not labeled for use on Christmas trees (Figures 1 and 2). The root cause of most chemical damage is applicator error.
Figure 1. Glyphosate damage to spruce. Courtesy of Rayanne D. Lehman, PDA
Figure 2. Distortion of spruce foliage due to herbicide application. Courtesy of Ricky Bates, Penn State
Typically, symptoms of chemical damage follow the path of application through the field (Figure 3). If drift or overspray is thought to be the culprit, damage would only occur in the portion of the field closest to the intended spray target (Figure 4). If the damage is throughout the field, mixing error, use of the wrong material, or equipment contamination may be the cause. Damage limited to new growth may result if an application is made before the growth has hardened off. These types of damage appear quickly during the growing season. During the rest of the year, damage appears at a slower rate. Some insecticide oils can burn new foliage, but most chemical injuries to trees are from herbicide applications (Figures 5 and 6).
Figure 3. Chemical burn all along the row on lower portion of the trees typical of improper sprayer adjustment. Courtesy of Rayanne D. Lehman, PDA
Figure 4. Chemical burn due to improper sprayer adjustment. Courtesy of Rayanne D. Lehman, PDA
Figure 5. Damage from simazine application. Courtesy of Ricky Bates, Penn State
Figure 6. Trees exhibiting glyphosate burn on left; healthy trees on right. Courtesy of Ricky Bates, Penn State