Importance. -- The total impact of chemical damage is unknown. Large losses have occurred in very localized areas. The losses normally can be traced to a point source such as a chemical spill or industrial waste. Not as easily recognized but perhaps more damaging are the non-point source pollutants such as those associated with a large city. Many oak species decline, dieback and succumb over a period of years. On the other hand, some oak species are relatively resistant to many pollutants and are not affected.

Identifying the Causal Agent. -- Chemicals reach the tree in a variety of forms through the air or soil. Several conditions must occur to cause damage. There must be a susceptible host in a receptive condition, and the chemical must arrive in a quantity and form that will affect the host. Some chemicals damage on contact, others interact with tree processes.

Identifying the Injury (figure 73). -- Most chemicals have certain characteristic symptoms. Ozone causes small bleached or pigmented spots on the upper leaf surface. Sulfur dioxide kills tissue between the leaf veins. Fluoride kills tissue on the leaf margin or between the veins. Ammonia causes faded leaf margins and dead or dying tissue with green islands mostly near veins. Herbicides cause blotchy dead areas on the surface of mature leaves; expanding leaves curl and become distorted. Because of great variation of susceptibility among trees and the combination of chemical and climatic factors, diagnosis is complex. Thus, proper diagnosis may require a person with extensive training and experience.

Control. -- Protect from chemicals or plant resistant trees.

Figure 73. -- Chemical damage -- due to ammonia