Plants exposed to acid rain can be damaged, leaving them vulnerable to disease, as well as having their growth hindered, even to the point of death in small plants. Acid rain is a result of air pollution when sulfur dioxide or nitrogen oxide levels raise too high and lower the rain's PH level.
When acid rain comes in contact with leaves, it can remove protective wax layers that prevent bacteria and viruses from infecting it and keeps the plants structural integrity intact. Even if the rain doesn't damage them directly, it leaves them open to being damaged or destroyed by weather, nature or insects. A secondary effect of acid rain is that it leaches into the soil. Acidic soil cannot deliver nutrients as effectively, which can stop plants from growing or kill them altogether if there are insufficient nutrients available.