Non-Infectious Diseases of Trees

Diseases of trees are caused by two major types of agents: 1) living pathogens and 2) nonliving or abiotic stress factors. Living pathogens of trees, such as fungi, bacteria, mycoplasmas, nematodes, parasitic seed plants and viruses are well known to anyone studying tree disease. Abiotic stresses, such as extremes of temperature and moisture, nutritional abnormalities, and people-pressures from a myriad of detrimental human activities, are often overlooked as causes of tree disease. These abiotic stresses are often called noninfectious diseases. Most shade trees exist in a high-stress environment unlike most forest trees which grow in an environment that is favorable because of natural species selection and minimal contact with people. Shade trees are usually people-planted trees, are commonly placed in the poorest imaginable sites, are exposed to extremes of weather, and are frequently contacted by people. Unfortunately, most of what we know about tree diseases comes from studies on forest trees (Roberts, 1977). Noninfectious diseases can best be understood by a closer look at some of the most common abiotic stresses affecting shade trees.