Infectious plant diseases differ from noninfectious or abiotic disorders because they are caused by specific plant pathogens. Plant pathogens are usually microorganisms that are on or inside the plant (the host). Many plant pathogens are common in the air or soil around the host plants. Although plant pathogens may be common in many landscape situations, they cannot infect the plant and cause disease unless the environment is favorable for the interaction to take place. Thus, in order to have an infectious plant disease, a susceptible host plant, a pathogen and a favorable environment for disease development are required. These three elements make up what is referred to as the disease triangle (Figure 1). Understanding this basic principle about infectious plant disease is important to developing successful disease management programs. Many of the cultural practices that are useful in disease management are directed toward changing or creating an environment that is less conducive to disease development. Another important difference between infectious and noninfectious disease is that infectious diseases can spread from plant to plant by means of pathogen dispersal. Noninfectious diseases do not spread in this manner.