11 ص Diseases, Pests, And Mechanical Injury . Like all living things, plants continuously subjected to a wide variety of potential disease-causing agents and to the stresses caused by mechanical injury. Generally speaking, the causes of disease fall into two broad groups, parasitic and nonparasitic. An organism that lives in or on another individual for part or all of its life, securing its food directly from the tissues of the host organism but not benefiting the host in any way, is referred ...