Book excerpt: Provides an explanation of how plant diseases are diagnosed, the 'plant disease triangle', how to determine the cause of a specific disease, what 'biotrophs' and necrotrophs are, disease cycles and how they can be utilized. Specific chapters address plant diseases caused by fungi, bacteria, nematodes, viruses, parasitic flowering plants, abiotic factors of the environment including light, temperature, and atmospheric gases, pathogens, how people influence plant disease epidemics, the prevention or management of plant disease epidemics, and more.