Plant diseases usually have the most colourful names, such as blight, downy mildew, powdery mildew, blast, rust, smut, smudge, wart, streak, blister, and scorch. Plant diseases are caused by parasitic organisms called pathogen, that are usually microscopic, and which include fungi, bacteria, phytoplasmas, viruses, and viroids. The development of a disease within a host population is called an epidemic.
Deficiency diseases are due to nutritional inadequacies and are best described as physiological disorders.
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