ABSTRACT

A general definition of plant diseases is that they are disturbances that affect the normal function of a plant. These disturbances commonly are divided into nonparasitic diseases and parasitic diseases. Nonparasitic diseases are referred to also as noninfectious diseases and are caused by environmental deficiencies or disturbances. Nutrient deficiency, cracking of fruits, damage from salinity, air pollution, repeated mechanical damage, and unfavorable light or temperature are examples of nonparasitic diseases. Parasitic diseases are infectious and are caused by organisms or by viruses. Infectious diseases are those that most people envision when plant diseases are mentioned. Principal parasitic organisms that cause infectious diseases are various fungi and bacteria. Fungi are widespread, multicellular, nongreen organisms. Several thousand species of fungi, among about 100,000 total fungal species, cause plants diseases. Fungal diseases are known as damping-off, rots, wilts, blights, rusts, mildews, smuts, scab, molds, and other names. Bacteria are virtually everywhere. A few hundred species cause plant diseases. Some common names for some bacterial diseases are spots, wilts, rots, blights, galls, and cankers. Mycoplasmas are unicellular organisms, classed as bacteria but having no cell walls and being smaller than common bacteria. They are like masses of protoplasm surrounded by an outer membrane. Mycoplasma cause several diseases known as yellows (carrot yellows, aster yellows). At one time, yellows diseases were attributed to viruses. Occasionally and usually in tropical regions, algae and protozoa cause plant diseases. Under taxonomic systems that divide living organisms into plant and animal kingdoms, some algae that might cause diseases are classed as unicellular plants having pigments of various kinds of chlorophyll. Under this taxonomic system, protozoa are unicellular animals. Mistletoe and dodder are flowering plants that are parasitic on other plants and thus may be classified as diseases. Infectious nematodes are roundworms that produce diseases on roots, leaves, stems, bulbs, and flowers. Soil-dwelling nematodes are the most common disease-producing species. Viruses are nonliving, noncellular matter that are composed of a core of nucleic acids and a sheath of protein. Viruses are only 1/100 to 1/1000 of the size of bacteria and may infect bacteria. Viral plant diseases cause malformations known as mosaics, ring spots, rosetting, curling, puckering, and other names.