spot

any one of several diseases of agricultural plants characterized by formation of blotches of dead cells on leaves, stems, fruits, and other plant parts; a specific type of necrosis. Spots are caused by inadequate soil nutrition; air pollution; burns; and pathogenic fungi (most often, imperfect hyphomycetic fungi of the genera Ramularia, Cercospora, and Macrosporium and pycnidial fungi of the genera Septoria, Ascochyta, and Phoma; less commonly such perfect fungi as ascomycetes of the genus Pseudopeziza), bacteria (genera Pseudomonas and X anthomonas), and viruses.

The outward appearance of the spots is determined by the relationship between and the specific features of the parasite and the plant host. The causative agents, which usually spread throughout the tissues, encounter opposition from the plant host in the form of mechanical and chemical barriers. A mechanical barrier is caused by the formation of a cork layer on the boundary between healthy and diseased tissue; the cork localizes the focus of infection. For example, in drupaceous crops infected with the fungus Clasterosporium carpophilum or the bacterium Xanthomonas pruni, the affected tissue, after the formation of the cork layer, falls out together with the pathogen. Chemical barriers form owing to the accumulation in the affected cells and the cells adjacent to them of phenolic substances that are toxic to the parasite (specifically anthocyanins and products of their oxidation). If intrusion of the parasite is accompanied by a severe protective reaction of the plant, a small necrotic spot is formed and development of the causative agent is curtailed. When, however, the protective action is insufficient to localize the infection, the spots slowly enlarge (for example, macrosporiosis of potatoes, tomatoes, cotton; phomosis of sugar beet). Sometimes check zones in the form of concentric rings are readily observable on the spots.

When the causative agents are pathogenic fungi, there arise variously shaped and colored dry spots, upon whose surface one may observe the spore carriers of the fungus. Bacterial spots are characterized by the formation of tiny blotches surrounded by chlorotic aureoles; sometimes droplets of resin emerge on the spots (for example, gummosis of cotton). Spots produced by viruses are localized along the veins or form characteristic rings and designs. Their coloration may be red (with accumulation of anthocyanins), dark brown, gray, black (with accumulation of melanins), or white (with decolorization of pigments).

The resulting atrophy of parts of leaves, fruits, and stems decreases the photosynthetic surface of the plant and the plant’s productivity. With widespread infection, the spots merge. This may result in the mass falling of leaves and fruits and the drying of stems, significantly decreasing the yield of agricultural crops. In some cases, even a minor infection may bring great harm. For example, a single infection of an alfalfa petiole with the pathogenic ascomycete Pseudopeziza medicaginis close to the place where the leaf blade is attached leads to leaf fall. For a discussion of control measures against spots of agricultural plants seeBACTERIAL DISEASES OF PLANTS, VIRAL PLANT DISEASES, and FUNGAL DISEASES OF PLANTS.