A number of different bacterial species incite diseases in plants. Some of the major bacterial diseases can seriously impair the economic value of such host crops as cotton, sugar cane, potato, apple and pear.
The first plant disease proven to be of bacterial origin was fire blight on apple, pear and related plants. The discovery was made a century ago when the disease was devastating orchards in the USA. It has since driven certain high-quality apples and pears out of cultivation in the eastern US.
There are more than 170 different kinds of bacteria which cause diseases in flowering plants belonging to 150 genera or 50 families. Bacterial diseases of plants occur in almost anywhere that is reasonably moist or warm. Their destructiveness varies from year to year and place to place, according to the presence or absence of a critical environmental condition under which the bacteria operate on the host plant.
Plants respond in many ways to invasion by bacteria. Among the symptoms of bacterial infections are galls, wilts, slow growth, dwarfing, imperfect fruits or ears, rots, colour changes of various plant parts, retarded ripening, distortion of leaves, cankers, brooming, fasciation, and leaf spots. Among the many well-known bacterial galls are olive knot, cane gall, beet pocket rot, sweet pea fasciation, hairy root, and crown gall. All contain large swollen cells and small, rapidly dividing cells along with vascular cells in a relatively disorganized arrangement. Eventually these gall structures may interfere with the normal transmission of water and food supplies, and the plants may die.
Bacterial wilt may be quite destructive - for example, in sweetcorn, cucumber, tobacco and related plants. Such bacteria may produce a slime, which plugs the water-conducting tissue of the invaded plant. Closely related are such diseases as black rot of cabbage, ring rot of potatoes, and tomato canker, which may start in the water-conducting tissue but subsequently result in disintegration of surrounding tissue. Cankers develop from extensive tissue destruction - for example, that caused by the fire blight bacteria or from the lesions of the tomato canker organism.
Local spots occur most commonly on the leaves, but sometimes appear elsewhere, as on many fruits. Symptoms of black arm of cotton show when the angular leaf spot bacteria enter the stem and girdle it. Bacterial blight of beans, halo blight of oats, potato scab, and many others appear primarily as local spots. The bacteria causing halo blight of oats and wildfire of tobacco produce toxic substances that are responsible for the yellowish areas immediately around the dead spots where the bacteria have invaded the tissue.
Soft rots develop in relatively fleshy tissues when certain bacteria invade them extensively. Such bacteria produce an enzyme that dissolves the pectic substance that cements plants' cell walls together. The result is a slimy, often foul-smelling, mass. The soft rots often follow and extend invasion and damage by some other pathogen. For example, black rot of cabbage and late blight of potatoes would be much less serious except for the subsequent soft rot.
Symptoms of disease appear at varying lengths of time after bacteria attack and grow in a plant. Soft rots are sometimes evident within a day or so, angular leaf spot of cotton within 10 days, corn wilt within 1 to 2 months. Crown gall of orange may take 2 years.