There are thousands of pathogens that exist in nature, although a particular plant may have only about forty of these. Also at any given time, this same plant is routinely attacked by only one or two pathogenic agents which can be bacterial pathogens. As a matter of fact, a plant’s resistance to a specific causative agent of disease is generally the rule rather than the exception.
There are several ways by which bacterial plant pathogens may be spread: by wind, water, and soil movements; by insect vectors; by infected seeds; and by infected farm tools. Once inside the plant tissue, bacterial plant pathogens usually grow intercellularly, that is, they grow between cells.
Some of the bacterial plant pathogens include agrobacterium spp., corynebacterium, pseudomonas, streptomyces spp., and xanthomonas spp. Agrobacterium spp. may be present in the soil or in the roots or stems of plants, where they cause galls to the plants they infect. The plant pathogens of the corynebacterium genus may live in the soil as well as in diseased plants. They cause ring rot of tomatoes and potatoes as well as a vascular disease of alfalfa. Erwinia spp., in particular, attack the tissues of living plants and cause wilts, soft rots, galls, and dry necrosis.
Pseudomonas produces wilt, leaf stripe, and leaf spot, while streptomyces spp. causes potato scab as well as a disease occurring specifically on the roots and rootlets of sweet potatoes. Xanthomonas spp. cause necrosis and most of them produce yellow colonies.
These bacterial plant pathogens cause a wide variety of diseases. The bacterial infections in plants are characterized by such host reactions as cankers, galls, rots, and wilts. Other host reactions to bacterial infections in plants include deformed fruits or retarded ripening of fruit, change in the color of plant parts, dwarfing, and leaf spots.
A disease common in stone fruits, bacterial cankers reduce the yield of fruit and even destroy the entire tree. They start in the water-conducting tissue of the plant and can spread into the surrounding tissue. The most typical symptom of this plant disease is the appearance of cankers coupled by another symptom, called gummosis – the exudation of gummy substance. Three bacterial plant pathogens cause cankers in certain plants: Erwinia amylovora causes cankers in pears and apples; Corynebacterium michiganense causes cankers in tomatoes; and Pseudomonas syringae is the cause of cankers and gummosis in drupes. This last pathogen is not killed during the winter.
Characterized by abnormal outgrowths of plant tissue, galls are caused by a number of bacterial plant pathogens, the most usual of which is Agrobacterium tumefaciens. This particular causative agent is a small, gram-negative, capsulated bacterium which does not undergo sporulation (the formation of spores) and bears polar flagella. Some of the abnormal outgrowths of plant tissue produced in galls are large and swollen, while others are small and divide rapidly. As galls develop, they may stop the passage of nutrients and water in the plant, causing death. Crown gall, in particular, may develop in sugar beets, fruit trees, and other plants with broad leaves and stems that come out of the ground.
Slime-producing bacteria are the causative agents of bacterial wilts. These bacterial plant pathogens plug the channels in the plant through which water flows. There are four wilt-causing bacterial plant pathogens that must be mentioned here: Corynebacterium insidiosum causes wilts in alfalfa; Erwinia tracheiphila causes wilts in cucumbers; Erwinia stewartii produces wilts in sweet corn; and Pseudomonas solanacearum causes wilts in tobacco.
The bacterial plant pathogens that produce leaf spots may also infect the stems and fruits of the diseased plant. Two spot-causing bacterial plant pathogens are named here: Pseudomonas striafaciens causes bacterial spots in oats, while Xanthomonas pruni produces bacterial spots in peaches.
Effective treatment of bacterial infections in plants may involve radical treatment or removal of the infected plant parts or removal of the diseased plant altogether. A more drastic measure may mean the removal of entire communities of susceptible or diseased plants.
Sources:
1. “Bacterial Plant Pathogens and Symptomology”, by Jim Cooper, Master Gardener, WSU County Extension, SJI, Edited by Dr. Tom Schultz, Copyright Mar. 21, 2006 –
2. “Bacteria as Plant Pathogens”, by Anne K. Vidaver and Patricia A. Lambrecht, Department of Plant Pathology, University of Nebraska, Lincoln, NE –
3. Laboratory 7 – “Plant Pathogenic Bacteria”,
