Bacteria are single-celled microorganisms and constitute a very wide group. Their single cells usually group in masses or in chains. The great majority of bacteria are obligate saprophytes and do not cause any disease in any living organism. But, there are many bacteria which enjoy parasitic mode of life and usually cause diseases in humans, animals and plants.
Some 80 species of bacteria consisting of numerous subspecies have been found to cause diseases in plants. Subspecies of plant pathogenic bacteria are also called pathovars, a pathovar (abbreviated as “pv.”) being a subspecies or group of strains of bacteria that can infect only plants within a certain genus or species. However, about half of all the plant pathogenic bacteria known belong to the genus Pseudomonas, about one third to the genus Xanthomonas, and about an eighth to the genus Erwinia.
The fact that a bacterium can cause disease in plant was first presented by Burrill (1878, 1880) by showing that the “fire blight” disease of pear is caused by a bacterium presently known as Erwinia amylovora.
At about the same time, Wakker (1883) proved that hyacinth yellows are caused by a bacterium. By 1895, when E.F. Smith demonstrated with proof the bacterial nature of cucurbit wilt disease, scientists were convinced with the fact that bacteria can cause diseases in plants. In this way, it took quite a considerable period of time in creating universal opinion in favour of plant pathogenic nature of bacteria.
Though bacteria are spherical, ellipsoidal, rod-shaped, spiral, comma-shaped, or filamentous in morphology, only the rod-shaped bacteria have been found plant pathogenic in nature, i.e., almost all bacteria that cause disease in plants are rod-shaped (only a filamentous bacterium, Streptomyces, being exception). Only a few of the plant pathogenic bacteria produce endospores.
Most plant pathogenic bacteria are flagellate. The flagellation on rod-shaped bacterial cell may be monotrichous (one flagellum at one end), lophotrichous (a tuft of flagella at one end), amphitrichous (a single or a tuft of flagella at each end), and peritrichous (flagella distributed all over the entire surface).
Almost all plant pathogenic bacteria grow parasitically in the host plant and partly in plant debris or in the soil as saprophytes. The ability of plant pathogenic bacteria to grow in soil as saprophytes differs among species. There are plant pathogenic bacteria like Agrobacterium tumefaciens (causing crown gall), Pseudomonas solanacearum (causing bacterial wilt of solanaceous plants), and Streptomyces scabies (causing common scab of potato) which have ability to survive indefinitely in soil as saprophytes in the absence of their host plants (soil-inhabitants).
But, most plant pathogenic bacteria have poor ability to grow in soil as saprophytes in the absence of their host plants; they persist in soil either as long as the host tissue resists decomposition by the saprophytes, or for varying durations afterwards, depending on the bacterial species and on the soil temperature and moisture conditions (soil-invaders).
Plant pathogenic bacteria reproduce asexually; the asexual process being mainly the ‘binary fission’. The unclear material (DNA) undergoes replication resulting in two DNA molecules in the parent cell.
Simultaneously, the cell membrane and the cell wall grow inward in the middle region of the cell dividing the parent cell into two equal daughter cells each possessing a DNA molecule. Finally, the two daughter cells get separated from each other at the partition-site and become independent.
This mode of reproduction completes rapidly usually within about 20 minutes in many of the bacteria. The daughter bacteria further follow the same process. In this way, at this rate, one bacterium conceivably could produce one million bacteria in ten hours provided all required conditions are favourable.
The production of bacteria in tremendous numbers in short period of time in an action-site brings great chemical changes therein. This property of causing quick changes in their environment makes the bacteria of great importance in the world of life in general and in the causation of diseases of plants in particular.
Classification of Plant Pathogenic Bacteria (Including Mollicutes):
Kingdom-Procaryotae
Bacteria-Possess cell membrane and cell wall
Division – Gracilicutes-Gram-negative bacteria
Class – Proteobacteria-Mostly single-celled bacteria
Family – Enterobacteriaceae
Genus – Erwinia, the cause of fire blight of pear and apple, soft rot of fleshy vegetables, and Stewart’s wilt in corn.
Family – Pseudomonadaceae
Genus – Acidovorax, the cause of leaf spots in corn, orchids, and watermelon. Pseudomonas, the cause of numerous leaf spots, blights, vascular wilts, soft rots, cankers, and galls.
Rhizobacter, the cause of bacterial gall of carrot.
Rhizobacter, the cause of corky root rot of lettuce.
Xantbomonas, the cause of numerous leaf spots, fruit spots, and blights of annual and perennial plants, vascular wilts, and citrus canker.
Xylophilus, the cause of bacterial necrosis and canker of grapevines.
Family – Rhizobiaceae
Genus – Agrobacterium, the cause of crown gall disease of plants.
Rhizobium, the cause of root nodules in legumes.
Family – Still unnamed
Genus – Xylella, the cause of leaf scorch and dieback diseases on trees and vines.
Division – Firmicutes-Gram-positive bacteria
Class – Firmibacteria-Mostly single-celled bacteria
Genus – Bacillus, the cause of rot of tubers, seeds, and seedlings, and white stripe of wheat.
Clostridium, the cause of rot of stored tubers and leaves and wet-wood of elm and poplar.
Class – Thallobacteria-Branching bacteria
Genus – Arthrobacter, the cause of bacterial blight of holly.
Clavibacter, the cause of bacterial wilts in alfalfa, potato, and tomato.
Curtobacterium, the cause of wilt in beans and other plants.
Rhodococcus, the cause of fasciation of sweet pea.
Streptomyces, the cause of common potato scab.
Mollicutes [Mycoplasma-like organisms (MLO)] Possess only cell membrane; cell wall absent
Division – Tenericutes
Class – Mollicutes
Family – Spiroplasmataceae
Genus – Spiroplasma, the cause of corn stunt, citrus stubborn disease.
Family(ies) – still unknown
Genus – Undefined, known as phytoplasmas [formerly as mycoplasma-like organisms (MLO)], the cause of numerous yellows, proliferation, and decline diseases in trees and some annuals.
Some of the Most Common Plant Pathogenic Bacteria:
1. Erwinia:
The bacteria are single-celled, straight rods, measuring 0.5-1.0 x 1.0-3.0 µm; motile (except E. stewartii and dissolvens) by means of several to many peritrichous flagella; are the only plant pathogenic bacteria that are facultative anaerobes; Gram-negative.
The genus has two major groups of pathogenic species:
(i) Amylovora group the members of which do not produce pectic enzymes and cause necrotic or wilt diseases and
(ii) Carotovora group the members of which have strong pectolytic activity and cause soft rots in plants.
2. Pseudomonas:
Single-celled, straight to curved rods, measuring 0.5-1.0 x 1.5-4.0 µm; motile by means of one or many polar flagella; non-spore forming; Gram-negative; strict aerobes; chemoorganotrophic; usually soil inhabitants; also found in fresh water and marine environments; non-fermentative.
Some plant pathogenic Pseudomonas species (e.g. P. syringae) are called fluorescent pseudomonads because, they produce yellow-green, diffusible, fluorescent pigments on a medium of low iron content, whereas other (e.g., P. solanacearum) are non-fluorescent pseudomonads because they do not produce fluorescent pigments.
3. Xanthomonas:
Single-celled, straight rods, measuring 0.4-1.0 x 1.2-3.0 µm; motile by means of a single polar flagellum; Gram negative, non-spore forming; strict aerobes; non-fermentative; positive to catalase, and negative to oxidase; form yellow coloured colonies on agar media; most are slow growing. All known species are plant pathogens and occur only in association with plants or plant materials.
4. Agrobacterium:
Rod-shaped, measuring 0.8 x 1.5-3.0 µm; motile by means of one to four peritrichous flagella; when only one flagellum is present, it is more often lateral than polar; Gram-negative; produce abundant polysaccharide slime when grown on carbohydrate containing media; colonies non-pigmented and usually smooth. These bacteria are rhizosphere and soil inhabitants and produce galls in plants.
5. Xylella:
Mostly single-celled, straight rods, measuring 0.3 x 1.0-4.0 µm; non-motile; aflagellate; Gram-negative; strictly aerobes; non-pigmented; produce long filamentous strands under certain cultural conditions; colonies small, with smooth or finely undulated margins; nutritionally fastidious; inhabit xylem of plant tissue.
6. Clavibacter (= Corynebacterium):
Straight or slightly curved rods, measuring 0.5-0.9 x 1.5-4.0 µm; Gram-positive; often arranged at an angle to give V-formation as a result of snapping or bending type of cell division; non-spore forming; usually non-motile, but some species are motile by means of one or two polar flagella; strict aerobes; rods pleomorphic; no coccoid cells seen. The genus was proposed by Davis et al. (1984) for those plant pathogenic species belonging to Corynebacterium michiganesis group that contain diaminobutyric acid as the major constituents of cell wall.
7. Streptomyces:
Slender, branched hyphae without cross walls (i.e., coenocytic), measuring 0.5-2.0 µm in diameter; aerial mycelium forms chains of 3 to many spores ; cell wall contains diaminopimelic acid; Gram-positive; aerobic; heterotrophs; generally reduce nitrates; slow growing; colonies formed on nutrient media are small (1-10 µm in diameter) in the beginning with a rather smooth surface but later with a weft of aerial mycelium that may appear angular, powdery, or velvety Streptomyces produce antibiotics and a variety of pigments that colour the mycelium and the substrate. All species are soil inhabitants.
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