General Characters of Plant Pathogenic Bacteria
1. Almost all plant pathogenic bacteria are rod shaped, the only exception being Streptomyces, which is filamentous.
2. The rod shaped bacteria are more or less short and cylindrical and in young cultures, they range from 0.6 to 3.5 m in length and from 0.5 to 1.0 m in diameter.
3. In older cultures or at high temperatures, the rods of some species are much longer and they may even appear filamentous.
4. Sometimes deviations from the rod shape in the form of a club , a Y or V shaped, and other branched forms occur, and some bacteria may occasionally occur in pairs or in short chains.
5. The cell walls of bacteria of most species are enveloped by a viscous, gummy material, which may be thin (Slime layer) or may be thick, forming a relatively large mass around the cell (Capsule).
6. Most plant pathogenic bacteria are equipped with delicate, thread like flagella.
7. In some bacterial species each bacterium has only the flagellum, others , have a tuff of flagella at one end of the cell (Polar flagella); some have a single flagellum or a tuft of flagella at each end, and till others have peritrichous flagella, that is, distributed over the entire surface of the cell.
8. In the filamentous Streptomyces species, the cells consist of non septate branched threads, which usually have a spiral formation and produce conidia in chains on aerial hyphae.
9. Single bacterium appears hyaline or yellowish white under the compound microscope.
10. Bacteria grow and produce colonies on solid medium.
11. Colonies of different species may vary in size, shape, form of edges, elevation and colour, and are sometimes characteristics of a given species.
12. Bacterial cells have thin, relatively tough, and somewhat rigid cell walls.
13. All the material inside the cell wall constitutes the protoplast.
14. The nuclear material consist of a large circular chromosome composed DNA and appear as spherical, ellipsoidal or dumbbell shaped body within the cytoplasm.
15. Often bacteria also have single or multiple copies of addition smaller circular chromosomes called ‘Plasmids’ that can move or be moved between bacteria or between bacteria and plants as for example in the crown gall disease.
16. Rod shaped Phytopathogenic bacteria reproduce by the asexual process known as binary fission or fission. Under favourable conditions bacteria may divide every 20 minutes.
17. Almost all plant pathogenic bacteria develop mostly in the host plant as parasites and partly in plant debris or in the soil as saprophytes.