Bacteria

Most bacteria that are associated with plants are actually saprotrophic, and do no harm to the plant itself. However, a small number, around 100 species, are able to cause disease. Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world. Most plant pathogenic bacteria are rod shaped (bacilli). In order to be able to colonise the plant they have specific pathogenicity factors. There are 4 main bacterial pathogenicity factors: (i) Cell wall degrading enzymes - used to break down the plant cell wall in order to release the nutrients inside. Used by pathogens such as Erwinia to cause soft rot (ii) Toxins-These can be non-host specific, and damage all plants, or host specific and only cause damage on a host plant (iii) Phytohormones - for example Agrobacterium changes the level of Auxin to cause tumours (iv) Exopolysaccharides - these are produced by bacteria and block xylem vessels, often leading to the death of the plant.