Bacteria are one of the three ‘domains’ of life, that is to say that their characteristics are different from any other lifeforms (eg different from fungi, viruses, animals and plants). Bacteria are single celled organisms which reproduce (often very rapidly, especially in warm conditions) by simply dividing themselves into two. There is a huge number of different types of bacteria (many millions), but relatively few are plant-pathogenic: living within the plant as a parasite. While not as numerous as fungal diseases, bacterial infections are often more serious.

Symptoms of bacterial diseases most commonly involve spots appearing on the stems, leaves, flowers or fruits of the plant. Bacterial diseases cause some of the most devastating damage to plants as they include many blights, soft rots and wilts. For example, the outbreak of fireblight among pear trees in 1930s America lead to the near elimination of commercial pear cultivation throughout the United States.

Unless caught early enough, and the affected plant parts removed and destroyed, there is generally no cure available for bacterial diseases. Bactericidal chemicals (usually known as antibiotics) could theoretically be used within the horticultural domain, but this increased use of bactericides would speed up the exposure of all bacteria (including those affecting people and animals) and therefore result in the development of resistant strains.