Bacterial diseases are found in all areas of the landscape, but they are generally more severe in areas of high to moderate rainfall. Bacterial diseases are controlled by resistant varieties, fungicides and cultural practices. (see: prevention)
Bacteria are microscopic single-cell organisms that reproduce by dividing in half. This process may occur as often as every 20 minutes, or it may take several hours. In some of the faster multiplying species, a single bacterium can produce over 47 million descendants in 12 hours.
Approximately 170 species of bacteria can cause disease on foliage plants. Bacteria cannot penetrate directly into plant tissue, but must enter through wounds or natural openings such as stomata (pores for air exchange) in leaves.
Bacteria are normally present on plant surfaces and will only cause problems when conditions are favorable for their growth and multiplication. These conditions include high humidity, crowding, and poor air circulation around plants. Misting plants will provide a film of water on the leaves where bacteria can multiply.
Just as with viruses, bacteria are a problem for plants and animals, although, in plants bacterial diseases are less common than fungal or viral diseases.
Unlike viruses many bacteria are free living; they can be parasites like viruses, saprophytes or autotrophs, like plants.
Bacterial diseases of plants tend to cause spotting of leaves, stems or fruits; sometimes bacteria cause soft rots in which tissue becomes a slimy mess. Generally speaking bacteria cannot invade healthy plant tissue; they need a wound or an area of dead or dying tissue to start an infection.
Plant bacterial diseases can be controlled with the same kinds of antibiotics used to control animal diseases, such as streptomycin. However, we are generally reluctant to use these on a large scale and control is usually based on avoidance or removal of sources of innoculum.
Fire blight of pear and apple, bacterial leaf spot and bacterial canker of peach and plum are three of the more frequently observed diseases in the home garden.
Bacterial pathogens cause leaf spots and stem rots on a variety of ornamental plants. Leaf spots are initially water-soaked or greasy in appearance, often angular, and concentrated along leaf veins or margins. In some cases, the leaf tissue surrounding the necrotic spots may be yellow.
Bacterial pathogens require films of water to enter plant tissues. Cool, wet conditions favor leaf spots caused by Pseudomonas species. Bacterial soft rots of stems are usually associated with wounds from handling or insect feeding.
Fungus gnat larvae can spread soft rot bacteria. Bacterial diseases of ornamental plants can cause wilt and death of plants. Any plant suspected of having a bacterial disease should be sent to a diagnostic laboratory for confirmation of the disease.