Bacteria are microscopic in nature, single-celled prokaryotic organisms, without a defined nucleus, that reproduce asexually by binary fission (one cell splitting into two). They occur singly or in colonies of cells. Bacteria are classified into two main groups based on cell wall structure, which can be determined by a simple staining procedure called the Gram stain.
Gram negative bacteria stain red or pink while Gram positive bacteria stain purple. The difference in color is directly related to the chemical composition and structure of their cell walls. The cells can be rod-shaped, spherical, spiral-shaped or filamentous. Only a few bacteria are known to cause diseases in plants. Most bacteria are motile and have whip-like flagella that enable them through films of water.
Bacteria that ranges from 1-2 µm in size that cannot be seen with the unaided eye. Plant associated bacteria may be beneficial or harmful. All plant surfaces have microbes on them (termed epiphytes), and some microbes live inside plants (termed endophytes). Some are residents and some are transient.
Bacteria are among the microbes that successively colonize plants as they mature. Individual bacterial cells cannot be seen without the use of a microscope; however, large populations of bacteria become visible as aggregates in liquid, as biofilms on plants, as viscous suspensions plugging plant vessels, or colonies on petri dishes in the laboratory.
For beneficial purposes or as pathogens, populations of 106 CFU (colony-forming units/milliliter) or higher are normally required for bacteria to function as biological control agents or cause infectious disease.
Phytoplasmas and spiroplasmas are bacteria that lack rigid cell walls, and infect plants. Phytoplasmas are round or oval in shape. As with viruses, many diseases caused by fastidious bacteria are named after the most important host plant or the one where the disease was first discovered, but some can also infect many other plants.
For example, the aster yellows phytoplasma also affects other ornamentals, such as gladiolus and phlox or tomato, spinach, onion, lettuce, celery, carrots and strawberry, and many weeds.
List Of Plant Diseases Caused By Bacteria
#1. ASTER YELLOW
ASTER YELLOW is a plant disease, caused by a phytoplasma bacterium, affecting over 300 species of herbaceous broad-leafed plants. Aster yellow is found over much of the world wherever temperature of air does not raise much above 32 °C (90 °F). As the name implies, members of plants belonging to the family Asteraceae are vulnerable to infection, though the disease can also affect a variety of common vegetables, cereals, garden plants, and wild species.
#2. BACTERIAL BLIGHT
Causes: The bacteria are spread by rain and wind so the disease can occur after long cool wet periods which penetrates into the plant through damaged leaves. Warm dry weather proves to curb the spread of bacterial blight.
Symptoms: Small brown spots on the upper leaves which eventually grow bigger as they cover the whole leaf. In some cases, the infected leaves can turn brown and become watery.
Treatment: infected part of the crop which happens to be the leaves, stems and branches should be cut off; make sure to disinfect the pruning tools after use. Avoid the use of overhead irrigation system for watering to avoid further spreading. When applicable, practice crop rotation to avoid overwintering of the bacterial blight. Use disease free seeds and practice antibiotic seed treatment.
#3. BLACK ROT
Causes: Black rot can be caused by wet and warm weather conditions. The bacteria can infest the plant through natural openings or wounds of the leaf. Once the bacterium is in the plant, it can infect the entire plant traveling through its water conducting system.
Symptoms: Yellow to dark brown discoloration appears on the margins of the leaves. As the disease spreads it infects the whole leaf which turns dark brown. Later stages of black rot can take over the plant’s fruits or vegetables which eventually decay and dry out.
Treatment: dead branches of trees should be pruned, dried out fruit/vegetables should be plucked out, weed regularly to prevent the spread of the disease to other plants. Use bactericides to treat the infected plants but beware that most of the bactericides may impose harm on the plant and its fruit or vegetables. One method of prevention is to use disease-free seeds or to wash them properly before planting them.
#4. LEAF SPOT
Causes: Wet and cool conditions hepls in the spread of bacteria. Once it has infected the plant, the leaf spot bacteria can multiply very quickly.
Symptoms: Dark spots on the plant’s leaves and leaf discoloration. In some extreme cases, these dark necrotic spots can spread to the whole leaf and kill it.
Treatment: Make sure to cut all infected leaves of the plant in order to prevent further spread of the leaf spot. A copper-based bactericide should be used in the early stage of the disease. Be sure to remove all debris from infected plants in the garden and then do not plant new ones in the very same place.
Cause: The wilt bacteria can be transmitted by insects. When insects bite off of a leaf from the plant, the bacteria start multiplying at the wound and then begin to spread. Dry hot weather conditions helps the bacteria spread and induce wilting.
Symptoms: When a part of the stem or the branch is cut, white slimy ooze extends from one cut to the other. Yellowing of the leaves can occur but not in every case. Sometimes the infected plant can wilt rapidly without any yellowing of the leaves.
Treatment: The most effective way to prevent the spread of bacterial wilt is to control over the insects spreading the disease. Once wilt has infected the plant there is no way to cure it. However, it is essential not to use the diseased plants for compost and to make sure to remove any remains of the infected plants from the soil.
Classification of Bacteria
The classification of plant pathogenic bacteria is currently based on recent advancement on how bacteria are classified.
Most plant pathogenic bacteria belong to the following genera: Erwinia, Pectobacterium, Pantoea, Agrobacterium, Pseudomonas, Ralstonia, Burkholderia, Acidovorax, Xanthomonas, Clavibacter, Streptomyces, Xylella, Spiroplasma, and Phytoplasma. Plant pathogenic bacteria cause many different kinds of symptoms that include galls and overgrowths, wilts, leaf spots, specks and blights, soft rots, as well as scabs and cankers.
In contrast to viruses, which are inside host cells, walled bacteria grow in the spaces between cells and do not invade them. The means by which plant pathogenic bacteria cause disease is as varied as the types of symptoms they cause. Some plant pathogenic bacteria produce toxins or inject special proteins that lead to the death of the host or they produce enzymes that break down key structural components of plant cells and their walls. An example is the production of enzymes by soft-rotting bacteria that degrade the pectin layer that holds plant cells together. Still others colonize the water-conducting xylem vessels causing the plants to wilt and die.
Agrobacterium species even have the capability to genetically modify or transform their hosts and bring about the formation of cancer-like overgrowths called crown gall. Bacteria that cause plant diseases are spread in many ways;they can be splashed about by rain or carried by the wind, birds or insects