This is the sixth fact sheet in a series of ten designed to provide an overview of key concepts in plant pathology. Plant pathology is the study of plant disease including the reasons why plants get sick and how to control or manage healthy plants.
Although considered structurally simple, bacteria are extremely diverse from a metabolic standpoint and are found almost everywhere on Earth in vast numbers—from living in jet fuel and on the rims of volcanoes to thriving in hydrothermal vents deep on the ocean floor. There are both beneficial and pathogenic bacteria. Beneficial bacteria are involved in such diverse processes as digestion in animals, nitrogen fixation in the roots of certain legumes, the decomposition of animal and plant remains, and sewage disposal systems. Pathogenic bacteria, on the other hand, cause severe and often fatal diseases in humans, animals and plants. The first bacterial disease ever discovered was anthrax (caused by Bacillus anthracis) of cattle and sheep in 1876. The discovery of anthrax in cattle was immediately followed by the discovery of fireblight of pear and apple (caused by Erwinia amylovora) by T. J. Burrill from the University of Illinois (1877–1885). Another group of bacterial pathogens are difficult or impossible to culture in the laboratory and are called fastidious vascular bacteria. They grow in either the xylem or phloem tissues and interfere with the transport of water and nutrients in the plant. Many of them are vectored by sucking insects such as leafhoppers, planthoppers and psyllids. Studies of corn stunt provide evidence that once the insect vectors establish the infective particles in their bodies, the insects retain the ability to transmit them the rest of their lives. Until their discovery in 1967, most of the diseases now known to be caused by fastidious vascular bacteria were believed to be caused by viruses and were initially described by virologists.
Bacteria are microscopic, 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 and 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 of the latter are known to cause diseases in plants. Most bacteria are motile and have whip-like flagella that propel them through films of water.
Phytoplasmas and spiroplasmas are bacteria that lack rigid cell walls, and infect plants. Phytoplasmas are round or ovoid. 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 characterized, 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.
Figure 1. Bacterial strand test on cut stems, with bacterial slime streaming from xylem tissues.
Image courtesy M. P. Hoffman, © The American Phytopathological Society.
|Figure 2. Bacterial leaf blight on wheat. Note the bacterial ooze. Image courtesy Sam Livingston, © The American Phytopathological Society.|
|Figure 3. Crown gall, caused by Agrobacterium tumefaciens, on a burning bush. Image courtesy Robert L. Forster, © The American Phytopathological Society.|
The taxonomy of plant pathogenic bacteria is currently in flux based on recent advances 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 host cell death 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 ability 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. People can unwittingly spread bacterial diseases by, for instance, pruning infected orchard trees during the rainy season. Water facilitates the entrance of bacteria carried on pruning tools into the pruning cuts. Propagation with bacteria-infected plant material is a major way pathogenic bacteria are moved over great distances. No matter how the bacterial pathogens are disseminated, they require a wound or natural opening, such as stomata, to get inside a plant host. Once inside they then kill host cells, by the means described above, so that they can grow. Between hosts they may grow harmlessly on plant surfaces and then can overwinter or survive unfavorable environmental periods or the absence of a susceptible host by either going dormant in infected tissue, infested soil or water, or in an insect vector.
Bacterial diseases in plants are difficult to control. Emphasis is on preventing the spread of the bacteria rather than on curing the plant. Integrated management measures for bacterial plant pathogens include:
1. Genetic Host Resistance
- Resistant varieties, cultivars or hybrids is the most important control procedure.
2. Cultural Practices
- Bacteria-free seed or propagation materials.
- Sanitation, particularly disinfestation of pruning tools.
- Crop rotation to reduce over-wintering.
- Preventing surface wounds that permit the entrance of bacteria into the inner tissues.
- Propagating only bacteria-free nursery stock.
- Prolonged exposure to dry air, heat, and sunlight will sometimes kill bacteria in plant material.
|Figure 4. Spiroplasmas in the phloem of an infected corn plant. Image courtesy R.E. Davis, © The American Phytopathological Society.||Figure 5. Citrus canker symptoms on fruit. Image courtesy Dr. Shabbir A. Rizvi, © The American Phytopathological Society.|
3. Chemical Applications
- Applications of copper-containing compounds or Bordeaux mixture (copper sulfate and lime).
- Antibiotics: streptomycin and/or oxytetracycline may also help kill or suppress plant pathogenic bacteria prior to infection and reduce spread of the disease, but they will not cure plants that are already diseased.
- Antibiotics are also used to treat diseases caused by fastidious vascular bacteria. Phytoplasmas and spiroplasmas are susceptible to certain antibiotics, particularly tetracycline, which has been used to treat pear trees with the pear decline disease. Tetracycline must be injected into mature trees on a routine or therapeutic schedule to be effective and even then only appears to suppress the development of symptoms rather than curing the infected plant. Applications made during the early stages of infection tend to be more effective than in the later stages of disease development.
- Insect control will help to eliminate vectors or reduce feeding wounds that can provide points of entry.
4. Biological Control
- The use of antagonistic or biological control products may also be effective for managing bacterial diseases of plants.
5. Government Regulatory Measures
- The implementation of strict quarantines that exclude or restrict the introduction or movement of fungal and FLO pathogens or infected plant material.
These fact sheets can be found at OSU Extension’s Ohioline website: ohioline.osu.edu.
Introduction to Plant Disease Series
- Plants Get Sick Too! An Introduction to Plant Diseases
- Diagnosing Sick Plants
- 20 Questions on Plant Diagnosis
- Keeping Plants Healthy: An Overview of Integrated Plant Health Management
- Viral Diseases of Plants
- Bacterial Diseases of Plants
- Fungal and Fungal-like Diseases of Plants
- Nematode Diseases of Plants
- Parasitic Higher Plants
- Sanitation and Phytosanitation (SPS): The Importance of SPS in Global Movement of Plant Materials
Search for these bacterial disease fact sheets on Ohioline.
- Fireblight of Apples, Crabapples and Pears
- Bacterial Spot, Speck and Canker of Tomatoes
- Bacterial Crown Gall of Ornamentals in the Landscape
- Blackleg, Aerial Stem Rot and Tuber Soft Rot of Potato
- Citrus Canker
- Stewart’s Wilt of Corn