Plant Diseases in History
Certain diseases have had tremendous impacts on our society. Perhaps foremost among these is Phytophthora late blight which caused the potato famine in Ireland (1845 - 1846). As a result, there was widespread famine in northern Europe and more than one and one half million people immigrated to the United States. In 1879, a new disease, downy mildew of grape, was introduced into Europe from the United States, spread rapidly, and threatened to ruin the vineyards of Europe. A mixture of copper sulfate and lime, used initially to deter pilferers, was discovered to control the disease. This discovery became known as "Bordeaux mixture" and stimulated the study of the nature and control of plant diseases. These examples are prominent because they caused so much damage. However, plant diseases cause variable amounts of damage from year to year, depending often on weather patterns.
Disease Defined
Plant disease is the rule rather than the exception. Every plant has disease problems of one sort or another. Fortunately, plants either tolerate these maladies, or they are not very serious in most years. A plant disease is defined as any alteration (of a plant) that interferes with its normal structure or function and renders it unfit for its normal use. Plant problems may be caused by either living or non-living factors. Living factors are infectious (they spread from plant to plant). These include pathogens (fungi, nematodes, bacteria, and viruses), insects, and other animals. Non-living factors that cause plant problems do not spread from plant to plant. They are caused by chemical, physical, or mechanical factors. Examples of these abiotic factors include nutrient deficiencies and water or temperature stress. One must distinguish between infectious disease, caused by biotic agents, and noninfectious disease (abiotic agents).
Infectious organisms can be defined as follows:
Fungus An organism (plant) with no chlorophyll, that reproduces by means of structures called spores, and usually has filamentous growth; e.g., molds, yeasts, mushrooms.
Bacterium A single-celled, microscopic organism with cell walls and no chlorophyll; reproduces by fission.
Phytoplasma A microscopic, bacteria-like organism that lacks a cell wall, and therefore appears filamentous.
Virus A submicroscopic, subcellular particle that requires a host cell in which to multiply; it is not known if a virus is a living or nonliving agent.
Nematode A microscopic roundworm, usually living in soil, which feeds on plant roots.
Parasitic seed plant A higher plant with chlorophyll that lives parasitically on other plants, e.g., mistletoe.
Fungi and bacteria cause such plant diseases as leaf spot and fruit, stem, or root rot. Plant viruses, viroids, and phytoplasmas often cause growth distortion, stunting, and abnormal coloration. Nematodes can cause stunting and root distortion. Parasitic seed plants cause a general weakening of the host plant.
Conditions Necessary For Disease Caused by Pathogens
In order for disease to occur, three conditions must be met. First, it is necessary to have a susceptible host plant. Each species of plant is capable of being infected by only certain organisms (pathogens). The plant must be in a stage of development susceptible to infection by the disease agent. The second requirement is the presence of an active pathogen in a stage of development conducive to infecting the host plant. If there is no pathogen present, there can be no disease. The third condition is an environment suitable for the pathogen to infect the plant. Temperature and moisture are important factors. The diseases of major importance are those which attack below the soil line where moisture and temperature conditions are favorable. Viruses, however, are plant pathogens that are little affected by climate.
Symptoms, Signs, Syndromes
A symptom is the physical expression of a change in the appearance and function of the plant. Examples of symptoms are:
A disease syndrome is the group of signs and symptoms which collectively characterize a disease. Familiarity with a disease's signs or symptoms is not enough to diagnose a disease; it is necessary to know the syndrome and case history. Seeing a spot on a leaf doesn't tell you much, but finding pycnidia in that spot and knowing the plant species and recent weather conditions might be sufficient information to diagnose the disease. Other times, laboratory work is necessary for diagnosis.
Disease Development
It is important to understand how plant diseases develop in order to control them. By the time it becomes obvious that a plant has a disease, it is generally too late to do anything about it in that growing season. Plants cannot be cured in the way people expect their own ills to be cured. The process by which diseases develop can be broken into five distinct phases:
Inoculation
This is the introduction of the pathogen to the host plant tissue. Wind, or rain, or running water can move pathogens and introduce them to a host plant, as can birds, insects, people, or equipment. Some pathogens move themselves short distances, but most rely on other means. Sources of inoculum include plant debris, seed, perennial plants, and soil.
Incubation
This is a period of development during which the pathogen undergoes changes to develop a form which can penetrate or infect the new host plant. Some fungi, for instance, grow a structure called a penetration peg that can grow through the cell walls of the plant.
Penetration
This is the process of getting inside the plant. It may be an active or passive process. Some pathogens produce enzymes to dissolve the cutin and cellulose layers of plant material between them and the cell contents. Some pathogens can swim through water on a plant's surface and into the plant through natural openings (such as stomata, lenticels, or hydathodes) or through wounds. Some pathogens are put inside the plant by insects, pruning tools, or driving rain.
Infection
When the pathogen invades the plant tissue and establishes a parasitic relationship between itself and the host, infection has occurred.
Disease
When the host plant responds to the presence of the pathogen, a disease exists. The host's response results in symptoms of the disease, such as blight or necrosis. As the pathogen matures, it produces inoculum in the form of spores, virus particles, and bacterial cells that can be spread or disseminated to other adjacent, healthy plants.
Control of Diseases
The importance of understanding the disease development process becomes obvious when considering control options. By the time symptoms are expressed, the pathogen (with few exceptions) is already inside the host plant and is relatively safe. Therefore, control efforts, in most cases, must occur before penetration has taken place. The overall principle in effective disease control is to keep the inoculum density of the pathogen at very low levels.
Success in controlling plant disease will occur when a combination of the following methods of control are used:
Avoidance
A grower can avoid certain diseases by choice of geographic area or choice of planting site. Disease can be avoided by planting at a time that does not favor disease. Using disease-free planting stock or modifying cultural practices also helps to avoid disease.
Exclusion
A grower can inspect stock for signs of disease and reject or treat any which is suspect. Plant quarantines are designed to exclude certain pests from areas that are free of that pest. Elimination of carrier insects can exclude a disease.
Eradication
Once a disease is established in an area, eradication is unlikely. However, significant reduction in disease inoculum can be attained by destroying diseased plants or alternate hosts, by rotating crops, or by soil solarization.
Protection
Spraying or dusting plants with fungicides or bactericides can protect them from disease. Sometimes modifying cultural practices or the environment may protect the crop. Control of carrier insects will also protect plants.
Resistance
Breeding and selection are used to develop resistant crops. Resistance can be enhanced through proper culture of a crop. Tolerance is another form of resistance in which the plant becomes infected but goes on to mature and yield normally.
Sanitation
Several disinfectants can be used to surface sterilize growing surfaces. Bleach, in a 1:1 solution, is effective against viruses, bacteria, and fungal spores. It is not effective against sclerotia or other resting fungal structures. Alcohol may be used as a disinfectant of tools, but is not effective against fungal sclerotia, viruses, and some bacteria.
Therapy
Removal of diseased parts of a plant will sometimes control the disease. Heat can be used to treat contaminated seeds and to eliminate viruses from certain types of fruit tree bud wood.
Familiarity with crops and the diseases and insects that affect them is useful in planning control programs. Some diseases occur every season; others occur sporadically. Some can be controlled easily by using proper methods; others must be tolerated. Knowing which problem falls into which category comes with experience. Knowing the proper method to use at the proper time is a part of integrated pest management.
Summary
Plant diseases are to be expected. Fortunately, there are few truly devastating diseases in most years. For disease to occur, there must be a susceptible host, a suitable environment, and a living pathogen. When all three conditions are met, disease occurs. Severity of the disease depends on the degree to which the conditions are met.
Disease development follows a precise course of events. Inoculation occurs first, usually followed by incubation. Penetration of the host is next. Infection occurs when the pathogen invades the host tissue. Only when the host responds has disease occurred. By this time, it is usually too late to control the disease. Control involves more than the use of chemicals for protection. Avoidance, eradication, exclusion, resistance, and therapy all have a role in disease control. A combination of these will give best results.