Plant pathology

(botany) The branch of botany concerned with diseases of plants.

The study of disease in plants; it is an integration of many biological disciplines and bridges the basic and applied sciences. As a science, plant pathology encompasses the theory and general concepts of the nature and cause of disease, and yet it also involves disease control strategies, with the ultimate goal being reduction of damage to the quantity and quality of food and fiber essential for human existence.

Kinds of plant diseases

Diseases were first classified on the basis of symptoms. Three major categories of symptoms were recognized long before the causes of disease were known; necroses, destruction of cell protoplasts (rots, spots, wilts); hypoplases, failure in plant development (chlorosis, stunting); and hyperplases, overdevelopment in cell number and size (witches'-brooms, galls). This scheme remains useful for recognition and diagnosis.

When fungi, and then bacteria, nematodes, and viruses, were recognized as causes of disease, it became convenient to classify diseases according to the responsible agent. If the agents were infectious (biotic), the diseases were classified as being “caused by bacteria,” “caused by nematodes,” or “caused by viruses.” To this list were added phanerogams and protozoans, and later mollicutes (mycoplasmas, spiroplasmas), rickettsias, and viroids. In a second group were those diseases caused by such noninfectious (abiotic) agents as air pollutants, inadequate oxygen, and nutrient excesses and deficiencies.

Other classifications of disease have been proposed, such as diseases of specific plant organs, diseases involving physiological processes, and diseases of specific crops or crop groups (for example, field crops, fruit crops, vegetable crops).

Symptoms of plant diseases

Symptoms are expressions of pathological activity in plants. They are visible manifestations of changes in color, form, and structure: leaves may become spotted, turn yellow, and die; fruits may rot on the plants or in storage; cankers may form on stems; and plants may blight and wilt. Diagnosticians learn how to associate certain symptoms with specific diseases, and they use this knowledge in the identification and control of pathogens responsible for the diseases.

Those symptoms that are external and readily visible are considered morphological. Others are internal and primarily histological, for example, vascular discoloration of the xylem of wilting plants. Microscopic examination of diseased plants may reveal additional symptoms at the cytological level, such as the formation of tyloses (extrusion of living parenchyma cells of the xylem of wilted tissues into vessel elements).

It is important to make a distinction between the visible expression of the diseased condition in the plant, the symptom, and the visible manifestation of the agent which is responsible for that condition, the sign. The sign is the structure of the pathogen, and when present it is most helpful in diagnosis of the disease.

All symptoms may be conveniently classified into three major types because of the manner in which pathogens affect plants. Most pathogens produce dead and dying tissues, and the symptoms expressed are categorized as necroses. Early stages of necrosis are evident in such conditions as hydrosis, wilting, and yellowing. As cells and tissues die, the appearance of the plant or plant part is changed, and is recognizable in such common conditions as blight, canker, rot, and spot.

Many pathogens do not cause necrosis, but interfere with cell growth or development. Plants thus affected may eventually become necrotic, but the activity of the pathogen is primarily inhibitory or stimulatory. If there is a decrease in cell number or size, the expressions of pathological activity are classified as hypoplases; if cell number or size is increased, the symptoms are grouped as hyperplases. These activities are very specific and most helpful in diagnosis. In the former group are such symptoms as mosaic, rosetting, and stunting, with obvious reduction in plant color, structure, and size. In the latter group are gall, scab, and witches'-broom, all visible evidence of stimulation of growth and development of plant tissues. See also Crown gall.

The primary agents of plant disease are fungi, bacteria, viruses and viroids, nematodes, parasitic seed plants, and a variety of noninfectious agents.

Fungi

More plant diseases are caused by fungi than by any other agent. The fungi that cause plant disease derive their food from the plant (host) and are called parasites. Those that can live and grow only in association with living plant tissues are obligate parasites. Some fungi obtain their food from dead organic matter and are known as saprobes or saprophytes. Still others can utilize food from either dead organic matter or from living plant cells, and are referred to as either facultative parasites or facultative saprophytes. The classes with plant disease-causing fungi are Plasmodiophoromycetes, Chytridiomycetes, Zygomycetes, Oomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes. See also Fungi.

Bacteria

Over 100 species of bacteria mainly in five genera cause disease in hundreds of different species of flowering plants. Destructive bacterial diseases affect the major cereal, vegetable, and fruit crops. None of the bacterial pathogens of plants causes serious diseases of humans or animals, and certain groups of green plants (mosses, ferns, conifers, and hardwood trees) have few or no major bacterial diseases. See also Bacteria.

Each species of bacteria produces a distinctive pattern of symptoms on those hosts that it attacks.

With a few exceptions, most of the bacteria that cause disease in plants are non-spore-forming, rod-shaped, gram-negative cells. It is not possible to separate plant pathogenic bacteria into species on the basis of colony characteristics, cell morphology, or staining characteristics. Therefore, biochemical and physical tests used to differentiate bacteria in general are also used in studies on plant pathogens. Helpful techniques include serological tests, DNA hybridization, sensitivity to phages, and gel electrophoresis of proteins. However, one of the most important of the tests for identification is the demonstration of pathogenicity to a specific plant.

Many foliage pathogens are dependent upon wind-driven splashing rain as the primary means of spread from plant to plant. Bacteria also may be spread from plant to plant by insects, in irrigation water, and by various cultural operations during the growing season. Bacteria can also survive the winter in insects such as flea beetles. Most bacterial plant pathogens survive adverse conditions in host plants. Only a small number of species survive for long periods of time in the soil in the absence of host plants. Bacterial plant pathogens are not capable of forcing their way through the cuticle f or bark of a stem. They must enter through wounds or natural openings.

The exact manner by which bacteria induce disease in plants is not fully understood. The surface area of hundreds of thousands of cells in intimate contact with the surrounding plant cells is very large, and enzymes, as well as toxic materials, can be released readily into the host tissue.

Certain vascular parasites produce gumlike substances or polysaccharides. Masses of bacterial cells embedded in the polysaccharide material and host responses to hormone imbalances reduce the rate of flow of water when such bacteria as the one causing bacterial wilt of tomato invade water-conducting tissue. The genetic control for tumor induction in the crown gall bacterium has been shown to reside in a plasmid, a nonchromosomal DNA element. The presence of the tumor-inducing plasmid ensures that genetic information in the bacterial cell is transferred and maintained in the transformed cells of the plant cancer resulting from infection.

Since primary sources of inoculum for new infections are often in undecomposed plant debris or seed, crop rotation and the use of pathogen-free seed or seed treatments are, in general, effective in reducing losses from a large number of bacterial diseases affecting foliage. Resistant varieties have been developed for a number of foliage diseases because spraying or dusting, in general, has been relatively ineffective. Certain antibiotics are effective, but not widely used because of the concern for the development of resistant strains. An insect-transmitted bacterium can be controlled if populations of the insect vector are reduced.

Viruses and viroids

Viruses and viroids are the simplest of the various causative agents of plant disease. The essential element of each of these two pathogens is an infective nucleic acid. The nucleic acid of viruses is covered by an exterior shell (coat) of protein, but that of viroids is not. See also Plant viruses and viroids.

Approximately 400 plant viruses and about 10 viroids are known. The nucleic acid of most plant viruses is a single-stranded RNA; a number of isometric viruses have a double-stranded RNA. A few viruses contain double-stranded DNA, and several containing single-stranded DNA have been reported. The nucleic acid of viroids is a single-stranded RNA, but its molecular weight is much lower than that of viruses.

Some viruses, such as tobacco mosaic virus (TMV) and cucumber mosaic virus, are found in many plant species; others, such as wheat streak mosaic virus, occur only in a few grasses. Viruses are transmitted from plant to plant in several ways. The majority are transmitted by vectors such as insects, mites, nematodes, and fungi which acquire viruses during feeding upon infected plants. Some viruses are transmitted to succeeding generations by infected seed. Viroids are spread mainly by contact between healthy and diseased plants or by the use of contaminated cutting tools.

The control or prevention of virus diseases involves breeding for resistance, propagation of virus-free plants, use of virus-free seed, practices designed to reduce the spread by vectors, and, in some cases, the deliberate inoculation of plants with mild strains of a virus to protect them from the deleterious effects of severe strains. See also Plant viruses and viroids.

Nematodes

All soils that support plant life contain nematodes living in the water films that surround soil particlesematodes feed primarily on microscopic plants, animals, and bacteria, but a few are parasites of animals; another relatively small group of nematodes parasitize plants. See also Nemata.

Plant-parasitic nematodes are distinguished by their small size, about 1 mm average length, and mouthparts that are modified to form a hollow stylet which is inserted into plant cells. All of the plant parasites are placed into two orders, Tylenchida and Dorylaimida.

Plant injury is of three general types and is related to feeding habits. Migratory endoparasites destroy tissues as they feed, producing necrotic lesions in the root cortex. Other migratory endoparasites invade leaf tissues and produce extensive brown spots. Sedentary endoparasites do not kill host cells, but induce changes in host tissues, which lead to an elaborate feeding site or gall. The third general type of symptom is produced by certain migratory ectoparasites, where root tips are devitalized and cease to grow without any associated swelling or necrosis.

In addition to the plant injury that they cause directly, nematodes are important factors in disease complexes. Lesions and galls provide entrance courts for soil fungi and bacteria, and many diseases caused by soil-borne pathogens are more severe when nematodes are present. Important viruses are transmitted by nematodes of the order Dorylaimida.

Control of plant-parasitic nematodes often is based on selection of nonhosts for crop rotations or nematode-resistant varieties. Some plant species release compounds into the soil that are toxic to nematodes. Animal manures, compost, and other organic amendments enhance the buildup of natural enemies of nematodes.

Many parasitic seed plants (estimated at nearly 3000) attack other higher plants. In some families (for example, the mistletoes Loranthaceae and Viscaceae) all members are parasitic; in others, only a single genus is parasitic in an otherwise autotrophic family.

Most parasitic plants are terrestrial; that is, the parasitic connection with the host plant is through the roots. Other parasitic plants grow on the above-ground parts of the host. Some plants are classed as semiparasites because they can live in the soil as independent plants for a time, but are not vigorous or may not flower if they do not become attached to a suitable host. The nutritional status of parasitic plants ranges from total parasites with no chlorophyll (for example, the broomrapes, Orobanchaceae) to plants that are well supplied with chlorophyll and obtain primarily water and minerals from their hosts (many mistletoes; see illustration).

Many small plants of a dwarf mistletoe (Arceuthobium vaginatum) parasitizing a ponderosa pine branch. This is the most damaging disease of ponderosa pine in many parts of the West.

Noninfectious agents of disease

Plants with symptoms caused by noninfectious agents cannot serve as sources of further spread of the same disorder. Such noninfectious agents may be deficiencies or excesses of nutrients, anthropogenic pollutants, or biological effects by organisms external to the affected plants. On the farm, plant-damaging pollution may be caused by careless use of pesticides. Mishandled herbicides are by far the most damaging to plants. Off the farm, anthropogenic air pollutants are generated by industrial processes, and by any heating or transportation method that uses fossil fuels. The most common air pollutants that damage plants are sulfur oxides and ozone. Sulfur oxides are produced when sulfur-containing fossil fuels are burned or metallic sulfides are refined. Human-generated ozone is produced by sunlight acting on clouds of nitrogen oxides and hydrocarbons that come primarily from automobile exhausts. See also Air pollution; Water pollution.

Epidemiology of plant disease

Epidemiology is the study of the intensification of disease over time and the spread of disease in space. The botanical epidemiologist is concerned with the interrelationships of the host plant (suscept), the pathogen, and the environment, which are the components of the disease triangle. With a thorough knowledge of these components, the outbreak of disease may be forecast in advance, the speed at which the epidemic will intensify may be determined, control measures can be applied at critical periods, and any yield loss to disease can be projected. The maximum amount of disease occurs when the host plant is susceptible, the pathogen is aggressive, and the environment is favorable.

Epidemiologically, there are two main types of diseases: monocyclic, those that have but a single infection cycle (with the rare possibility of a second or even third cycle) per crop season; and polycyclic, those that have many, overlapping, concatenated cycles of infection per crop season. For both epidemiological types, the increase of disease slows as the proportion of disease approaches saturation or 100%.

Control of plant disease is defined as the maintenance of disease severity below a certain threshold, which is determined by economic losses. Diseases may be high in incidence but low in severity, or low in incidence but high in severity, and are kept in check by preventing the development of epidemics. The principles of plant disease control form the basis for preventing epidemics. However, the practicing agriculturist uses three approaches to the control of plant disease: cultural practices affecting the environmental requirement of the suscept-pathogen-environment triangle necessary for disease development, disease resistance, and chemical pesticides.