Pathogens

Types of Pathogens

Fungi.

Like all eukaryotic organisms, fungal cells have nuclei, a well-defined endoplasmic reticulum with ribosomes, and cell organelles such as mitochondria. The fungal body consists of filamentous strands called hyphae that collectively make up a mycelium. Sometimes the hyphae become compressed, forming a tissue such as that found in a mushroom fruiting body.

Fungi are classified in the kingdom Fungi, separate from all other organisms. There are many different groups within the kingdom, and most groups have prominent plant pathogens. Of particular note are the Ascomycetes, Fungi Imperfecti, and some of the Basidiomycetes. Ascomycetes produce sexual spores called ascospores that are vital in survival between hosts. They also produce asexual spores called conidia that play a major role in the spread of disease during the growing season. The Fungi Imperfecti usually are ascomycetes that have lost the ascospore (sexual) stage. Sometimes they produce ascospores but have been classified according to the coni-dial stage because of its importance in the disease cycle. It is valid to classify a fungus based either on its perfect (sexual) state or on its imperfect (asexual) state.

The Basidiomycetes are extremely common in nature, but only the rusts and smuts are notable plant pathogens. While most basidiomycetes produce basidiospores in a fruiting body such as a mushroom, in rusts and smuts the basidiospores are produced from a specialized, overwintering spore called a teliospore. The rust fungi are especially common and usually have a complex sexual cycle with four spore stages and two different hosts required for completion of the sexual cycle. Rusts also produce an asexual spore called a uredospore that is responsible for spread of disease during the growing season.

A fourth group of fungi, the Oomycetes, has long been recognized to be very different from other fungi in both morphology and chemistry. They produce overwintering sexual spores called oospores and asexual motile spores (zoospores) that spread disease during the season. The current trend is to place these fungi in a kingdom other than Fungi. The Oomycetes contain the Pythium and Phytophthora species and the downy mildews, plant pathogens that are of worldwide importance. Regardless of classification, these pathogens will continue to be treated much the same as true fungi by those who work with plant diseases.

The potato late blight disease caused by Phytophthora infestans that resulted in famine in Ireland in 1845 and 1846 first brought the attention of the world to plant diseases. At that time many thought that fungi arose spontaneously in diseased plants and did not themselves cause disease. However, publications by the German scientist Anton de Bary beginning in 1853 convincingly demonstrated the prominent role fungi play as plant pathogens.

Bacteria.

The plant pathogenic bacteria are rod-shaped eubacteria. They are prokaryotes, having a chromosome but no nucleus. The cytoplasm has ribosomes but no endoplasmic reticulum and no organelles. The cells have a cell wall and may or may not have flagella.

The first bacterium shown to cause a plant disease, fire blight of pome fruits, was reported by Thomas Burrill in Illinois in 1878. However, it was the research by Erwin F. Smith from 1890 to 1915 that demonstrated the importance of these agents as plant pathogens.

Viruses.

Viruses are nucleoprotein macromolecules. They contain genetic information, either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), which is covered by protein subunits. Plant pathogenic viruses usually contain RNA rather than DNA. Since viruses are not cellular organisms, they express lifelike characteristics only when within a susceptible host cell. Additionally, since they are not cellular, they do not obtain organic nutrients directly from the host. Instead, the RNA or DNA of the virus directs the metabolic machinery of the host cell to use organic nutrients present in the cell. Various chemical reactions lead to symptom expression as well as production of new virus particles.

Tobacco mosaic disease was shown in the 1890s to be caused by a sub-microscopic infectious agent later determined to be a virus, now called tobacco mosaic virus.

Viroids.

A viroid is a small, infectious piece of RNA. Unlike a virus, it has no protein, but it behaves as a plant parasite much the same as a virus. In 1967, Theodor Diener and William Raymer reported on the basic characteristics of the agent causing spindle tuber of potato, and in 1971 Diener named these agents "viroids." About twenty diseases have been shown to be caused by this type of pathogen.

Phytoplasmas.

Phytoplasmas are prokaryotic cellular organisms. They represent a separate group of bacteria, having no cell wall or flagella. Much like viruses, they are transmitted by insects and cause phloem necrosis-type diseases. These diseases, having yellows witches'-broom-type symptoms, were thought to be caused by viruses until 1967 when Yoji Doi and others in Japan showed that the disease-inducing agents are mycoplasma-like organisms. Some of these pathogens, especially the aster yellows phytoplasma, have a wide host range and attack plants in many families.

For many years these plant pathogens were identified as mycoplasma-like organisms, but they now are called phytoplasmas. Although viruses and phytoplasmas are very different biological agents, similarities in transmission and in host-parasite interactions and symptoms make it easy to understand why researchers before 1967 thought all these diseases were caused by viruses.

Nematodes.

Plant pathogenic nematodes (roundworms) are usually found in soil. These plant pathogens have a stylet (spearlike) mouthpart that is used to penetrate and feed on roots. Although root knot nematode diseases have been well known since the 1850s, it was not until the period between 1920 and 1940 that research by many investigators showed the full significance of these agents as plant pathogens.

Angiosperms.

Like typical flowering plants, parasitic angiosperms have stems, leaves, flowers, and seeds. They do not have true roots but produce a structure that penetrates stems and unites with the vascular system of the host. The leaves may or may not have chloroplasts, but these pathogens are completely dependent on the host for water and mineral nutrients. Dwarf mistletoe is a prominent pathogen of coniferous forest trees, and the dodders attack many crops worldwide. Leafy mistletoe, known as a popular household Christmas decoration, attacks hardwood trees but is seldom a leading pathogen.

Insects.

Insects do not merely feed on plants; they often produce toxins and growth substances that cause diseaselike symptoms. Some of the biological interactions are similar to those that occur with nematodes. However, there are hundreds of thousands, perhaps millions, of insect species, and their life cycles and behavior may be very complex. Although the phenomenon may be much the same, insect problems are worked on by experts (entomologists), and the insects usually are not thought of as pathogens. They are, however, major vectors of diseases, including those caused by viruses and phytoplasmas and some fungi and bacteria.

How Pathogens Cause Disease

Pathogens impede normal growth of plants in many ways. They attack and kill seeds and seedlings (damping off diseases). They invade and kill roots, preventing absorption of water and mineral nutrients (root rots). They invade and plug xylem tissue, preventing movement of water and minerals to leaves and growing points (vascular wilts). They kill leaves, preventing photosynthesis and production of carbohydrates (leaf spots and blights, downy mildews, powdery mildews, and rusts). The phloem tissue may be invaded and killed, preventing translocation of the carbohydrate produced in photosynthesis to other parts of the plant (phloem necrosis). After the crop has been produced, pathogens may rot fruits and vegetables in transit or storage or in the marketplace (fruit and vegetable rots). A few pathogens cause disease by inducing abnormal growth, thus stunting normal growth (galls).

Damping Off.

Many fungi that live in the soil invade and kill seeds or seedlings. This is called damping off. Beyond the seedling stage, a plant is no longer susceptible to damping off. There is no resistance but seed treatments with fungicides usually provide effective control.

Root Rots.

Roots may be rotted by many fungi living in the soil. The resulting lack of water and mineral nutrients stunts growth and causes a general yellowing due to lack of chlorophyll. There usually is little resistance, and chemical controls are ineffective. Crop rotation may hold crop losses to acceptable levels.

Vascular Wilts.

Vascular wilts, diseases of the xylem tissue, are caused by fungi and bacteria. Fungi causing this disease are soilborne. They infect roots and grow through the plant, colonizing the xylem. Bacteria that cause vascular wilts are transmitted by insects or penetrate leaves through stomatal openings. They invade xylem tissue through pits in the xylem vessel cell walls and become systemic in the plant. Vascular wilts caused by both fungi and bacteria reduce movement of water and mineral nutrients to stems and leaves, resulting in symptoms of wilting and yellowing. There is no chemical control but genetic resistance may be helpful. Crop rotation is important in combating vascular wilts.

Leaf Diseases.

Many fungi and bacteria attack leaves, causing leaf spots and blights. The downy and powdery mildews also cover the leaves with fungal structures further reducing photosynthesis. Both of the mildews and the rusts eventually kill leaf tissue, but since all three are obligate parasites, they no longer can obtain nutrients after the leaves are dead. Resistance to these fungal diseases often is available, and chemical controls usually are effective. Most of the fungicides applied to crops are used to control leaf diseases. There are fewer leaf diseases caused by bacteria, and this is fortunate because genetic resistance is seldom available and chemical controls are usually ineffective.

Phloem Necrosis.

Viruses and phytoplasmas are transmitted by insect vectors that feed on leaves. The vectors often feed on the phloem tissue, directly depositing the pathogen in the tissue. These agents then become systemic in the phloem tissue, killing the phloem cells (necrosis) and preventing translocation of organic nutrients throughout the plant. Typical symptoms are stunting, yellowing, mosaic (different shades of green and yellow), and mottling (blotches of different colors). There is no chemical control and often little resistance. Cultural practices such as use of healthy planting stock often limit disease incidence.

Fruit and Vegetable Rots.

Postharvest rots by fungi and bacteria often cause serious losses. Chemical treatments help control these diseases, but more significant prevention tactics include sanitation and use of proper storage conditions, particularly reduced temperature and increased air circulation.

Galls.

Abnormal growth is an extremely common phenomenon and can be caused by many biological agents. The most notable abnormal growth diseases of crops are crown gall, caused by the bacterium Agrobacterium, club root of crucifers caused by the fungus Plasmodiophora brassicae, and root knot caused by the nematode Meloidogyne. Abnormal growth results from action of the same kinds of growth substances that are responsible for normal growth: auxins, cytokinins, and gibberellins.

Recognition and Penetration

Relatively little is known of the biochemistry of recognition of a susceptible host by a pathogen. Fungal and bacterial inoculum (infectious material) is spread at random to both hosts and nonhosts. Mucilagenous substances on the surface of the inoculum facilitates adherence to host surfaces. Some host chemicals that serve as signals leading to penetration are known, and some pathogen chemicals that serve as elicitors in disease development have been identified. In some cases penetration occurs but growth in the host is limited, and disease does not develop. Either the agent does not produce the elicitors that lead to infection, or the host produces chemicals that prevent infection. Since viruses and phytoplasmas are brought to hosts by insect vectors, disease may result from an adaptive sequence in which the vector feeds preferentially on the hosts that are susceptible to the pathogen.

Fungi usually penetrate leaves by production of a specialized structure called an appressorium. As a fungal hypha grows over the surface of a leaf, the hyphal tip mounds up and becomes cemented to the leaf, forming an appressorium. A specialized hypha, called a penetration peg, grows from the appressorium and penetrates the leaf, largely by mechanical pressure. The penetration peg also may produce cutinase and cellulose enzymes that soften the tissue. The leaf epidermis is covered by a cuticle made primarily of a waxy substance called cutin, and the epidermal cell walls have a high cellulose content. Sometimes a fungus penetrates through a stoma, a hole in the lower epidermis of the leaf formed by two guard cells. Even when a fungus penetrates through a stoma, an appressorium is usually produced. Of course, the penetration peg meets no resistance.

Inside the leaf, fungal hyphae grow between cells (intercellular) and through cells (intracellular) to obtain nutrients. When the leaf dies, the fungus is able to obtain nutrients from the dead cells. The fungi that are obligate parasites (downy mildews, powdery mildews, and rusts) grow inter-cellularly and produce haustoria (specialized hyphal structures) that penetrate the host cells. The haustoria produce enzymes and obtain nutrients from the host cells. Eventually the cells die, and these fungi are no longer able to obtain nutrients.

Bacteria that attack leaves are disseminated in splashing rain and penetrate through stomata or wounds. The bacteria are found between cells in the host and never penetrate the living cells. Nutrients leaking from the host cells provide sufficient food for the bacteria. After the death of leaves, the bacteria continue to obtain nutrients from the dead cells.

Viruses and phytoplasmas are usually transmitted by insects. Feeding by the insects deposits these agents into the phloem or parenchyma tissues. Some viruses can be transmitted by workers in the field. Handling plants causes small wounds and transmits small amounts of contaminated sap. Many viruses attack crops that are propagated vegetatively (by bulbs, corms, budding, etc.), and the diseases are transmitted through use of infected planting stock. Both viruses and phytoplasmas are obligate parasites and cannot obtain nutrients from tissues that have died.

Nematodes penetrate roots mechanically by use of the stylet mouth part. Once inside they insert the stylet into parenchyma cells of the cortex and obtain nutrients. Some nematodes have a long stylet and feed on plant roots while the body is outside the root. Plant pathogenic nematodes are all obligate parasites, capable of obtaining nutrients only from living host cells.

Role of Enzymes, Toxins, and Phytoalexins

Because cutin and cellulose provide tough, protective barriers for the plant, cutinase and cellulase enzymes are necessary to the penetration of plant hosts by pathogenic fungi. They break down the cutin in the cuticle and the cellulose in the primary cell wall. Hydrolytic (digestive) enzymes also play important roles in pathogenesis. The organic food in the host is usually in the form of complex carbohydrates, fats, and proteins. To be absorbed by pathogens, they must be broken down to their simpler units: simple sugars, fatty acids, glycerol, and amino acids. Common digestive enzymes—amylases, cellulases, lipases, and proteases—produced by pathogens break down these complex foods.

The middle lamella, the area between cells in parenchyma tissue, has a high pectin content. For many diseases pathogenesis involves production of pectolytic enzymes that break down pectin. This causes dissolution and eventually death of the cells. Damping off, root rots, vascular wilts, and fruit and vegetable rots are caused by pathogens that produce large amounts of pectolytic enzymes.

Several toxins have been shown to be produced by plant pathogenic fungi and bacteria. Most of them are nonhost-specific toxins. They usually kill cells but may act on the permeability of the cytoplasmic membrane. Although they are involved in pathogenesis, in some cases strains of the pathogen that are unable to produce the toxin still can cause disease. In a few cases the toxin is host-specific and only affects that host at normal toxin concentrations.

Most plants are resistant to infection because of the presence of preexisting chemicals. However, there are many cases where chemicals that ward off infection are produced by the host only after the pathogen is present. These chemicals are called phytoalexins. This is a rather common phenomenon, with about three hundred chemicals from thirty different families of plants having been identified as phytoalexins.

SEE ALSO AGRICULTURE, MODERN; CHESTNUT BLIGHT; DEFENSES, CHEMICAL; DUTCH ELM DISEASE; EUBACTERIA; FUNGI; HERBICIDES; HORMONES; INTERACTION, PLANT-FUNGAL; POTATO BLIGHT.

Ira W. Deep

Bibliography

Agrios, George N. Plant Pathology, 4th ed. New York: San Diego, CA: Academic Press, 1997.

Bove, Joseph M. "Wall-less Prokaryotes of Plants." Annual Review of Phytopathology22 (1984): 361-96.

Doi, Yoji, et al. "Mycoplasma- or PLT Group-like Microorganisms Found in the Phloem Elements of Plants Infected with Mulberry Dwarf, Potato Witches'-Broom, Aster Yellows, or Paulownia Witches'-Broom." Annual Phytopathological Society of Japan 33 (1967): 259-66.

Diener, Theodor O. "The Viroid—A Subviral Pathogen." American Scientist 71 (1983): 481-89.

Schumann, Gail L. Plant Diseases: Their Biology and Social Impact. St. Paul, MN: American Phytopathological Society Press, 1991.