Diseases and disorders

Categories: Diseases and disorders; pests and pest control

Plant disease is as old as land plants themselves, as shown by the fossil record. Several biblical accounts of plagues have been attributed to plant diseases, and in Roman times cereal rust was so serious that an annual ritual, the Robigalia, was performed to appease the Rust God, Robigo. In the mid-nineteenth century the Irish Potato Famine, a result of potato late blight disease, caused the deaths of some 800,000 persons and the emigration of about 1.5 million more, mostly to North America. Similarly, but to a lesser extent, brown spot disease of rice caused the Bengal famine of 1943 in India. Plant diseases continue to cost billions of dollars annually worldwide. The combined costs of lost yield, reduced quality, and costs of pesticides and other control measures are inevitably passed on to consumers. No type of plant is free from disease.

Causes and Types

Plant disease results from the continuing action of an irritant that can be either physical or biological. Physical, or abiotic, causes of disease include water stress (either from excess or insufficient quantities), poor nutrition, improper soil acidity, and other environmental factors. Brief, damaging effects such as hail, wind, and lightning are considered injuries, not diseases.

Biological, biotic, causes of disease include bacteria, viruses, fungi, nematodes, and other microorganisms acting as disease-causing agents, or pathogens. Such pathogens infect plants, colonize tissues, and extract nutrients by living as parasites. Those actions often result in disruptions of normal physiological processes in plants, including photosynthesis, water uptake and movement, nutrient transport, and reproduction. In some instances anatomical abnormalities such as stunting, growth distortions, and gall formation are induced. The resulting physiological and anatomical abnormalities constitute disease symptoms.

Typically, and most important to producers and consumers, yield is reduced, both in quantity and in quality. In addition to diseases of growing plants, there are postharvest diseases. Those include fruit and vegetable rots and decay of stored grains. They may begin either before or after harvest, but in either case such diseases can continue long after harvest, further reducing the quality and value of food, fiber, and feed products. Some pathogens also produce toxins in plants they infect. Those toxins can be injurious, sometimes even fatal, to humans or animals that eat the contaminated plant materials.

Three interacting components are required for any plant disease to occur: (1) a susceptible host plant, (2) either a biotic pathogen or an abiotic, nonliving, causal agent, and (3) environmental conditions favorable for development of disease. Each of those components may vary in their contribution to overall severity of the resulting disease. The most important influences are the existing level of genetic resistance or susceptibility of the host plant; the existing level of virulence, or ability to cause disease, of the pathogen; for abiotic causes, the plant’s degree of nutrient deficiency or water deficiency; and the degrees of favorable or unfavorable temperature, humidity, light intensity or other environmental factors that influence the health and vitality of both host and pathogen. A fourth component, a vector, may be required for some diseases. A vector is a second organism, most commonly an insect, that transmits the pathogen from a diseased plant to a healthy plant and injects or otherwise introduces the pathogen into the plant while feeding or laying eggs. Most pathogens do not require vectors.

Kinds of Plant Pathogens and Typical Diseases

Most fungi, bacteria, and nematodes are free-living organisms in nature. They generally contribute to the ecosystem and cause no deleterious effects on plants. Only a small percentage of microorganisms are plant-parasitic or pathogenic. The most important plant pathogens include fungi, bacteria and related forms, viruses, viroids, and nematodes; each group contains species that cause serious, economically important diseases. Fungi constitute the largest number of plant pathogens. By their very nature, viruses and viroids function only as parasites within plant or animal cells and cannot exist as free-living organisms. Nematodes are microscopic, wormlike animals that most commonly reside in soil and feed externally or internally on plant roots.

Plant Disease Control and Management

Plant pathology, in contrast to human and veterinary medicine, focuses very little on diseases of individuals but rather on large populations. Exceptions include certain high-value individuals, such as trees of historic or particular aesthetic or economic value. Emphasis is largely on prevention of plant diseases rather than curative therapy. Control of plant disease, in absolute terms, is usually impossible or economically impractical to achieve. The realistic goal is more commonly management of plant disease, with the goal of achieving a level of disease prevention or reduction that is both economically and environmentally sound. Briefly, most plant disease management practices fall within one of five broad categories: exclusion, eradication, resistance, protection or therapy, and adaptation to or modification of cultural conditions or practices.

Exclusion is the practice of keeping pathogens separated from their host plants and can be accomplished in several ways. Quarantines between countries are commonly used, with varying degrees of success, to prevent importation of pathogens into countries where they currently do not exist. Success of quarantines often depends on the extent and physical nature of the separation between the countries and whether the pathogen is wind-, soil-, or seed-borne. A variant of exclusion is evasion or avoidance of the pathogen: for example, growing plants susceptible to certain bacterial pathogens only in irrigated, semiarid regions unfavorable to the pathogen. The use of pathogen-free, certified seed or pathogen-free nursery stock are other examples of exclusion aimed at preventing or limiting introduction of pathogens into new areas.

Eradication is the practice of killing pathogens either in the environment or in and on the diseased plant. This limits infection, helps prevent further spread to healthy plants in the area, and helps prevent introduction of pathogens by shipment of diseased plant parts into regions where they currently are not present. Eradication practices such as chemical treatment of soil prior to planting, sanitation procedures such as removing infected crop debris, and long-duration crop rotations can all be successful to various degrees. Eradication and destruction of infected, living plants are often practiced for diseases of orchard crops and landscape trees, such as citrus canker and Dutch elm disease.

Resistance, resulting from innate genetic properties of plants or by directed plant breeding, is usually the most cost-effective means of managing plant diseases. Unfortunately, resistant cultivars may succumb to new, more virulent, or more aggressive strains of the pathogen that can arise by natural selection from populations of the pathogen in nature. Several examples also exist of successful application of genetic modification or genetic engineering to the development of resistant varieties, by introducing genes from other species, including nonrelated plants, viruses, or bacteria. Successful breeding of plants resistant or more tolerant to plant diseases provides billions of dollars’ worth of benefit annually to producers of all types of plants.

Protection is the practice of treating susceptible plants in some manner, often chemically, to prevent infection once a pathogen reaches the plant surface by growth of the pathogen through the soil to seeds or roots, by deposition of windborne fungal spores, or by splash-drop dispersal of bacteria onto aerial plant surfaces. Protective fungicides or bactericides must be on the seed or plant surface prior to arrival of a pathogen to be effective. Protective fungicides do not kill pathogens; rather, they prevent spore germination, host penetration, or other early stages of infection, if spore germination does occur. Protective fungicides and bactericides are widely used throughout plant agriculture. Some recently developed protective fungicides also have limited eradicative activity. Those fungicides not only function as surface protectants but also enter plant tissue and kill pathogens already inside the plant—an example of therapeutic chemical action.

Adaptation to and modification of cultural conditions or practices are used to take advantage of conditions that benefit the plant but are also detrimental to development and spread of the pathogen. Examples include planting when soil temperatures are favorable for germination and subsequent growth of seedlings, altering soil pH by addition of lime or sulfur, not overfertilizing, increasing or lowering temperature, lowering humidity, and increasing airflow in greenhouses. The goal of such practices is to foster plant health and survival as well as prevent or reduce plant disease.

Often it is advantageous to use several of these approaches simultaneously. By combining natural host resistance, even if incomplete, with manipulation of environmental conditions unfavorable to the pathogen, by utilizing pathogen-free seed or nursery stock, or other practices, growers can often reduce significantly the amount of chemical pesticides required to manage a disease. Such a combined approach to disease and insect management is known as integrated pest management (IPM). Major goals of IPM are to reduce significantly the amount of pesticides going into the ecosystem while limiting plant disease at an economically acceptable level.

Bibliography

Agrios, George N. Plant Pathology. 4th ed. San Diego: Academic Press, 1997. The standard, most comprehensive college-level textbook of plant pathology in the United States. Covers basic concepts of the discipline plus information on all types of plant diseases and their pathogens; well illustrated.

Schumann, Gail L. Plant Diseases: Their Biology and Social Impact. St. Paul: APS Press, 1991. Overview of a wide range of plant diseases and pathogens, with emphasis on basic biology and economic, social, and historical impacts of plant diseases.

Symptoms of Plant Diseases