Plant disease
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What is a plant disease?
A plant disease is any abnormal condition that impairs a plant's normal functions. All plant species are susceptible to diseases, which vary based on the pathogen's presence, environmental factors, and the plant varieties grown.
Plant diseases are classified by their causal agent, either infectious or noninfectious. Infectious diseases are caused by pathogens like fungi, bacteria, or viruses, while noninfectious diseases result from unfavorable growing conditions or nutrient imbalances. The study of plant diseases is called plant pathology.
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What causes diseases in plants?
Plant diseases arise from infectious and non-infectious agents. Infectious diseases are caused by pathogens like fungi, bacteria, viruses, and nematodes, which can spread between susceptible hosts. Non-infectious diseases stem from unfavorable growing conditions such as temperature extremes, lack of nutrients, or toxic substances, and are not transmissible.
Fungi cause most infectious plant diseases, followed by bacteria. Viruses are transmitted by insects or through infected sap, while bacteria enter through wounds or natural openings. Plant pathology, the study of plant diseases, is essential for recognizing and addressing these issues to reduce crop losses and prevent economic and ecological damage.
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How can you tell if a plant is sick?
To identify a sick plant, look for telltale symptoms in its leaves, stems, and roots. Rapid and accurate diagnosis is needed before control measures can be taken.
- Leaves Lesions, spots, or blight may indicate disease. Discoloration, holes, or deformations may indicate the presence of insects.
- Stems, shoots, branches, and trunk Sunken or swollen areas can indicate infection or injury. Holes and sawdust-like debris are evidence of boring insects.
- Roots Stunted leaves and twigs, discolored foliage, and dieback may indicate root problems.
Besides symptoms, also recognize signs characteristic of specific diseases. Signs are either structures formed by the pathogen or the result of interaction between pathogen and host.
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What are some common pathogens that infect plants?
Plant pathogens are infectious agents that cause disease in plants. They consist of microscopic organisms, and a few macroscopic ones, that can be grouped as viruses, bacteria, fungi, nematodes, and parasitic seed plants.
- Viruses and Viroids Examples of plant diseases caused by viruses and viroids are tobacco mosaic, cucumber mosaic, barley yellow dwarf, tomato spotted wilt, prunus necrotic ring spot, potato spindle tuber, and citrus exocortis.
- Bacteria Examples of bacterial diseases include Granville wilt, fire blight, wildfire of tobacco, blight of beans, soft rot, crown gall, aster yellows, and citrus stubborn disease. The principal genera of plant pathogenic bacteria are Agrobacterium, Clavibacter, Erwinia, Pseudomonas, Xanthomonas, Streptomyces, and Xylella.
- Fungi Fungi cause the majority of plant diseases. Examples of fungal diseases are late blight of potato, chestnut blight, Dutch elm disease, black stem rust of wheat, coffee rust, white-pine blister rust, corn smut, loose smut, downy mildew, powdery mildew, apple scab, black spot of rose, anthracnose of grape, nectria canker, black knot of plum and cherry, brown rot, soft rot, fusarium wilt of tomato, and wilts of vegetables, flowers, and some trees.
- Nematodes About 1,200 species of nematodes cause disease in plants.
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How do plant diseases spread from one plant to another?
Plant diseases spread through various ways, depending on the pathogen involved. Fungi, which cause the majority of plant diseases, spread via spores carried by wind, water, soil, insects, birds, or infected plant remains. Viruses are often transmitted by insects and mites, known as vectors, or through grafting, seeds, and pollen. Bacteria and viruses can also spread through contaminated soil or crop residue via machines, irrigation water, and insects. Additionally, human activities can inadvertently spread plant diseases around the world.
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What methods are used to prevent or control plant diseases?
Plant disease control integrates biological, cultural, and chemical methods targeting pathogens and pests. Control measures focus on inoculum, using exclusion, avoidance, eradication, protection, host resistance, and therapy.
Eradication eliminates disease agents via crop rotation, destroying diseased plants, eliminating alternate hosts, pruning, disinfection, and heat treatments. Protection involves barriers like environmental regulation, cultural practices, and chemical pesticides. Biological control uses beneficial organisms to reduce infection. Disease-resistant plant varieties offer a safe and inexpensive control method. Systemic fungicides and antibiotics can also treat plants post-infection.
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Plant disease, an impairment of the normal state of a plant that interrupts or modifies its vital functions. All species of plants, wild and cultivated alike, are subject to disease. Although each species is susceptible to characteristic diseases, these are, in each case, relatively few in number. The occurrence and prevalence of plant diseases vary from season to season, depending on the presence of the pathogen, environmental conditions, and the crops and varieties grown. Some plant varieties are particularly subject to outbreaks of diseases while others are more resistant to them. See also list of plant diseases.
General considerations
Nature and importance of plant diseases
Plant diseases are known from times preceding the earliest writings. Fossil evidence indicates that plants were affected by disease 250 million years ago. The Bible and other early writings mention diseases, such as rusts, mildews, and blights, that have caused famine and other drastic changes in the economy of nations since the dawn of recorded history. Other plant disease outbreaks with similar far-reaching effects in more recent times include late blight of potato in Ireland (1845–60); powdery and downy mildews of grape in France (1851 and 1878); coffee rust in Ceylon (now Sri Lanka; starting in the 1870s); Fusarium wilts of cotton and flax; southern bacterial wilt of tobacco (early 1900s); Sigatoka leaf spot and Panama disease of banana in Central America (1900–65); black stem rust of wheat (1916, 1935, 1953–54); southern corn leaf blight (1970) in the United States; Panama disease of banana in Asia, Australia, and Africa (1990 to present); and coffee rust in Central and South America (1960, 2012 to present). Such losses from plant diseases can have a significant economic impact, causing a reduction in income for crop producers and distributors and higher prices for consumers.
Loss of crops from plant diseases may also result in hunger and starvation, especially in less-developed countries where access to disease-control methods is limited and annual losses of 30 to 50 percent are not uncommon for major crops. In some years, losses are much greater, producing catastrophic results for those who depend on the crop for food. Major disease outbreaks among food crops have led to famines and mass migrations throughout history. The devastating outbreak of late blight of potato (caused by the water mold Phytophthora infestans) that began in Europe in 1845 brought about the Great Famine that caused starvation, death, and mass migration of the Irish. Of Ireland’s population of more than eight million, approximately one million (about 12.5 percent) died of starvation or famine-related illness, and 1.5 million (almost 19 percent) emigrated, mostly to the United States, as refugees from the destructive blight. This water mold thus had a tremendous influence on economic, political, and cultural development in Europe and the United States. During World War I, late blight damage to the potato crop in Germany may have helped end the war.
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Diseases—a normal part of nature
Plant diseases are a normal part of nature and one of many ecological factors that help keep the hundreds of thousands of living plants and animals in balance with one another. Plant cells contain special signaling pathways that enhance their defenses against insects, animals, and pathogens. One such example involves a plant hormone called jasmonate (jasmonic acid). In the absence of harmful stimuli, jasmonate binds to special proteins, called JAZ proteins, to regulate plant growth, pollen production, and other processes. In the presence of harmful stimuli, however, jasmonate switches its signaling pathways, shifting instead to directing processes involved in boosting plant defense. Genes that produce jasmonate and JAZ proteins represent potential targets for genetic engineering to produce plant varieties with increased resistance to disease.
Humans have carefully selected and cultivated plants for food, medicine, clothing, shelter, fibre, and beauty for thousands of years. Disease is just one of many hazards that must be considered when plants are taken out of their natural environment and grown in pure stands under what are often abnormal conditions.
Many valuable crop and ornamental plants are very susceptible to disease and would have difficulty surviving in nature without human intervention. Cultivated plants are often more susceptible to disease than are their wild relatives. This is because large numbers of the same species or variety, having a uniform genetic background, are grown close together, sometimes over many thousands of square kilometres. A pathogen may spread rapidly under these conditions.