Introduction to Plant Pathology

Introduction to Plant Pathology

• Plant pathology connects entomology and weed science.
• Diseases, insects, and weeds cause an estimated 31–42% loss in crop production globally each year.
• Diseases account for 14.1% of total crop losses, costing about $220 billion annually.
• Post-harvest crop losses can add another 6–12%, which are higher in tropical countries due to lack of resources like refrigeration.
• Environmental factors like freezes, droughts, pollution, and nutrient issues also contribute to crop losses.

Estimated Annual Crop Losses Worldwide

• Attainable crop production with 2002 prices: $1.5 trillion.
• Actual crop production is at $950 billion, reflecting a 36.5% loss.
• Production without crop protection would only reach $455 billion.
• Crop protection helps prevent $415 billion in losses.
• Actual annual losses to world crop production amount to $550 billion.
• Diseases alone account for $220 billion in losses, which is 14.1% of the total.

Plant Pathology and Its Scope

• Directly or indirectly, humans and animals depend on plants for sustenance.
• Plants convert sunlight into stored chemical energy in carbohydrates, proteins, and fats.
• It remains unknown whether diseased plants can feel discomfort or pain.
• Plant diseases are caused by similar disease agents to those in humans and animals.
• Agents that cause disease in plants are pathogenic microorganisms and unfavorable environmental conditions like nutrient imbalance, moisture issues, pollution and toxic chemicals.
• Plant pathology does not usually include plant damage from insects, humans, or other animals.
• Plant pathology studies organisms and environmental factors, disease mechanisms, prevention, control, and damage reduction.
• Plant pathology is an integrative science using knowledge from botany, mycology, bacteriology, virology, nematology, plant anatomy/physiology, genetics, biochemistry, horticulture, agronomy, tissue culture, among other sciences.
• Diseased plants can be poisonous and unfit for consumption.
• Plant disease control has increased use of toxic chemicals (pesticides) in the last 100 years.
• Conventional breeding, genetic engineering, cultural practices, RNA and gene-silencing, nontoxic substances, and biological agents show promise in controlling plant diseases.
• Efforts in plant pathology aim to reduce food losses, improve quality, and protect the environment.

The Concept of Disease in Plants

• Plant diseases start localized and invisible then become widespread with visible changes.
• Visible changes are the symptoms of the disease.
• Disease in plants involves responses of plant cells/tissues to pathogenic organisms or environmental factors that result in form, function, or integrity leading to impairment/death.
• Physiological disruptions depend on affected cells and tissues such as root rots due to root infections.
• Vascular wilts and cankers interfere with water and mineral translocation.

More on the Concept of Disease in Plants

• Diseases caused by viruses, mollicutes, and protozoa, interfere with photosynthetic product translocation.
• Infections of flowers and fruits interferes with leaf spots, blights, rusts, mildews, mosaics that interfere with photosynthesis.
• Pathogens disrupt plant cell metabolism through enzymes, toxins, and growth regulators.
• Plant diseases are grouped by symptoms (root rots, wilts, leaf spots) or pathogen type (infectious/biotic, noninfectious/abiotic).

History of Plant Pathology

• Thread-like parasitic higher plant dodder (Cuscuta sp.) parasitizing pepper seedlings.
• Plant parasitic nematodes (Ditylenchus sp.) were isolated from infected onion bulbs.
• Theophrastus wrote "The Nature of Plants" and "Reasons of Vegetable Growth.”
• Theophrastus is considered the "father of botany.”
• Mistletoe was the first plant pathogen to be recognized.
• Robert Hooke invented the compound microscope in England.
• Antonius van Leeuwenhoek used the microscope to examine fungi, algae, protozoa, sperm, blood cells, and bacteria.
• Carl von Linne published “Systema Naturae” in 1735 to diagnose plant species and create binomial nomenclature.
• Charles Darwin published “The Origin of Species” in 1859.
• Needham observed nematodes inside wheat kernels in 1743.
• Leeuwenhoek first saw microbes in 1674 using his microscope.
• Adolph Mayer discovered the TMV virus in 1886.
• Ivanowski showed tobacco mosaic disease could pass through a filter.

Viroids, Koch's Postulates, and Plant Diseases

• Viroids are small, circular infectious RNA molecules.
• Viroids only infect plants (more than 40 have been found).
• Prions have not been found to infect plants.
• Robert Koch (1843-1910) showed anthrax was caused by Bacillus anthracis in 1876.
• Viroids cause more than 40 diseases in plants.
• Prions only affect humans and animals.

Viruses and Losses Caused by Plant Diseases

• Viruses are submicroscopic, spherical, rod-shaped or filamentous entities with protein coats.
• Viruses infect and multiply inside cells of humans, animals, plants and other organisms.
• Viroids exclusively infect plants.
• The Irish Potato Famine caused 1.25 million deaths in 1845.
• French wine production decreased by 80% in 1854, due to a new disease.
• Powdered lime and sulfur protected leaves and berries from powdery mildew on vines.
• Bordeaux mixture (8-8-100) controlled downy mildew and was an excellent fungicide and bactericide (1882).

Dutch Elm Disease, Oak Wilts, and Grape Disease

• Dutch elm disease is caused by the fungus Ophiostoma ulmi.
• O. ulmi is carried by two elm bark beetles.
• Oak wilt is caused by the fungus Ceratocystis fagacearum.
• Pierce’s disease of grape is caused by Xylella fastidiosa, a xylem-inhabiting bacterium.

Plant Diseases Making Plants Poisonous

• Some diseases, like ergot of rye and wheat, make plant products unfit due to poisonous fruiting structures

Fungal Diseases

• “Holy fire” disease is caused by consuming ergot-contaminated rye or grasses (ergotism).
• Mycotoxins are toxic compounds produced by fungi.
• Ergotism is an example of mycotoxicosis caused by Claviceps purpurea.
• Aflatoxins in “turkey X disease” are toxic, appear in contaminated feed, attack the liver, and are mutagenic, teratogenic, and carcinogenic.
• Aspergillus infects corn and bread.
• Penicillium infects bread and orange fruit.
• Techniques for growing microorganisms (fungi and bacteria) were introduced by Brefeld, Koch, and Petri.

Etiological Phase, Chemical Control, and Public Concern

• The etiological phase focuses on the causes of plant diseases.
• The Etiological phase began with pathogenicity proof for late blight fungus on potatoes; rust and smut fungi of cereals.
• Bordeaux mixture controls late blight of potato, downy mildews, and leaf spots and blights.
• Alexander Fleming discovered penicillin.
• Streptomycin was first used to control bacterial plant diseases.
• Public concern: Pesticide means "pest killer" and includes bacteria, fungi, insects, weeds, and rodents.
• DDT and chlorinated hydrocarbons are persistent and remain toxic for years.
• Rachel Carson's “Silent Spring” (1962) showed dangers of poisonous chemicals polluting the environment.

Epidemiology

• Epidemiology of Plant Disease came of age in 1963, when Vanderplank wrote “Plant Diseases: Epidemics and Control."
• Epidemiology was established as key in plant pathology.
• Molecular plant pathology began with W. Stanley’s 1935 isolation of tobacco mosaic virus as a crystalline protein.
• Plant biotechnology uses tissue culture and genetic engineering for genetically modified plants (GMOs).
• Cool, humid climates; annual epidemics (e.g., Irish famine of 1845) leads to the Potato Late Blight.
• Asia Epidemics (e.g., Bengal famine of 1943) leads to the Brown spot of rice.
• Worldwide European epidemics (1840s-1850s) leads to the Powdery mildew of grapes.
• U.S., Europe European epidemic (1870s–1880s) leads to the Downy mildew of grapes.
• Diagnosing plant diseases requires determining between pathogen and environmental factors.
• Parasitic higher plants (e.g., dodder, mistletoe, witchweed, or broomrape) lead to diseases that are caused by parasitic higher plants.

Parasitism and Pathogenicity

• Pathogens attacking plants are in the same groups as those causing diseases in humans and animals.
• Plants are attacked by other plants.
• A parasite lives on another organism and obtains food from it.
• Removal of food by a parasite is parasitism.
• Parasitism is tied to pathogenicity, which is a pathogen's ability to cause disease.
• Symbiosis is when both the plant and microorganism benefit from their relationship.
• Some parasites live as parasites but may grow on dead matter under certain conditions (semibiotrophs) and are called facultative saprophytes.
• Some thrive on dead matter (necrotrophs) but can attack living plants (facultative parasites).
• Plants become diseased by pathogens or abiotic agents.
• Disease interactions involve the “disease triangle” of the host, pathogen, and environment.
• Disease needs a susceptible host plant, a virulent pathogen, and favorable environmental conditions.
• If any of these components are missing, there is no disease.

Stages in Disease Development

• A disease cycle is a series of events that occur in succession leading to disease.
• The primary events in a disease cycle are inoculation, penetration, establishment of infection, colonization, invasion, and dissemination.
• Inoculation is the initial contact of a pathogen with a plant.
• The inoculum is the contact made by the pathogen and is any part that can initiate infection.
• A propagule is one unit of inoculum
• Primary inoculum survives dormancy and causes the original infections.
• Secondary inoculum comes from primary infections and causes secondary infections.
• Inoculum is carried to host plants passively by wind, water, and insects.
• Many pathogens have enzymes, including cutinases, needed for spore attachment.
• Fungi penetrate and invade through direct penetration, natural openings (stomata), and natural wounds.
• An appressorium generates turgor pressure measured to be 40 times greater than a car tire pressure.
• high turgor pressure is due to the accumulation of glycerol

Germination

• Spores germinate producing a typical mycelium, infecting and growing into host plants or producing a short germ tube producing a specialized infectious structure, the haustorium
• Some fungal spores germinate by producing other spores, e.g., sporangia produce zoospores and teliospores produce basidiospores.
• high spore concentration leads to “quorum sensing”
• Fungi penetrates the host plant by means of specialized feeding organs called haustoria.

Pathways of Pathogen Entry

• Bacteria enter plants mostly through wounds, less through natural openings, and never directly through unbroken cell walls.
• Viruses, viroids, mollicutes, fastidious bacteria, and protozoa enter through wounds made by vectors or tools.
• Parasitic higher plants enter their hosts by direct penetration.
• Nematodes enter through direct penetration or natural openings.
• All bacteria, most fungi, viruses, and viroids enter plants through wounds.
• Many fungi and bacteria enter through stomata, hydathodes, and lenticels.
• Hydathodes are pores at leaf margins, and lenticels are openings on fruits, stems, and tubers.

Infection

• Infection is the process by which pathogens establish contact with susceptible cells or tissues of the host and procure nutrients from them.
• Successful infections lead to symptoms.
• Symptoms encompass all visible changes in infected plants and change over time
• Symptoms appear quickly (2-4 days) or as late as 2-3 years post inoculation.
• The time interval between inoculation and visible symptoms is the incubation period.

Spread and Invasion

• Fungi spread through plant tissues either growing directly through the cells (intracellular mycelium) or between cells (intercellular mycelium).
• Bacteria invade tissues intercellularly, or intracellularly.
• Most nematodes invade tissues intercellularly, intracellularly, and using stylets.
• Viruses, viroids, mollicutes, fastidious bacteria, and protozoa invade tissues moving from cell to cell
• Bacteria divide (double their numbers) every 20 to 30 minutes.
• Virus particles may be present in a single cell at a rate of 100,000 to 10,000,000.
• Nematode females lay 300 to 500 eggs

Environmental impact

• Plant diseases occur in all parts of the world where plants grow.
• The most severe fungal, bacterial, or nematode diseases are not as prevalent in Dry areas.
• Wet, warm days and nights are the conditions most diseases appear and develop best.
• Higher chance of plants fertilized heavily with nitrogen being attacked by pathogens.
• Environmental conditions greatly affect disease.
• Viruses showing yellow or leaf-roll symptoms are most severe in the summer.
• Late blight of potato, apple scab, downy mildew of grapes, and fire blight are found or are severe only in areas with high precipitation or high relative humidity during their respective growing seasons
• High moisture is needed for fungal spore germination and penetration of the host.
• Moisture enhances the succulence of host plants, as well as their susceptibility to particular pathogens and the severity of disease.
• Powdery mildews are more common and severe in drier areas.
• Wind is important for disease development during rainfall.
• Light duration affects plant susceptibility to infection.
• Soil pH impacts the occurrence and severity of plant diseases that are caused by soilborne pathogens.
• Clubroot of crucifers, caused by Plasmodiophora brassicae, is prevalent at pH 5.7.
• Cotton root rot fungus (Phymatotrichopsis omnivora) favours high pH, around range 7.2–8.0.
• High nitrogen results in higher susceptible, succulent growth, prolonged vegetative period, and the plant will grow with delayed maturity