Unit 1: Importance, history, and concepts of Plant Pathology

1. Introduction to Plant Pathology

Plant Pathology, also known as Phytopathology, is the branch of agricultural, botanical, or biological science that deals with the study of plant diseases.

• Etymology: Derived from three Greek words:
  • Phyton = Plant
  • Pathos = Suffering/Disease
  • Logos = Study/Knowledge
• Definition: The study of the nature, cause, and control of plant diseases. It involves the study of the living entities (pathogens) and environmental conditions that cause disease, the mechanisms by which these factors produce disease in plants, and the methods of preventing or controlling disease.

Objectives of Plant Pathology

1. Etiology: To study the living (biotic) and non-living (abiotic) causes of plant diseases.
2. Pathogenesis: To study the mechanism of disease development and host-pathogen interactions.
3. Epidemiology: To study the interaction between the causal agent and the diseased plant in relation to environmental conditions.
4. Management: To develop management systems to reduce losses caused by diseases.

2. Basic Terms and Concepts

To understand plant pathology, one must be familiar with the following terminologies:

• Disease: A physiological malfunction or abnormality in the normal structure, function, or development of a plant caused by continuous irritation by a primary causal agent (pathogen or environmental factor).
• Pathogen: Any entity, usually a microorganism, capable of causing disease (e.g., fungi, bacteria, viruses).
• Parasite: An organism that lives on or in another organism (host) and obtains food from it. (Note: All pathogens are parasites, but not all parasites are pathogens).
• Host: A plant that is invaded by a parasite and from which the parasite obtains its nutrients.
• Pathogenicity: The relative capability of a pathogen to cause disease.
• Virulence: The degree of pathogenicity of a specific isolate of a pathogen.
• Inoculum: The part of the pathogen (e.g., spores, bacterial cells, viral particles) that contacts the plant and initiates infection.
• Infection: The establishment of a parasite within a host plant.
• Incubation Period: The time interval between infection (penetration) and the appearance of the first symptom.
• Hypersensitivity: The extreme sensitivity of plant tissues to the pathogen, resulting in rapid death of cells at the infection site to restrict the pathogen's spread.

The Disease Triangle (and Tetrahedron)

For a disease to occur, three critical factors must interact simultaneously. This is known as the Disease Triangle:

1. Susceptible Host: A plant with a genetic makeup that allows the pathogen to invade.
2. Virulent Pathogen: A pathogen capable of attacking the specific host.
3. Favorable Environment: Conditions (temperature, humidity, moisture) that favor the pathogen's growth and host susceptibility.

The Disease Tetrahedron: Modern pathology adds a fourth factor: Time. The duration for which the three factors interact determines the severity of the disease.

3. Importance of Plant Diseases

Plant diseases are significant because they reduce the quantity and quality of plant produce, leading to economic losses and, historically, massive famines.

Major Historical Famines Caused by Plant Diseases

1. Irish Potato Famine (1845):
   • Disease: Late Blight of Potato.
   • Pathogen: Phytophthora infestans.
   • Impact: Destroyed the potato crop in Ireland, leading to the death of over 1 million people due to starvation and the migration of 1.5 million to the USA.
2. Coffee Rust in Sri Lanka (1867):
   • Disease: Coffee Rust.
   • Pathogen: Hemileia vastatrix.
   • Impact: Wiped out coffee plantations in Sri Lanka (Ceylon), forcing the island to switch from coffee to tea production.
3. Bengal Famine (1943):
   • Disease: Brown Spot of Rice.
   • Pathogen: Helminthosporium oryzae.
   • Impact: Caused yield losses of 40–90% in rice, contributing to the death of approximately 2 million people in Bengal, India.

Economic and Biological Importance

• Yield Reduction: Direct loss of food and fiber.
• Quality Reduction: Blemishes on fruits/vegetables reduce market value; production of toxins (e.g., Aflatoxins by Aspergillus flavus).
• Cost of Control: Farmers spend billions annually on fungicides, pesticides, and resistant variety breeding.
• Limitation of crop types: Diseases may prevent the cultivation of certain crops in specific areas (e.g., Pear cultivation is difficult in parts of the US due to Fire Blight).

4. History of Plant Pathology

A. Pre-Modern Era

• Theophrastus (370–286 B.C.): Known as the "Father of Botany." He recorded observations on plant diseases in his book Enquiry into Plants, noting that diseases were more severe in lowlands than highlands.
• Romans: Created a holiday "Robigalia" to appease the rust god Robigus to protect grain from rust disease.

B. The Microscope and Proof of Pathogenicity

• Robert Hooke (1665): Illustrated plant cells and arguably the first fungus (Teliospores of Phragmidium) in Micrographia.
• Anton van Leeuwenhoek (1675): Invented the microscope and observed bacteria.
• P.A. Micheli (1729): Described fungi and their reproduction in Nova Plantarum Genera.
• Tillet (1755): Demonstrated that Bunt of Wheat is a contagious disease and can be reduced by seed treatment, though he believed it was a poisonous fluid, not a fungus.
• Prevost (1807): Proved that the bunt of wheat is caused by a fungus and that copper sulfate inhibits spore germination. This laid the foundation for the germ theory of disease.

C. The Golden Era (Foundational Scientists)

• Anton de Bary (1853): Known as the Father of Modern Plant Pathology. He confirmed that Phytophthora infestans was the cause of Late Blight of Potato (not the result of it). He also worked out the heteroecious nature of wheat rust.
• Robert Koch (1876): Established Koch's Postulates, a set of criteria to prove a specific organism causes a specific disease.
• T.J. Burrill (1882): Proved that bacteria cause plant disease (Fire Blight of Pear/Apple caused by Erwinia amylovora).
• Millardet (1885): Discovered the Bordeaux Mixture (Copper Sulfate + Lime + Water) to control Downy Mildew of Grapes.
• Beijerinck (1898) & Ivanowski (1892): Discovered viruses (Tobacco Mosaic Virus), describing them as Contagium vivum fluidum (contagious living fluid).
• Doi et al. (1967): Discovered Mycoplasma-like organisms (now Phytoplasmas) cause Yellows type diseases.

D. History with Special Reference to Indian Scientists

1. E.J. Butler:

   • Known as the Father of Plant Pathology in India.
   • First Imperial Mycologist at IARI (Pusa, Bihar).
   • Authored the classic book: Fungi and Disease in Plants (1918).
   • Initiated the study of fungal diseases in India.

2. J.F. Dastur:

   • First Indian plant pathologist.
   • Worked under Butler.
   • Known for his comprehensive study on the genus Phytophthora (P. parasitica) on castor.

3. K.C. Mehta:

   • Professor at Agra College.
   • Famous for discovering the Annual Recurrence of Wheat Rusts in India. He proved that the rust inoculum survives in the Himalayas (Barberry bush does not play a functional role in the rust cycle in the Indian plains due to high heat).

4. B.B. Mundkur:

   • Worked on the control of Cotton Wilt.
   • Founded the Indian Phytopathological Society (IPS) in 1948.
   • Authored the textbook Fungi and Plant Diseases.

5. M.J. Thirumalachar:

   • World-renowned for work on Rusts and Smuts.
   • Developed antibiotics like Aureofungin for plant disease control.

6. T.S. Sadasivan:

   • Established a school of research on the mechanism of wilts (Physiological Plant Pathology) at the University of Madras.

5. Important Plant Pathogens (Groups)

Plant diseases are primarily caused by mesobiotic (viruses) and biotic agents.

6. Classification of Plant Diseases

Plant diseases can be classified based on several criteria:

A. Based on Cause

1. Biotic (Infectious): Caused by living pathogens (Fungi, Bacteria, etc.). They are transmissible.
2. Abiotic (Non-infectious/Physiological): Caused by environmental factors (Temperature, Mineral deficiency, Toxicity, Pollution). Not transmissible.
3. Mesobiotic: Caused by Viruses and Viroids (entities on the border of living and non-living).

B. Based on Occurrence (Geographic Distribution)

1. Endemic: The disease is constantly present in a specific locality in moderate to severe form (e.g., Early blight of potato, Wart disease of potato in Darjeeling).
2. Epidemic (Epiphytotic): Occurs widely but periodically in severe form (e.g., Red rot of sugarcane, Late blight of potato).
3. Sporadic: Occurs at very irregular intervals and locations (e.g., Green ear disease of bajra).
4. Pandemic: Occurs all over the world causing mass mortality (e.g., Late blight of potato in 1845).

C. Based on Plant Part Affected

• Localized: Affects specific parts (e.g., Leaf spots, Galls).
• Systemic: Pathogen spreads throughout the plant (e.g., Viruses, Wilts).

7. Symptoms and Signs of Plant Diseases

It is crucial to distinguish between a symptom and a sign for diagnosis.

• Sign: The physical evidence of the pathogen itself appearing on the host surface.
  • Examples: Mycelial growth, spores (rust pustules), sclerotia, bacterial ooze, nematode cysts.
• Symptom: The external or internal reaction/alteration of the host plant as a result of the disease.
  • Examples: Yellowing, wilting, rotting.

Classification of Symptoms

Symptoms are generally grouped into three morphological categories:

1. Necrotic Symptoms (Death of tissues)

• Spot: Localized death of leaf tissue (Leaf spot).
• Blight: Sudden, severe, and extensive scorching/browning of leaves, blossoms, or twigs (e.g., Late blight).
• Rot: Decomposition/disintegration of tissue (Soft rot, Root rot).
• Canker: Dead, sunken lesion on a stem or branch surrounded by callus tissue.
• Damping-off: Collapse and death of seedlings near the soil line.
• Die-back: Progressive death of shoots/twigs from the tip downward.

2. Hypoplastic Symptoms (Underdevelopment)

• Chlorosis: Failure of chlorophyll development (yellowing).
• Stunting/Dwarfing: Reduced plant size.
• Mosaic: Irregular pattern of light and dark green areas on leaves (Viral).
• Rosetting: Shortening of internodes resulting in crowding of leaves.

3. Hyperplastic Symptoms (Overgrowth)

• Gall/Tumor: Uncontrolled cell division causing swelling (e.g., Crown gall).
• Curl: Rolling or arching of leaves due to unequal growth.
• Scab: Roughened, crust-like lesion on fruit or leaf surface (e.g., Apple Scab).
• Witches' Broom: Proliferation of many twigs from a single point giving a broom-like appearance.
• Phyllody: Transformation of floral parts into leafy structures.