Unit1 - Subjective Questions

Definition:
Plant Pathology (Phytopathology) is the branch of agricultural, botanical, or biological science which deals with the study of plant diseases. The term is derived from Greek words: phyton (plant), pathos (suffering), and logos (study). It covers the study of the cause of the disease, the resultant losses, and control measures.

Objectives:

1. Etiology: To study the living (biotic), non-living (abiotic), and environmental causes of plant diseases.
2. Pathogenesis: To study the mechanism of disease development by pathogens.
3. Epidemiology: To study the interaction between the causal agent and the diseased plant in relation to environmental conditions.
4. Management: To develop methods of management/control of plant diseases to reduce economic losses.

While often used interchangeably, there is a technical distinction:

• Parasite: An organism that lives on or within another living organism (host) and obtains its nutrition from it. Parasitism refers strictly to the method of nutrition.
• Pathogen: An entity (living or non-living) capable of causing disease.

Key Differences:

1. Scope: All plant pathogens are not parasites (e.g., abiotic factors like nutrient deficiency are pathogens but not parasites). Conversely, some parasites might not cause significant disease (though rare in pathology contexts).
2. Interaction: A parasite focuses on drawing nutrition, whereas a pathogen disrupts the physiological functions of the host, leading to symptoms.
3. Examples:
   • Fungi causing rusts are both parasites and pathogens.
   • Viruses are obligate parasites and pathogens.
   • Sulfur dioxide toxicity is a pathogen (cause of disease) but not a parasite.

The Disease Triangle is a fundamental concept in plant pathology illustrating that disease occurrence requires the simultaneous interaction of three factors:

1. Susceptible Host: A plant that has the genetic makeup to be infected by the pathogen.
2. Virulent Pathogen: An agent capable of overcoming the host's defense mechanisms.
3. Favorable Environment: Conditions (temperature, humidity, moisture) that support pathogen growth and reproduction while stressing the host.

Influence of Environment:
Even if a virulent pathogen and a susceptible host are present, disease will not occur if the environment is unfavorable. For example, fungal spores may require specific leaf wetness duration to germinate. If the weather is dry, infection fails. Therefore, the total amount of disease is proportional to the area of the triangle formed by these three factors.

Sir Edwin John Butler (1874–1943) is widely regarded as the "Father of Indian Plant Pathology."

Key Contributions:

• Establishment: He initiated mycological and pathological studies in India at the Imperial Agricultural Research Institute (IARI), Pusa, Bihar, in 1905.
• Research: He conducted an exhaustive study on Indian fungi and the diseases they cause.
• Literature: He wrote the classic book "Fungi and Disease in Plants" (1918), which served as a primary textbook for generations.
• Identification: He identified and described the causal organisms of wilt of cotton and pigeon pea, and downy mildew of cereals.
• Global Impact: Later, he returned to England and established the Imperial Mycological Institute (now CABI) in Kew.

Symptoms:
Symptoms are the visible external or internal expressions of the host plant resulting from the pathological activity of a pathogen. It is the plant's reaction to the disease.

• Examples: Wilting (drooping), Chlorosis (yellowing), Necrosis (death of tissue), Cankers, Stunting.

Signs:
Signs represent the physical presence of the pathogen itself (vegetative or reproductive structures) seen on or in the host tissue.

• Examples: Mycelial growth on the leaf surface (Powdery mildew), Sclerotia (black resting bodies in Stem rot), Bacterial ooze, Rust pustules (containing spores).

Summary:

• Symptom = Host's response.
• Sign = Pathogen's physical body.

Robert Koch (1876) proposed a set of criteria to establish the causal relationship between a specific microorganism and a specific disease. These are known as Koch's Postulates:

1. Association: The suspected pathogen must be constantly associated with the disease in all diseased plants examined.
2. Isolation: The pathogen must be isolated from the diseased plant and grown in a pure culture on nutrient media.
3. Inoculation: The pathogen from the pure culture must be inoculated into healthy, susceptible plants of the same species, and it must produce the same symptoms as observed originally.
4. Re-isolation: The pathogen must be re-isolated from the artificially inoculated plants, and it must be identical to the original isolate.

Note: These postulates generally apply to fungi and bacteria but are modified for obligate parasites (like viruses and downy mildews) which cannot grow on artificial media.

Plant diseases are classified based on the frequency and geographical extent of their occurrence:

1. Endemic Diseases:

   • Prevalent in a particular area or country year after year in a moderate to severe form.
   • Example: Wart disease of potato in Darjeeling, Citrus canker in Asia.

2. Epidemic (Epiphytotic) Diseases:

   • Occur periodically but in a severe form, affecting a large population of plants over a vast area causing heavy losses.
   • Example: Red rot of sugarcane, Wheat rusts, Late blight of potato.

3. Sporadic Diseases:

   • Occur at irregular intervals and distinct locations. They occur only when environmental conditions are highly favorable.
   • Example: Leaf blight of wheat, Green ear disease of bajra.

4. Pandemic Diseases:

   • When an epidemic becomes prevalent throughout a continent or the whole world.
   • Example: Late blight of potato in the 1840s (Global), Wheat rusts.

The Irish Potato Famine (1845-1846):

• Event: A devastating disease outbreak destroyed the potato crop across Ireland.
• Causal Agent: The fungus-like organism Phytophthora infestans (Late blight of potato).
• Impact: Since potatoes were the staple food, the famine led to the starvation and death of approximately 1 million people and the migration of 1.5 million people to North America.
• Scientific Significance: This disaster was a turning point in Plant Pathology. It triggered intense scientific inquiry, leading Anton de Bary (in 1861) to prove that the fungus was the cause of the disease, not the result. This helped establish the Germ Theory of Disease in plants, disproving the theory of Spontaneous Generation.

Prof. K.C. Mehta (1892–1950):

• Focus: He is renowned for his work on the Cereal Rusts in India.
• Discovery: He discovered the "Wheat Rust Cycle" in India. He proved that the barberry bush (alternate host), which functions in Europe, does not play a role in the annual recurrence of Black Stem Rust in the plains of India due to the high summer temperatures.
• Mechanism: He demonstrated that the rust inoculum survives the summer in the cooler hills (Himalayas) on self-sown crops or collateral hosts and is blown down to the plains by winds to cause infection in winter.
• Laboratory: He established the Rust Research Laboratory at Agra.
• Impact: His findings laid the foundation for breeding rust-resistant wheat varieties suited for Indian conditions.

Necrotic Symptoms:
These symptoms involve the degeneration and death of cells and tissues. The protoplast dies, and the tissue turns brown or black.

Types of Necrotic Symptoms:

1. Leaf Spots: Localized lesions on host leaves consisting of dead and collapsed cells. The spots can be circular, angular, or irregular. (e.g., Cercospora leaf spot of groundnut).
2. Blight: A general and rapid browning and death of leaves, branches, twigs, or floral organs. It implies sudden death. (e.g., Late blight of potato).
3. Canker: A necrotic, often sunken lesion on a stem, branch, or twig. It typically involves the death of the bark and cambium. (e.g., Citrus canker).
4. Rot: Decomposition and putrefaction of plant tissue, which can be soft, wet, or dry. (e.g., Soft rot of vegetables).

Both are symptoms resulting from the overgrowth of plant tissues (Hyperplastic symptoms), but the cellular mechanism differs:

Hypertrophy:

• Definition: Abnormal increase in the size of the cells.
• Mechanism: The pathogen stimulates the host cells to enlarge beyond normal limits.
• Example: In Club root of cabbage, the giant cells are formed due to hypertrophy.

Hyperplasia:

• Definition: Abnormal increase in the number of cells (uncontrolled cell division).
• Mechanism: The pathogen stimulates cell division (mitosis) at a rapid rate.
• Example: Warts, galls, and tumors (e.g., Crown gall of stone fruits).

Note: Often, both processes occur simultaneously to produce symptoms like galls or knots (e.g., Root-knot nematode galls).

1. Biotic (Infectious) Diseases:

   • Caused by living organisms (animate causes) capable of reproducing and spreading from diseased to healthy plants.
   • Agents: Fungi, Bacteria, Phytoplasmas, Nematodes, Parasitic flowering plants, Algae.
   • Example: Wheat rust (Fungi), Citrus canker (Bacteria).

2. Mesobiotic Diseases:

   • Caused by infectious agents that share characteristics of both living and non-living entities. They contain genetic material (RNA/DNA) but requires a host to replicate.
   • Agents: Viruses and Viroids.
   • Example: Tobacco Mosaic Virus (TMV), Potato Spindle Tuber Viroid.

3. Abiotic (Non-infectious) Diseases:

   • Caused by non-living (inanimate) environmental factors. These are not transmissible.
   • Agents: Temperature extremes, soil moisture imbalance, nutrient deficiency/toxicity, air pollutants.
   • Example: Black heart of potato (Oxygen deficiency), Khaira disease of rice (Zinc deficiency).

Anton de Bary (1831–1888) is known as the Father of Modern Plant Pathology.

Contributions:

1. Late Blight Proof: He confirmed that the fungus Phytophthora infestans was the cause of the Irish Potato Famine, establishing the pathogenic nature of fungi.
2. Rust Life Cycle: He worked out the complex life cycle of wheat rust fungus (Puccinia graminis). He discovered that it is heteroecious, requiring two hosts (Wheat and Barberry) to complete its life cycle.
3. Terminology: He coined the term "Symbiosis" to describe the association between unlike organisms (in lichens).
4. Training: He trained many students who became leading plant pathologists, spreading scientific methodology across Europe and America.

General Characteristics:

1. Cellular Structure: They are unicellular, prokaryotic organisms (lacking a true nucleus and membrane-bound organelles).
2. Shape: Most plant pathogenic bacteria are rod-shaped (bacilli). (Exception: Streptomyces are filamentous).
3. Cell Wall: Most have rigid cell walls. The majority are Gram-negative (e.g., Pseudomonas, Xanthomonas, Erwinia), though some are Gram-positive (e.g., Clavibacter).
4. Motility: Many are motile via flagella (hair-like appendages). Flagellation patterns vary (monotrichous, peritrichous, etc.).
5. Reproduction: They reproduce primarily by binary fission (asexual), occurring very rapidly under favorable conditions.
6. Dissemination: They do not produce spores for wind dispersal; they rely on water (rain splash), insects, seeds, or human tools for spread.

Definition:
Pathogenesis is the chain of events that leads to the development of a disease in the host. It covers the life cycle of the pathogen from the initial contact with the host to the production of new inoculum.

Stages of Pathogenesis:

1. Inoculation: Arrival of the pathogen (inoculum) on the host surface.
2. Pre-penetration: Germination of spores or multiplication of bacteria on the host surface.
3. Penetration: Entry of the pathogen into the host (direct, through pores, or wounds).
4. Infection: Establishment of the pathogen within the host tissue and obtaining nutrients.
5. Colonization (Invasion): Spread of the pathogen within the host tissues.
6. Reproduction: Production of new spores or propagules.
7. Dissemination: Spread of the pathogen to new hosts.

Mollicutes (Phytoplasmas/Spiroplasmas):
These are prokaryotic organisms resembling bacteria but lacking a cell wall. They were formerly known as Mycoplasma-like Organisms (MLOs).

Key Features & Differences from Bacteria:

1. Cell Wall: Mollicutes lack a rigid cell wall (they are bounded only by a unit membrane), whereas true bacteria have a cell wall (peptidoglycan).
2. Shape: Due to the lack of a wall, they are pleomorphic (variable shape: spherical, ovoid, or filamentous), whereas bacteria have definite shapes.
3. Habitat: They are restricted to the phloem sieve tubes of plants, whereas bacteria can colonize xylem, parenchyma, or intercellular spaces.
4. Transmission: They are typically transmitted by phloem-feeding insects (leafhoppers) or grafting, not by wind or rain splash.
5. Antibiotic Sensitivity: They are sensitive to Tetracycline but resistant to Penicillin (since Penicillin attacks cell walls).

Plant Pathology is critical for agricultural sustainability and food security for several reasons:

1. Loss Reduction: Biotic stresses (diseases) cause an estimated 14% to 20% loss in global agricultural production annually. Managing these preserves the harvest.
2. Famine Prevention: Historical events (e.g., Irish Potato Famine, Bengal Famine of 1943 due to Brown spot of rice) demonstrate that unchecked diseases can lead to mass starvation.
3. Food Safety: Some fungal pathogens produce mycotoxins (e.g., Aflatoxin by Aspergillus flavus) in grains, which are toxic or carcinogenic to humans and animals.
4. Economic Stability: Disease outbreaks can destroy the livelihood of farmers and disrupt trade (e.g., quarantine restrictions on exporting diseased produce).
5. Biodiversity: Invasive pathogens (e.g., Chestnut blight) can wipe out entire tree species, altering ecosystems.

Discovery:
The Bordeaux Mixture was accidentally discovered by P.M.A. Millardet in 1882 in the Bordeaux region of France.

• Context: Farmers applied a paste of Copper Sulfate () and Lime () to grapevines along roadsides to deter pilferers (thieves) by making the grapes look poisonous.
• Observation: Millardet noticed that vines treated with this mixture retained their leaves, while untreated vines were defoliated by the Downy Mildew of Grapes (Plasmopara viticola).

Significance:

1. First Fungicide: It was the first successful broad-spectrum chemical fungicide developed.
2. Disease Control: It saved the French wine industry from collapse.
3. Chemical Era: It marked the beginning of the era of chemical control in plant pathology, leading to the development of other copper-based and eventually organic fungicides.

This classification is based on the mode of primary infection and survival:

1. Soil-borne Diseases:

   • The pathogen survives in the soil (on debris or as resting spores) and attacks the roots or lower stem.
   • Examples: Damping-off (Pythium), Wilt (Fusarium), Root rot (Rhizoctonia).

2. Seed-borne Diseases:

   • The pathogen survives on or inside the seed. The infection starts when the seed germinates.
   • Examples: Loose smut of wheat (Ustilago tritici - internal), Bacterial blight of paddy (external).

3. Air-borne Diseases:

   • The inoculum (spores) is spread by wind currents. These often cause epidemics.
   • Examples: Rusts of wheat, Powdery mildews, Blast of rice.

Disease Tetrahedron:
The Disease Tetrahedron is an expansion of the Disease Triangle concept. While the triangle includes Host, Pathogen, and Environment, the Tetrahedron adds a fourth dimension.

The Fourth Factor:
Most commonly, the fourth factor is identified as Time or Human Intervention.

1. Time: Disease is not an instantaneous event; it is a process. The duration of favorable environmental conditions and the timing of host susceptibility (growth stage) are crucial. If the three factors of the triangle align but not for a sufficient duration, disease will not develop.
2. Human Factor: Sometimes Human is considered the 4th factor because agricultural practices (irrigation, fertilization, variety selection) directly manipulate the Host, Environment, and Pathogen.

Difference:

• Triangle: Shows the conditions necessary for disease.
• Tetrahedron: Shows the dynamics and quantity of disease development over time/management.