🦠 Plant Pathogens
Introduction to plant pathology, disease triangle, fungi classification, bacteria, phytoplasma, archaea, virus, viroid, prion, spiroplasma for CUET Agriculture
Introduction to Plant Pathology (पादप रोग विज्ञान)
Plant pathology is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions. It is also called Phytopathology, a term derived from Greek where Phyton means Plant, Pathos means Disease or Ailment, and Logos means Study. Understanding plant pathology is essential because plant diseases can devastate crops, cause famines, and lead to massive economic losses.
- The Father of Modern Plant Pathology is Anton de Bary (Germany), who pioneered the study of fungi as causal agents of plant diseases.
- The Father of Indian Plant Pathology is E.J. Butler, who made foundational contributions to understanding plant diseases in the Indian subcontinent.
Historical Milestones
The history of plant pathology is marked by devastating epidemics and groundbreaking discoveries. Each milestone below shaped our modern understanding of how plants get sick and how we can protect them.
| Year | Event |
|---|---|
| 1839-1916 | T.J. Burrill — First scientist to prove that bacteria cause plant disease. He demonstrated that Erwinia amylovora causes fire blight in apple and pear trees. This was a landmark because, until then, only fungi were known plant pathogens. |
| 1845 | Irish Famine (Great Potato Famine) — Late blight of potato caused by Phytophthora infestans destroyed potato crops across Ireland, leading to mass starvation and emigration. This event highlighted the catastrophic potential of plant diseases. |
| 1885 | Bordeaux mixture invented by Millardet in France for controlling downy mildew of grape. This was the world's first fungicide. Composition: Copper sulphate + Lime + Water @ 5:5:50 gallons (1 lb = 0.45 kg = 1 Pond; 1 gallon = 3.78 litres). |
| 1929 | Alexander Fleming discovered Penicillin from the fungus Penicillium notatum. Though primarily used in human medicine, this discovery opened the door to antibiotic-based disease management. |
| 1943 | Bengal Famine — Brown spot disease of rice caused by Helminthosporium oryzae devastated rice crops in Bengal, contributing to one of India's worst famines. |
| 1944 | Waksman discovered Streptomycin, the first antibiotic effective against tuberculosis, derived from Streptomyces. Streptomycin later became important in managing bacterial plant diseases too. |
| 1946 | H.H. Flor proposed the Gene for Gene Hypothesis — the idea that for each resistance gene in the host plant, there is a corresponding avirulence gene in the pathogen. This concept is foundational to plant breeding for disease resistance. |
| 1977 | Clark & Adams developed the ELISA (Enzyme-Linked Immunosorbent Assay) technique, which became the standard method for virus detection in plants. |
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Introduction to Plant Pathology (पादप रोग विज्ञान)
Plant pathology is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions. It is also called Phytopathology, a term derived from Greek where Phyton means Plant, Pathos means Disease or Ailment, and Logos means Study. Understanding plant pathology is essential because plant diseases can devastate crops, cause famines, and lead to massive economic losses.
- The Father of Modern Plant Pathology is Anton de Bary (Germany), who pioneered the study of fungi as causal agents of plant diseases.
- The Father of Indian Plant Pathology is E.J. Butler, who made foundational contributions to understanding plant diseases in the Indian subcontinent.
Historical Milestones
The history of plant pathology is marked by devastating epidemics and groundbreaking discoveries. Each milestone below shaped our modern understanding of how plants get sick and how we can protect them.
| Year | Event |
|---|---|
| 1839-1916 | T.J. Burrill — First scientist to prove that bacteria cause plant disease. He demonstrated that Erwinia amylovora causes fire blight in apple and pear trees. This was a landmark because, until then, only fungi were known plant pathogens. |
| 1845 | Irish Famine (Great Potato Famine) — Late blight of potato caused by Phytophthora infestans destroyed potato crops across Ireland, leading to mass starvation and emigration. This event highlighted the catastrophic potential of plant diseases. |
| 1885 | Bordeaux mixture invented by Millardet in France for controlling downy mildew of grape. This was the world's first fungicide. Composition: Copper sulphate + Lime + Water @ 5:5:50 gallons (1 lb = 0.45 kg = 1 Pond; 1 gallon = 3.78 litres). |
| 1929 | Alexander Fleming discovered Penicillin from the fungus Penicillium notatum. Though primarily used in human medicine, this discovery opened the door to antibiotic-based disease management. |
| 1943 | Bengal Famine — Brown spot disease of rice caused by Helminthosporium oryzae devastated rice crops in Bengal, contributing to one of India's worst famines. |
| 1944 | Waksman discovered Streptomycin, the first antibiotic effective against tuberculosis, derived from Streptomyces. Streptomycin later became important in managing bacterial plant diseases too. |
| 1946 | H.H. Flor proposed the Gene for Gene Hypothesis — the idea that for each resistance gene in the host plant, there is a corresponding avirulence gene in the pathogen. This concept is foundational to plant breeding for disease resistance. |
| 1977 | Clark & Adams developed the ELISA (Enzyme-Linked Immunosorbent Assay) technique, which became the standard method for virus detection in plants. |
IMPORTANT
The two most exam-relevant famines are: Irish Famine (1845) caused by Phytophthora infestans (potato late blight) and Bengal Famine (1943) caused by Helminthosporium oryzae (rice brown spot).
Important Pioneers
These scientists laid the groundwork for modern plant pathology. Their contributions range from establishing basic principles of disease proof to discovering entirely new categories of pathogens.
| Scientist | Contribution |
|---|---|
| Robert Koch | Formulated Koch's Postulates — a systematic, four-step method to prove that a specific organism causes a specific disease. These postulates remain the gold standard for establishing disease causation. |
| E.J. Butler | Father of Indian Plant Pathology — conducted pioneering research on crop diseases in India. |
| Mitcheli | Father of Mycology (the study of fungi). |
| K.C. Mehta | Studied rust diseases of wheat in India extensively, tracing the epidemiology and spread patterns. |
| T.O. Diener | Discovered Viroids — the smallest known plant pathogens, consisting of naked RNA without any protein coat. |
| Herelle (d'Herelle) | Coined the term Bacteriophage (viruses that infect bacteria). |
| Y.L. Nene | Studied wilt disease of pigeon pea (Arhar/Cajanus cajan), an economically important pulse crop in India. |
Disease Triangle (रोग त्रिकोण)
The Disease Triangle was proposed by Stevens (1960). It is a fundamental concept in plant pathology that explains why diseases occur. For a plant disease to develop, three factors must be present simultaneously and interact over a sufficient period of time:
- Susceptible Host (परपोषी) — The plant must be genetically vulnerable to the pathogen. If the host carries resistance genes, infection may fail.
- Virulent Pathogen (रोगजनक) — The pathogen (fungus, bacterium, virus, etc.) must be present and capable of causing disease. Its virulence determines the severity.
- Favourable Environment (पर्यावरण) — Conditions like temperature, humidity, moisture, and wind must favour the pathogen's growth and spread.
NOTE
If any one factor is missing or unfavourable, the disease will NOT occur. This principle is the basis for all disease management strategies — we try to modify at least one corner of the triangle (e.g., use resistant varieties to change the host factor, apply fungicides to target the pathogen, or adjust planting dates to avoid favourable environmental conditions).
Classification of Plant Diseases
Plant diseases can be classified in multiple ways depending on the criterion used — by cause, by spread pattern, by disease cycle, or by source of infection. Understanding these classifications helps in diagnosis and choosing the right management approach.
A. Based on Causal Organism
The causal agents of plant disease fall into three broad categories:
| Biotic (जैविक) | Meso-Biotic (मिजो-बायोटिक) | Abiotic (अजैविक) |
|---|---|---|
| 1. Fungi (कवक) — Most common plant pathogens | 1. Virus (विषाणु) — Non-cellular, require living host | 1. Nutrient deficiency |
| 2. Bacteria (जीवाणु) | 2. Viroid (वाइरोइड) — Smallest pathogens | 2. Low oxygen |
| 3. Algae (शैवाल) | — | 3. Temperature extremes |
| 4. Protozoa (प्रोटोज़ोआ) | — | 4. Environmental pollutants |
| 5. Nematode (सूत्रकृमि) | — | — |
| 6. Phytoplasma (फाइटोप्लाज़्मा) | — | — |
| 7. Spiroplasma | — | — |
| 8. Parasitic flowering plants | — | — |
TIP
Biotic pathogens are living organisms that can reproduce. Meso-biotic agents (viruses and viroids) are on the boundary between living and non-living — they need a living host cell to replicate. Abiotic diseases are caused by non-living environmental factors and are not transmissible.
B. Based on Spread (रोग का भौगोलिक वितरण)
The geographic spread and intensity of a disease determine its epidemiological classification. This is important for surveillance and policy decisions.
| Type | Spread | Examples |
|---|---|---|
| Sporadic (छिटपुट/दबाचित) | Limited area, at irregular intervals — appears unpredictably | Fusarium wilt of cotton, Peanut clump virus, Loose smut of wheat |
| Endemic (स्थानिक) | Moderate spread, confined to a specific region or country — always present at low levels | Wart disease of potato |
| Epidemic (महामारी/व्यापक) | Sudden outbreak covering large areas with severe damage | Wheat rust (काली रोली), Potato late blight, Grape downy mildew |
| Pandemic (सर्वव्यापी/विश्वव्यापी) | Spreads across an entire country or worldwide | Wheat rust, Potato late blight |
C. Based on Disease Cycle
This classification is based on how many infection cycles a pathogen completes in a single growing season. It directly affects how quickly a disease can spread.
| Type | Cycles/Year | Rate | Examples |
|---|---|---|---|
| Monocyclic (एकल चक्रीय) | 1 life cycle per year | Slow spread — follows simple interest model (linear increase) | Wilt (Fusarium), Root rot (Rhizoctonia), Damping off (Pythium) |
| Polycyclic (बहु चक्रीय) | 2 or more cycles per year | Fast spread — follows compound interest model (exponential increase) | Powdery mildew (Erysiphe), Stem/black rust of wheat (Puccinia graminis tritici), Blast of rice (Pyricularia oryzae) |
WARNING
Polycyclic diseases are far more dangerous because each cycle produces new inoculum that immediately re-infects crops, leading to explosive epidemics. Rust, powdery mildew, and blast are classic examples that can destroy entire fields within weeks.
D. Based on Source of Infection
Knowing where the pathogen survives between crop seasons (its primary inoculum source) is critical for choosing the right management strategy.
| Source | Examples |
|---|---|
| Soil borne (मृदा जनित) — Pathogen survives in soil as spores or resting structures | Wilt (Fusarium), Root rot (Rhizoctonia), Damping off (Pythium) |
| Seed borne (बीज जनित) — Pathogen carried on or inside the seed | Covered smut (external), Loose smut (internal), DM of bajra, Blast of rice |
| Air borne (वायु जनित) — Spores spread through wind | Tikka/leaf spot of groundnut (Cercospora) |
1. Fungi (कवक)
Fungi are the most important group of plant pathogens, responsible for approximately 70-80% of all known plant diseases. They range from simple single-celled yeasts to complex multicellular organisms with elaborate fruiting bodies.
Key Characteristics
- Eukaryotic organisms — they have a true nucleus with membrane-bound organelles
- Thallus (body) = mycelium (a network of thread-like structures called hyphae); hyphae may be septate (with cross-walls) or non-septate/coenocytic (without cross-walls)
- Chlorophyll absent — fungi are heterotrophic, meaning they cannot photosynthesize and must obtain nutrition from other organisms (living or dead)
- Reproduce by asexual spores (conidia, sporangia) and sexual spores (ascospores, basidiospores, oospores)
- Lifestyle can be parasitic (feeding on living organisms) or saprophytic (feeding on dead organic matter)
Fungi as Parasites
Fungi can be classified by their nutritional relationship with the host. This determines how they survive and how we manage them.
| Category | Type | Genus | Disease |
|---|---|---|---|
| Obligate parasite | Must have a living host to survive and reproduce — cannot be cultured on artificial media in the lab | Albugo | White rust of mustard |
| Erysiphe | Powdery mildew | ||
| Puccinia | Rust | ||
| Sclerospora | Downy mildew | ||
| Facultative parasite | Primarily parasitic but can also live on dead matter — more versatile survival | Fusarium | Wilt disease |
| Rhizoctonia | Root rot | ||
| Pythium | Damping off | ||
| Saprophytic (मृतोपजीवी) | Lives on dead organic matter — some can become opportunistic pathogens | Fusarium | Wilt (can survive in soil on dead plant debris) |
| Agaricus | White button mushroom (not a pathogen — commercially cultivated) |
TIP
Obligate parasites like Puccinia (rust) and Erysiphe (powdery mildew) are the hardest to manage because they co-evolve rapidly with their hosts. They also cannot be studied easily in labs since they require living plant tissue.
Symbionts (सहजीवी)
Not all fungal associations are harmful. Some fungi form mutually beneficial relationships with other organisms:
- Lichen = Fungi + Algae (or Cyanobacteria). The algal partner produces food via photosynthesis, while the fungus provides structure and protection. Lichens are indicators of air quality.
- Mycorrhiza = Fungi + Plant roots. First described by Frank (1885).
- The fungus helps the plant absorb Phosphorus & Nitrogen more efficiently from the soil, while the plant provides sugars to the fungus.
- Examples: Glomus (forms arbuscular mycorrhiza — fungal hyphae penetrate root cells); Pinus, Eucalyptus (form ectomycorrhiza — fungal hyphae surround root cells externally)
Classification of Fungi (कवक का वर्गीकरण)
The five-kingdom classification system was proposed by Whittaker, which placed fungi in their own kingdom (Kingdom Mycota). P.A. Saccardo (1866) classified fungi based on septation (presence of cross-walls in hyphae) and spore production patterns.
Kingdom Mycota → Division: Eumycota (True Fungi)
i. Subdivision: Mastigomycotina
These are primitive fungi, many of which are aquatic or require water for spore dispersal. Their spores are motile (flagellated).
Class: Oomycetes (water moulds) — These fungi produce oospores (sexual) and zoospores (asexual, motile in water):
| Family | Genus | Disease |
|---|---|---|
| Pythiaceae | Pythium | Damping off (आर्द्रगलन) — kills seedlings at soil level |
| Phytophthora | Late blight of potato — caused the Irish Famine | |
| Albuginaceae | Albugo | White rust of mustard — white pustules on leaves |
| Peronosporaceae | Sclerospora | Downy mildew of bajra — green ear symptom |
| Peronospora | Downy mildew (general — affects many crops) | |
| Plasmopora | Downy mildew of grape — led to Bordeaux mixture discovery |
ii. Subdivision: Zygomycotina
These fungi reproduce sexually by forming zygospores (fusion of two similar gametangia).
- Rhizopus (Mucor) — common bread mould. Not a major plant pathogen but important in food spoilage.
iii. Subdivision: Ascomycotina (Sac Fungi)
Named "sac fungi" because their sexual spores (ascospores) are produced inside sac-like structures called asci (singular: ascus).
Class: Plectomycetes:
- Erysiphe → Powdery mildew (चूर्णिल आसिता) — white powdery growth on leaf surfaces. An obligate parasite.
Class: Pyrenomycetes:
- Claviceps → Ergot disease (अर्गट रोग) — infects grain crops, producing toxic alkaloids (ergotin/ergotoxin) that cause ergotism in humans and animals who consume contaminated grain.
iv. Subdivision: Basidiomycotina
These are advanced fungi producing sexual spores (basidiospores) on club-shaped structures called basidia. This group includes some of the most economically important plant pathogens.
Class: Teliomycetes (Rust & Smut fungi) — All obligate parasites of crop plants:
| Order | Genus | Disease |
|---|---|---|
| Uredinales | Puccinia | Rust (रोली) — orange/brown/black pustules on leaves and stems |
| Ustilaginales | Ustilago | Smut (कण्डवा) — grains replaced by black powdery spore mass |
| Tilletiaceae | Tilletia | Bunt (बन्ट) — grains filled with dark, foul-smelling spore mass |
Class: Hymenomycetes — Includes commercially cultivated edible mushrooms:
- Agaricus → White button mushroom (सफेद बटन मशरूम) — most widely cultivated mushroom globally
- Pleurotus → Oyster mushroom (ओएस्टर मशरूम) — grows on agricultural waste, rich in protein
v. Subdivision: Deuteromycotina (Fungi Imperfecti)
Called "imperfect" because their sexual stage is unknown — only asexual spores (conidia) have been observed. Many major crop pathogens belong here.
Class: Coelomycetes:
- Colletotrichum → Red rot of sugarcane — one of the most destructive sugarcane diseases
Class: Hyphomycetes — Contains several genera responsible for widespread crop diseases:
| Genus | Disease |
|---|---|
| Alternaria | Blight (झुलसा) — causes large dead patches on leaves with concentric ring pattern |
| Cercospora | Tikka of groundnut — circular leaf spots |
| Fusarium | Wilt (उखटा) — blocks xylem vessels, causing wilting |
| Rhizoctonia | Root rot (जड़ गलन) — attacks roots and stem base |
2. Bacteria (जीवाणु)
Bacteria are single-celled prokaryotic organisms (lacking a true membrane-bound nucleus). Although they cause fewer diseases than fungi, bacterial plant diseases can be very destructive and are often difficult to control because bacteria multiply rapidly and have limited chemical management options.
Key Characteristics
- Prokaryotic — no true nucleus; genetic material (circular DNA) is free in the cytoplasm
- Cell wall composed of peptidoglycan (also called murein)
- Reproduce asexually by binary fission — one cell divides into two identical daughter cells, enabling very rapid population growth
- Contain both DNA & RNA; some carry plasmids (small circular DNA molecules) — notably Ti plasmid (tumor-inducing) and Ri plasmid (root-inducing) in Agrobacterium
- Classification by Gram staining (developed by H.C. Gram, 1884):
| Feature | Gram Positive (+Ve) | Gram Negative (-Ve) |
|---|---|---|
| Cell wall | Single layer (thick peptidoglycan) | Double layer (thin peptidoglycan + outer membrane) |
| Peptidoglycan content | 40-90% | 1-10% |
| Teichoic acid | Present | Absent |
| Plant disease | Few plant pathogens | Most plant pathogenic bacteria are Gram-negative |
| Examples | Bacillus, Clostridacter, Clavibacter, Streptomyces | Xanthomonas, Erwinia, Pseudomonas |
IMPORTANT
Most bacteria that cause plant diseases are Gram-negative. The three most important genera to remember are: Xanthomonas (leaf blights, cankers), Erwinia (fire blight, soft rots), and Pseudomonas (leaf spots, blights).
Disease-causing Bacteria
- Xanthomonas: Causes Bacterial leaf blight of rice (Paddy) — one of the most devastating rice diseases; Citrus canker — quarantine disease; Cotton angular leaf spot (कोणीय पर्णचीती)
- Erwinia: Causes Fire blight of apple — highly destructive in temperate fruit orchards; also causes various soft rot diseases in vegetables
- Pseudomonas: Causes various leaf spots and blights across multiple crops
Beneficial Bacteria
Not all bacteria harm plants. Some are essential for agriculture:
- Nitrogen fixation: Rhizobium — forms symbiotic nodules on roots of legume crops (pulses, groundnut, soybean) and converts atmospheric nitrogen (N₂) into plant-usable ammonium (NH₄⁺). This biological nitrogen fixation reduces the need for chemical fertilizers.
- Genetic engineering: Agrobacterium tumefaciens carries the Ti plasmid (Tumor-inducing plasmid) and is known as "Nature's genetic engineer" because it naturally transfers DNA into plant cells. Scientists exploit this ability to create transgenic crops. A. rhizogenes carries the Ri plasmid (Root-inducing) and is used for hairy root culture research.
3. Phytoplasma / Mycoplasma / MLOs
Phytoplasma (earlier called Mycoplasma-Like Organisms or MLOs) are a unique group of pathogens discovered by Doi et al. (1967). They are among the smallest and most unusual plant pathogens.
- Cell wall-less prokaryotes — unlike bacteria, they lack a rigid cell wall, making them pleomorphic (variable in shape). This also means they are resistant to antibiotics that target cell wall synthesis (like penicillin).
- Contains both dsDNA & RNA
- Requires sterol (a lipid) for growth — unusual for prokaryotes
- Found specifically in the phloem (food-conducting tissue) of host plants, which is why they cause symptoms related to food transport disruption
- Vectors: Transmitted by Leafhoppers (most common vector), which acquire the phytoplasma while feeding on phloem sap
Diseases caused by Phytoplasma
- Sesame phyllody / Witch's broom (तिले का फाईलोडी/वीच ब्रूम) — floral parts transform into leaf-like structures
- Brinjal little leaf (बैंगन का छोटी पत्ती रोग) — leaves remain abnormally small
- Sugarcane grassy shoot (गन्ने की ग्रासी तना रोग) — excessive tillering with thin, grass-like shoots
What is Phyllody?
**Phyllody** is a condition where floral organs (petals, sepals, stamens) are transformed into **leaf-like structures**. This is a hallmark symptom of phytoplasma infection. The affected plant produces no viable seeds, leading to complete yield loss in the infected plant. The phytoplasma manipulates plant hormones (especially cytokinins) to cause this abnormal growth.4. Archaea bacteria (आर्कीजीवाणु)
Archaea are a special domain of life, distinct from both bacteria and eukaryotes. While they superficially resemble bacteria (both are prokaryotic and single-celled), their molecular biology and biochemistry are fundamentally different.
- Found in extreme environments — hot springs (thermophiles), salt lakes (halophiles), acidic environments (acidophiles). These are called extremophiles.
- Cell wall lacks murein (peptidoglycan) — a key difference from true bacteria. Instead, they may have pseudopeptidoglycan or protein-based cell walls.
- Contains dsDNA & RNA
- Some are obligate anaerobic and methanogenic: Methanobacterium produces methane gas (CH₄) — relevant to biogas production and greenhouse gas emissions from flooded rice fields.
NOTE
Archaea are not significant plant pathogens, but they are important in CUET exams for understanding the diversity of microorganisms and their ecological roles, especially in extreme environments and methane production.
5. Virus (विषाणु)
Viruses are submicroscopic, non-cellular infectious agents that can only replicate inside living host cells. They are on the boundary between living and non-living matter — they show no metabolic activity outside a host cell but can hijack the host's cellular machinery to reproduce.
Key Characteristics
- 1886: Adolf Mayer first described Tobacco Mosaic disease — showed it could be transmitted from sick to healthy plants
- 1898: Beijerinck named the causal agent "virus" (Latin for poison), calling it a "contagious living fluid" that could pass through bacteria-proof filters
- Stanley (1935): Crystallized TMV (Tobacco Mosaic Virus), proving that viruses are particulate, not fluid. He won the Nobel Prize in 1946 for this work.
- Father of Plant Virology: Beijerinck
- Virus coat protein = Capsid (made of protein subunits called capsomeres that protect the nucleic acid)
- Infectious particle = Virion (the complete virus particle outside a host cell)
- Most plant viruses that cause disease contain ssRNA (single-stranded RNA) as their genetic material
- Notable exceptions:
- Cauliflower mosaic virus → dsDNA (double-stranded DNA)
- Banana bunchy top virus → ssDNA (single-stranded DNA)
- Common virus-caused symptoms: Mosaic (light and dark green patches on leaves), Leaf curl (upward or downward curling), Yellow vein (yellowing along veins)
- Viruses are transmitted in plants primarily by insect vectors: aphids (most common), whiteflies, leafhoppers, thrips
Important Virus Diseases
| Disease | Virus/Vector |
|---|---|
| Sesame phyllody / Witch broom | Vector: Leafhopper |
| Brinjal little leaf | Vector: Leafhopper |
| Sugarcane grassy shoot | Vector: Aphid |
TIP
For CUET, remember the virus discovery timeline: Mayer (1886) described the disease → Ivanovsky (1892) showed it passed through filters → Beijerinck (1898) named it "virus" → Stanley (1935) crystallized TMV.
6. Viroid (वाइरोइड)
Viroids are the smallest known pathogens — even simpler than viruses. They were discovered by T.O. Diener while studying potato spindle tuber disease.
- A viroid is simply naked ssRNA (single-stranded RNA) — it has no protein coat, no capsid, unlike viruses
- Despite being just a small RNA molecule (246-401 nucleotides), viroids can cause serious diseases because they interfere with the host plant's gene expression
- Diseases caused by viroids:
- Potato spindle tuber viroid — tubers become elongated and spindle-shaped
- Coconut cadang-cadang viroid — lethal disease of coconut palms in the Philippines
- Chrysanthemum stunt viroid — causes stunting in chrysanthemum plants
IMPORTANT
Key difference: Virus = RNA or DNA + protein coat (capsid) vs Viroid = naked RNA only (no protein coat). Viroids are much smaller than viruses.
7. Prion (प्रियोन)
Prions are unique among infectious agents because they contain no nucleic acid (no DNA, no RNA). A prion is simply an infectious protein particle — a misfolded version of a normal protein that can force other normal proteins to misfold too, creating a chain reaction.
- Causes Mad cow disease (Bovine Spongiform Encephalopathy / गाय का मेड रोग) in animals
- Prions are relevant to animal science rather than plant pathology, but they are included in CUET syllabus for completeness as a category of infectious agents
8. Spiroplasma (स्पाइरोप्लाज़्मा)
Spiroplasma are spiral-shaped, wall-less prokaryotes closely related to phytoplasma. They were discovered by Davis et al. (1972).
- Coiled/spiral-shaped — this is their distinguishing morphological feature (phytoplasma are pleomorphic, but spiroplasma have a consistent helical shape)
- Cell wall absent — like phytoplasma, they lack a rigid cell wall
- Requires cholesterol for growth (phytoplasma require sterol — both need lipids for their cell membrane)
- Diseases:
- Corn stunt disease (Vector: leafhopper) — causes stunting and reddening in maize
- Citrus stubborn disease (Vector: leafhopper) — causes lopsided, small fruits in citrus
Key Points for CUET
These are the most frequently tested concepts. Review them as a quick checklist before your exam:
- Disease Triangle: Host + Pathogen + Environment (Stevens, 1960)
- Anton de Bary = Father of Modern Plant Pathology
- E.J. Butler = Father of Indian Plant Pathology
- Koch's Postulates = Method to prove disease causation (4 steps)
- Bordeaux mixture = First fungicide (1885, Millardet); CuSO₄ + Lime + Water
- Bengal Famine (1943) = Brown spot of rice (Helminthosporium oryzae)
- Irish Famine (1845) = Late blight of potato (Phytophthora infestans)
- Puccinia = Rust; Ustilago = Smut; Tilletia = Bunt
- Fusarium = Wilt; Pythium = Damping off; Rhizoctonia = Root rot
- TMV = First virus identified; Stanley crystallized (1935)
- Viroid = Naked ssRNA; discovered by T.O. Diener; Potato spindle tuber
- Phytoplasma = Cell wall-less prokaryote in phloem; vector: leafhoppers
- Gene for Gene Hypothesis = H.H. Flor (1946)
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Plant Pathology | Study of plant diseases; also called Phytopathology (Greek: Phyton=Plant, Pathos=Disease, Logos=Study) |
| Father of Modern Plant Pathology | Anton de Bary (Germany) |
| Father of Indian Plant Pathology | E.J. Butler |
| Father of Mycology | Mitcheli |
| Koch's Postulates | 4-step method to prove a specific organism causes a specific disease; formulated by Robert Koch |
| Irish Famine (1845) | Late blight of potato caused by Phytophthora infestans; mass starvation in Ireland |
| Bengal Famine (1943) | Brown spot of rice caused by Helminthosporium oryzae; devastated Bengal rice crops |
| Bordeaux Mixture (1885) | First fungicide; invented by Millardet; CuSO₄ + Lime + Water @ 5:5:50 gallons; for downy mildew of grape |
| Penicillin (1929) | Discovered by Alexander Fleming from Penicillium notatum |
| Streptomycin (1944) | Discovered by Waksman from Streptomyces |
| Gene for Gene Hypothesis | Proposed by H.H. Flor (1946); each resistance gene in host has a corresponding avirulence gene in pathogen |
| ELISA (1977) | Developed by Clark & Adams; standard method for plant virus detection |
| T.J. Burrill (1839-1916) | First proved bacteria cause plant disease (Erwinia amylovora → fire blight of apple) |
| Disease Triangle | Proposed by Stevens (1960); requires 3 factors: Susceptible Host + Virulent Pathogen + Favourable Environment |
| Disease spread — Sporadic | Limited area, irregular intervals (e.g., Fusarium wilt of cotton) |
| Disease spread — Endemic | Confined to specific region (e.g., Wart disease of potato) |
| Disease spread — Epidemic | Sudden outbreak, large area (e.g., Wheat rust, Potato late blight) |
| Monocyclic disease | 1 cycle/year; simple interest model; slow spread (e.g., Wilt, Root rot, Damping off) |
| Polycyclic disease | 2+ cycles/year; compound interest model; fast/explosive spread (e.g., Rust, Powdery mildew, Blast) |
| Source — Soil borne | Fusarium (Wilt), Rhizoctonia (Root rot), Pythium (Damping off) |
| Source — Seed borne | Covered smut (external), Loose smut (internal), DM of bajra, Blast of rice |
| Source — Air borne | Tikka/leaf spot of groundnut (Cercospora) |
| Fungi — Key traits | Eukaryotic, chlorophyll absent (heterotrophic), body = mycelium (hyphae); septate or coenocytic |
| Obligate parasites (fungi) | Albugo (White rust), Erysiphe (Powdery mildew), Puccinia (Rust), Sclerospora (Downy mildew) |
| Facultative parasites (fungi) | Fusarium (Wilt), Rhizoctonia (Root rot), Pythium (Damping off) |
| Mycorrhiza | Fungi + Plant roots; described by Frank (1885); helps absorb P & N; e.g., Glomus (arbuscular) |
| Mastigomycotina — Oomycetes | Motile zoospores; Pythium (Damping off), Phytophthora (Late blight), Albugo (White rust), Sclerospora (Downy mildew) |
| Zygomycotina | Sexual spore = zygospore; Rhizopus (bread mould) |
| Ascomycotina (Sac Fungi) | Sexual spore = ascospore in asci; Erysiphe (Powdery mildew), Claviceps (Ergot — ergotin toxin) |
| Basidiomycotina | Sexual spore = basidiospore; Puccinia → Rust, Ustilago → Smut, Tilletia → Bunt |
| Edible mushrooms | Agaricus (White button mushroom), Pleurotus (Oyster mushroom) |
| Deuteromycotina (Fungi Imperfecti) | Sexual stage unknown; Alternaria (Blight), Cercospora (Tikka), Fusarium (Wilt), Rhizoctonia (Root rot), Colletotrichum (Red rot of sugarcane) |
| Bacteria — Key traits | Prokaryotic, cell wall = peptidoglycan/murein, reproduce by binary fission |
| Gram staining (H.C. Gram, 1884) | Gram +ve: thick peptidoglycan (40-90%); Gram -ve: thin (1-10%) + outer membrane; most plant pathogenic bacteria are Gram -ve |
| Important bacterial genera | Xanthomonas (BLB of rice, Citrus canker), Erwinia (Fire blight of apple), Pseudomonas (leaf spots) |
| Agrobacterium tumefaciens | "Nature's genetic engineer"; carries Ti plasmid (tumor-inducing); used for making transgenic crops |
| Phytoplasma (Doi et al., 1967) | Cell wall-less prokaryote, pleomorphic, found in phloem; requires sterol; vector: leafhoppers |
| Phytoplasma diseases | Sesame phyllody/Witch's broom, Brinjal little leaf, Sugarcane grassy shoot |
| Archaea | Extremophiles; cell wall lacks murein; Methanobacterium produces methane; not plant pathogens |
| Virus — Discovery timeline | Mayer (1886) described TMV disease → Ivanovsky (1892) filter-passing → Beijerinck (1898) coined "virus" → Stanley (1935) crystallized TMV (Nobel 1946) |
| Virus — Key traits | Non-cellular; coat = capsid; particle = virion; most plant viruses = ssRNA |
| Virus exceptions | Cauliflower mosaic virus = dsDNA; Banana bunchy top virus = ssDNA |
| Father of Plant Virology | Beijerinck |
| Viroid (T.O. Diener) | Smallest pathogen; naked ssRNA (no protein coat); Potato spindle tuber viroid, Coconut cadang-cadang |
| Prion | Infectious protein (no DNA/RNA); causes Mad cow disease (BSE) |
| Spiroplasma (Davis et al., 1972) | Spiral-shaped, cell wall absent, requires cholesterol; Corn stunt disease, Citrus stubborn disease; vector: leafhopper |
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