🧫 Important Groups of Plant Pathogens
Understand the major biotic groups that cause plant disease and how they differ in nature, spread, and examples.
Plant diseases are classified by causal agents so diagnosis and control can target the exact pathogen group instead of only visible symptoms.
Classification of Causal Agents
Broadly, causes are:
- Biotic (infectious): fungi, bacteria, phytoplasmas, spiroplasmas, viruses, viroids, nematodes, parasitic plants.
- Abiotic (non-infectious): nutrient imbalance, temperature stress, moisture stress, toxicity, pollution, and adverse pH.
Prokaryotic Plant Pathogens
Main prokaryotic groups:
- True bacteria (example: citrus canker).
- Fastidious vascular bacteria / RLB.
- Mollicutes: phytoplasmas and spiroplasmas.
Typical effects include wilting, chlorosis, stunting, little leaf, and vascular decline.
Eukaryotic Plant Pathogens
Major eukaryotic pathogen groups:
- Fungi (largest group of plant pathogens).
- Oomycete-like pathogens in practical disease lists.
- Algae (example: red rust).
- Protozoan-like pathogens in limited diseases.
- Parasitic flowering plants (broomrape, dodder).
Mesobiotic Agents
Viruses and viroids are infectious but replicate only inside living host cells.
- Viruses: nucleoprotein particles, usually vector-transmitted.
- Viroids: very small circular RNA without protein coat.
Examples:
- Yellow mosaic of blackgram (virus).
- Potato spindle tuber (viroid).
Fastidious Vascular Bacteria (RLB)
RLB are nutritionally exacting, usually vector-borne, and often restricted to xylem or phloem.
Important points:
- Difficult to culture on routine media.
- Cause chronic decline and systemic symptoms.
- Spread via insect vectors, grafting, and sometimes vegetative propagation.
Summary Cheat Sheet
| Group | Nature | Example Disease |
|---|---|---|
| Fungi | Eukaryotic filamentous pathogens | Wilt of cotton |
| Bacteria | Unicellular prokaryotes | Citrus canker |
| Phytoplasmas | Wall-less mollicutes | Sesame phyllody |
| Spiroplasmas | Helical mollicutes | Citrus stubborn |
| Viruses | Obligate intracellular particles | Yellow mosaic |
| Viroids | Naked circular RNA | Potato spindle tuber |
| Parasitic plants | Phanerogamic parasites | Broomrape of tobacco |
References
1 source • [1]
References
Used for: Standard BSc-level classification of plant pathogenic organisms and examples.
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