🍄 Fungi, Algae, Viruses & Mycoplasma — Structure, Features, and Agricultural Significance
Key features, comparisons, and exam-critical facts about fungi, algae, viruses, mycoplasma, and their roles in agriculture with mnemonics and summary tables
From Field to Lab — Four Groups That Shape Every Farm
Picture a wheat field in Punjab after the monsoon. The leaves are covered with orange-brown pustules — that is rust, caused by a fungus. In a nearby rice paddy, the standing water has a greenish sheen — that is blue-green algae, a natural biofertiliser fixing nitrogen for free. A tomato plant in the kitchen garden has curled, yellowing leaves — the culprit is leaf curl virus, transmitted by whiteflies. And in the neighbouring brinjal field, plants with tiny, bunched leaves signal little leaf disease, caused by mycoplasma (phytoplasma).
These four groups — fungi, algae, viruses, and mycoplasma — together account for the vast majority of crop diseases and biological processes in agriculture. Understanding their differences is essential for both farm management and competitive exams.
Fungi — The Largest Group of Plant Pathogens
Fungi are eukaryotic, heterotrophic organisms. They cannot make their own food and depend on organic matter — either as saprophytes (feeding on dead matter) or as parasites (feeding on living hosts).
Structure and Key Features
| Feature | Detail |
|---|---|
| Cell type | Eukaryotic |
| Nutrition | Heterotrophic (cannot photosynthesize) |
| Size | 1.5–10 microns |
| Single filament | Hypha (plural: hyphae) |
| Group of hyphae | Mycelium |
| Root-like structures of Rhizopus | Rhizoids |
| Nutrient-absorbing structures in host | Haustoria |
| Best culture media | Potato Dextrose Agar (PDA) |
| Solidifying agent in media | Agar-agar |
| Yeast reproduction (asexual) | Budding |
| Yeasts are responsible for | Fermentation |
IMPORTANT
All fungi are heterotrophs while all algae are autotrophs. This is the single most important distinction between these two groups for exams.
TIP
Mnemonic — "Fungi Hunt, Algae Auto": Fungi are Heterotrophs (they hunt for food), Algae are Autotrophs (they auto-produce food via photosynthesis).
Algae — The Photosynthetic Microorganisms
Algae are photosynthetic microorganisms that are nutritionally Autotrophs. They are also called primary producers of organic matter because they convert sunlight into food.
Key Features
| Feature | Detail |
|---|---|
| Nutrition | Autotrophic (photosynthetic) |
| Growth requirement | Can grow only in Light |
| Cell type | Eukaryotic (except BGA which are prokaryotic) |
Blue-Green Algae (BGA) — A Special Case
BGA (Cyanobacteria) are unique because they are prokaryotic organisms that contain Chlorophyll — making them the only prokaryotes capable of oxygenic photosynthesis.
| Feature | Detail |
|---|---|
| Cell type | Prokaryotic (unlike other algae) |
| Blue pigment | Phycocyanin |
| Agricultural use | Biofertiliser in rice fields |
| N-fixation association | Anabaena (BGA) + Azolla (water fern) |
TIP
BGA in rice: The Anabaena-Azolla association fixes nitrogen in flooded rice fields. Anabaena is the BGA that lives inside the water fern Azolla. This symbiosis can fix 20–40 kg N/ha/season — a frequently asked fact.
Viruses — The Smallest Infectious Agents
Viruses are smaller than bacteria and are obligate intracellular parasites — they can only multiply inside living host cells. Outside a host, they exist as inert chemical particles.
Key Features
| Feature | Detail |
|---|---|
| Size | 0.06–0.14 microns (smallest of all microorganisms) |
| Parasitism | Obligate intracellular (must be inside a living cell) |
| Crystalline nature proved by | Stanley |
| Viruses that infect bacteria | Bacteriophages |
Virus Composition — A Critical Exam Table
Different types of viruses have different chemical compositions. This is one of the most frequently tested topics.
| Type | Composition | Memory Aid |
|---|---|---|
| Plant virus | RNA + Protein | Plant = RNA (both have "a" sound) |
| Animal virus | DNA + Protein | Animal = DNA |
| Lipo-virus | Nucleic acid + Protein + Lipid | Has a lipid envelope |
| Viroid | RNA only (naked nucleic acid, no protein coat) | Viroid = "void" of protein |
IMPORTANT
Plant viruses mostly contain RNA, while animal viruses (and bacteriophages) contain DNA. Viroids are even simpler — just naked RNA without any protein coat. Viroids cause diseases like potato spindle tuber and coconut cadang-cadang.
Mycoplasma (MLO / Phytoplasma) — The Wall-less Pathogens
Mycoplasma (Mycoplasma-Like Organisms) are unique pathogens that are larger than viruses but smaller than bacteria. They cause many important yellowing diseases in plants.
Key Features
| Feature | Detail |
|---|---|
| Size | 0.1–0.3 microns |
| Cell wall | Devoid of cell wall (no cell wall) |
| Shape | Highly Pleomorphic (variable shape) |
| Nucleic acid | Contains both DNA and RNA |
| Resistant to | Penicillin (targets cell wall — mycoplasma has none) |
| Sensitive to | Tetracycline (targets protein synthesis) |
WARNING
Penicillin vs Tetracycline — The No. 1 Mycoplasma Exam Question:
- Mycoplasma has no cell wall
- Penicillin works by disrupting cell wall synthesis — so it is ineffective against mycoplasma
- Tetracycline inhibits protein synthesis — so it works against mycoplasma
- This is tested in nearly every agriculture competitive exam.
Important Mycoplasma Diseases
| Crop | Disease |
|---|---|
| Brinjal | Little leaf |
| Sesamum | Phyllody |
| Sandal | Spike disease |
| Sugarcane | Grassy shoot |
| Potato | Purple top / Witches' broom |
Comparison Table — Fungi vs Algae vs Viruses vs Mycoplasma
| Feature | Fungi | Algae | Viruses | Mycoplasma |
|---|---|---|---|---|
| Cell type | Eukaryotic | Eukaryotic (BGA: Prokaryotic) | Acellular | Prokaryotic |
| Nutrition | Heterotrophic | Autotrophic | Obligate parasite | Parasitic |
| Size | 1.5–10 microns | 0.1+ microns | 0.06–0.14 microns | 0.1–0.3 microns |
| Cell wall | Present (chitin) | Present (cellulose) | Absent | Absent |
| Nucleic acid | DNA + RNA | DNA + RNA | DNA or RNA | DNA + RNA |
| Reproduction | Spores, budding | Binary fission, spores | Inside host only | Binary fission |
| Culture on artificial media | Yes (PDA) | Yes | No (obligate parasite) | Difficult |
| Antibiotic sensitivity | Fungicides | — | No antibiotic works | Tetracycline |
Oxidative vs Photophosphorylation
These two processes are the main ways living cells produce ATP (energy currency).
| Feature | Oxidative Phosphorylation | Photophosphorylation |
|---|---|---|
| Occurs in | Mitochondria | Chloroplasts |
| Energy source | Chemical energy (from food breakdown) | Light energy (from sun) |
| Process | Part of respiration | Part of photosynthesis |
| Common mechanism | Chemiosmotic coupling | Chemiosmotic coupling |
TIP
Both processes use chemiosmotic coupling (proton gradient across a membrane drives ATP synthase). The difference is the energy source: food (mitochondria) vs light (chloroplasts).
Summary Table — Key Facts at a Glance
| Fact | Answer |
|---|---|
| All fungi are | Heterotrophs |
| All algae are | Autotrophs |
| BGA cell type | Prokaryotic |
| Blue pigment in BGA | Phycocyanin |
| N-fixation in rice fields | Anabaena + Azolla |
| Smallest microorganism | Viruses (0.06–0.14 microns) |
| Plant virus composition | RNA + Protein |
| Animal virus composition | DNA + Protein |
| Viroid composition | RNA only |
| Crystalline nature of viruses | Stanley |
| Viruses infecting bacteria | Bacteriophages |
| Mycoplasma has no | Cell wall |
| Mycoplasma resistant to | Penicillin |
| Mycoplasma sensitive to | Tetracycline |
| Haustoria are | Nutrient-absorbing fungal structures in host |
| Best media for fungi | PDA (Potato Dextrose Agar) |
| Solidifying agent | Agar-agar |
| Yeast reproduction | Budding |
| Both phosphorylation types use | Chemiosmotic coupling |
Summary Cheat Sheet
| Fact | Answer |
|---|---|
| Fungi cell type | Eukaryotic |
| Fungi nutrition | Heterotrophic |
| Single fungal filament | Hypha |
| Group of hyphae | Mycelium |
| Root-like structures in Rhizopus | Rhizoids |
| Nutrient-absorbing fungal structures | Haustoria |
| Best media for fungi | PDA (Potato Dextrose Agar) |
| Solidifying agent in media | Agar-agar |
| Yeast reproduction | Budding |
| All algae are | Autotrophs |
| BGA cell type | Prokaryotic |
| Blue pigment in BGA | Phycocyanin |
| N-fixation in rice fields | Anabaena (BGA) + Azolla (water fern) |
| Smallest microorganism | Viruses (0.06–0.14 microns) |
| Viruses are obligate | Intracellular parasites |
| Plant virus composition | RNA + Protein |
| Animal virus composition | DNA + Protein |
| Viroid composition | RNA only (no protein coat) |
| Crystalline nature of viruses proved by | Stanley |
| Viruses infecting bacteria | Bacteriophages |
| Mycoplasma has no | Cell wall |
| Mycoplasma resistant to | Penicillin |
| Mycoplasma sensitive to | Tetracycline |
| Mycoplasma size | 0.1–0.3 microns |
| Little leaf disease (brinjal) caused by | Mycoplasma (Phytoplasma) |
| Both phosphorylation types use | Chemiosmotic coupling |
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