Identifying Common Plant Diseases – Study Notes
1. Why Disease Identification Matters
- Early detection → timely interventions, reduced crop loss.
- Accurate diagnosis → appropriate control measures (cultural, chemical, biological).
- Economic impact → protects yield, marketability, and farmer income.
2. General Diagnostic Approach
| Step |
Action |
Key Tools / Techniques |
| 1. Observation |
Scan the whole plant and surrounding area. |
Hand lens (10×–30×), smartphone macro camera. |
| 2. Symptom Recording |
Note type, location, and pattern of symptoms. |
Field notebook, photo log, GPS tag. |
| 3. Sample Collection |
Take representative plant parts (leaf, stem, root, fruit). |
Clean scissors, labeled bags, cool storage. |
| 4. Laboratory Tests (if needed) |
Microscopy, culturing, molecular assays. |
Light microscope, PDA medium, PCR kits. |
| 5. Compare |
Match findings with reference guides or databases. |
Extension bulletins, online portals (e.g., CABI, PlantVillage). |
| 6. Confirm & Report |
Validate diagnosis, document for future reference. |
Diagnostic report, disease code (e.g., EPPO, USDA). |
3. Major Categories of Plant Diseases
| Category |
Causative Agents |
Typical Symptoms |
Example Diseases |
| Fungal |
Botrytis, Fusarium, Puccinia, Rhizoctonia |
Spots, blights, wilts, rust pustules, powdery mildew |
Gray mold (Botrytis cinerea), Wheat rust (Puccinia spp.) |
| Bacterial |
Xanthomonas, Pseudomonas, Erwinia |
Water‑soaked lesions, ooze, wilting, cankers |
Bacterial leaf spot (Xanthomonas spp.), Fire blight (Erwinia amylovora) |
| Viral |
RNA/DNA viruses (e.g., TMV, CMV) |
Mosaic, chlorosis, stunting, leaf curling |
Tobacco mosaic virus, Cucumber mosaic virus |
| Nematode |
Root‑knot (Meloidogyne), cyst (Heterodera) |
Galls, root knots, stunted growth, yellowing |
Root‑knot nematode disease |
| Phytoplasma / Mycoplasma |
Wall‑less bacteria |
Yellowing, witches’ broom, phyllody |
Aster yellows, Peanut stunt disease |
| Abiotic Mimics |
Nutrient deficiency, toxicity, environmental stress |
Similar visual signs but no pathogen |
Iron deficiency (chlorosis) vs. fungal leaf spot |
4. Recognizing Common Diseases (Crop‑Specific Highlights)
4.1. Cereal Crops (Wheat, Barley, Maize)
| Disease |
Pathogen |
Diagnostic Clues |
| Septoria leaf blotch |
Zymoseptoria tritici (fungus) |
Small, angular brown lesions with pale halos; appears after heading. |
| Powdery mildew |
Blumeria graminis (fungus) |
White, powdery colonies on leaf surfaces; thrives in high humidity, moderate temps. |
| Northern corn leaf blight |
Exserohilum turcicum (fungus) |
Long, cigar‑shaped lesions, gray‑green to tan, often on |