Plant Pathology: Diseases of Field and Horticultural Crops
Plant Pathology: Diseases of Field and Horticultural Crops
This domain tests the disease triangle, symptom versus sign, major pathogen groups and field diagnosis. Questions often ask you to separate infectious disease from nutrient or water stress.
Core rules
- The disease triangle requires a susceptible host, a virulent pathogen and a favourable environment; time is sometimes added.
- Fungi cause rusts, blights, mildews and wilts. Bacteria cause cankers, blights and vascular wilts. Viruses cause mosaic, leaf curl and stunting. Nematodes cause root knots and patches.
- Symptoms are the host's visible response, such as chlorosis or wilting. Signs are pathogen structures, such as rust pustules, mycelium or bacterial ooze.
- Diagnosis uses field pattern, microscopy, isolation and Koch's postulates for proof of pathogenicity.
- Management combines resistant varieties, clean seed, crop rotation, sanitation, biological agents and labelled fungicides.
Worked example
Wheat leaves show yellow-orange pustules in stripes. Reasoning: stripes and powdery pustules are signs of yellow rust, favoured by cool, moist weather. A nutrient deficiency would not produce raised pustules. Action: note the field pattern, choose a resistant variety next season and apply a labelled fungicide at first sign if pressure is high.
Traps
Diagnosing from one leaf, calling a virus a fungus, using a bactericide against a virus, and ignoring varietal resistance are common errors.
Key distinction
A leaf with mosaic pattern and no fungal growth is likely viral; a leaf with ooze in water is likely bacterial.