How do environmental factors affect resistance levels?

The environment is the critical third side of the disease triangle, along with a susceptible host and a virulent pathogen. Unfavourable environmental conditions can weaken or even break down a plant’s genetic resistance.

Temperature

• Pathogen: Each pathogen has an optimum temperature range for growth, sporulation, and infection.
• Plant: Elevated temperatures are a major stress that can suppress immunity. Several resistance (R) genes function poorly at high temperatures. For example, a wheat line resistant to stem rust at 20 °C (68 °F) may become susceptible at 30 °C (86 °F).

Humidity and Moisture

• Leaf wetness: Many fungal and bacterial pathogens require high humidity and a film of water on leaf surfaces to germinate, penetrate, and spread.
• Disease favouring conditions: Prolonged dew, frequent rains, overhead irrigation, or poor air circulation extend wetness duration and promote infection cycles.

Light

• Photosynthetic balance: Low light reduces carbohydrate availability and overall vigour, leaving fewer resources for defence responses (e.g., synthesis of defence compounds, wound sealing).
• Canopy effects: Dense shading and long periods without direct light can increase humidity within the canopy, indirectly favouring pathogens.

Soil and Nutrition

• Nitrogen: Excess N often leads to soft, succulent growth that is more easily colonised by pathogens and pests.
• Potassium and Calcium: Adequate K and Ca strengthen cell walls and support signalling pathways essential for effective immune responses.
• Soil health: Good structure, drainage, and balanced microbiota reduce stress and suppress soil-borne pathogens.

Even genetically resistant plants need a supportive environment to defend themselves. Heat, prolonged wetness, low light, or imbalanced nutrition can tip the balance towards the pathogen and overwhelm resistance.