Encyclopaedia

Environmental thresholds for infection

Most plant pathogens do not infect continuously.
They require specific environmental conditions, often for a minimum duration, before infection can occur.

These conditions are known as infection thresholds.

Understanding thresholds shifts disease management from reaction to risk prediction.

Infection is a process, not a moment

For most diseases, infection involves several steps:

Spore or propagule arrives on plant tissue
Moisture allows germination or activation
Suitable temperature enables growth
Pathogen penetrates plant tissue
Infection becomes established (often invisibly)

If any step is interrupted, infection fails.

Leaf wetness duration (often the limiting factor)

For many foliar diseases, leaf wetness is more important than rainfall.

Sources of leaf wetness include: - Dew - Fog - Condensation in glasshouses - Overhead irrigation - Poor airflow within dense canopies

Typical patterns: - Short wetness periods → no infection - Extended wetness (hours) → infection possible - Repeated wetness events → high risk

Leaf wetness is strongly influenced by microclimate, not regional weather.

Relative humidity thresholds

Many pathogens do not require free water, but do require very high humidity.

Common features: - Infection or sporulation above ~90–95% RH - Sporulation increases rapidly once a threshold is crossed - Small RH increases near saturation can have large effects

This explains why disease often accelerates: - Overnight - During still, humid periods - After ventilation is reduced to conserve heat

Temperature windows

Pathogens typically have: - A minimum temperature for activity - An optimum temperature range - A maximum temperature above which activity slows or stops

Important points: - Infection may occur at temperatures where growth is slow - Sporulation may require different temperatures than infection - Short periods at optimal temperature can be sufficient if moisture is present

Temperature interacts strongly with leaf wetness duration — cooler conditions usually require longer wetness periods.

Duration matters more than peaks

Disease risk is driven more by: - How long conditions persist than by: - How extreme conditions become

Examples: - 10 hours of moderate wetness can be riskier than 1 hour of heavy rain - Sustained 92% RH can be riskier than brief condensation events - Moderate temperatures over long periods can drive infection

This is why disease models often use time-above-threshold, not daily averages.

Why infection thresholds are often misunderstood

Common mistakes: - Using daily weather summaries - Reacting to visible symptoms - Focusing on rainfall instead of leaf wetness - Ignoring night-time conditions - Treating all diseases as if they behave the same

Thresholds are pathogen-specific, but the principles are consistent.

Practical implications for management

You reduce infection risk by interrupting thresholds:

Reduce leaf wetness duration
Increase air movement
Avoid late-day irrigation
Manage canopy density
Use ventilation strategically
Avoid creating humid “dead zones”

Disease prevention is often about minutes and hours, not days.

Why environment-led control outperforms calendar control

Calendar-based control assumes: - Disease pressure is constant - Infection timing is predictable

In reality: - Infection windows are episodic - Risk varies daily and spatially - Missed windows cannot be “fixed” later

Environmental awareness allows: - Better timing - Fewer interventions - More reliable outcomes

Key takeaways

Infection requires threshold conditions
Duration matters as much as intensity
Leaf wetness and humidity are critical drivers
Temperature defines when thresholds are active
Managing environment breaks the disease triangle