Inoculum Pressure

Inoculum pressure & carry-over

Disease risk is not determined by whether a pathogen is present, but by how much viable pathogen is present.

This is known as inoculum pressure.

Low inoculum pressure may result in little or no disease even under favourable conditions.
High inoculum pressure can overwhelm plant defences during very short risk windows.

What is inoculum?

Inoculum refers to any viable form of a pathogen capable of causing infection, including:

• Spores
• Mycelium
• Sclerotia
• Bacterial cells
• Viral particles
• Resting or survival structures

The form varies by pathogen, but the principle is the same.

Inoculum pressure vs disease outcome

Inoculum pressure influences:

• Likelihood of infection
• Speed of disease development
• Severity of symptoms
• Effectiveness of preventative measures

General pattern:

• Low pressure → disease may fail to establish
• Moderate pressure → disease depends strongly on environment
• High pressure → disease likely whenever thresholds are met

This explains why the same conditions produce different outcomes in different sites.

Carry-over: where inoculum comes from

Inoculum rarely appears spontaneously. It is carried over from:

• Crop debris
• Old leaves and prunings
• Substrates and growing media
• Benches, floors, and structures
• Tools, hands, clothing
• Irrigation systems
• Volunteer plants and weeds

Protected systems often accumulate inoculum year after year unless actively disrupted.

Primary vs secondary inoculum

Primary inoculum

• Initiates the first infections
• Often originates from carry-over
• Determines when disease starts

Secondary inoculum

• Produced after infection is established
• Drives rapid spread within the crop
• Is harder to control

Most epidemics