Introduction

The relationship between a plant and a pest is very complex. The terms that describe the reaction of a plant variety to a pest are determined by tests with known and characterized biotypes, pathotypes, races or strains of the pest in question.

In practice however, the ability of a pest to cause disease in or damage to a plant depends on environmental conditions, the properties of the organism itself and the capacity of the plant to defend itself. Plant varieties within a species can differ in their ability to defend themselves. Under different conditions, such as age of the plant, pest pressure and degree of virulence or adverse environmental conditions, the interaction between the same plant variety and pest may have different outcomes.

Pests are known to develop and form new biotypes, pathotypes, races or strains that can cause disease in or damage to plants that remain unaffected by the original form of the pest.

Definitions

• Susceptibility: the inability of a plant variety to restrict the growth and/or development of a specified pest.
• Immunity: when a plant is not subject to attack or infection by a specified pest.
• Resistance: the ability of a plant variety to restrict the growth and/or development of a specified pest and/or the damage it causes when compared to susceptible plant varieties under similar environmental conditions and pest pressure. Resistant varieties may exhibit some disease symptoms or damage under heavy pest pressure. Two levels of resistance are defined.