Resistance terminology

What does it mean?

Below you will find the definitions of the terms describing the reaction of plants to pests or pathogens and to abiotic stresses for the vegetable seed industry (based upon an ISF approved version, May 2017).

1. Preamble

The relationship between a plant and a pest or pathogen is very complex. The terms that describe the reaction of a plant variety to a pest or pathogen are determined by tests with known and characterized biotypes, pathotypes, races or strains of the pest or pathogen in question.
In practice however, the ability of a pest or pathogen 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 or pathogen pressure and degree of virulence or adverse environmental conditions, the interaction between the same plant variety and pest or pathogen may have different outcomes.
Pests and pathogens 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 or pathogen.

2. Definitions

• Immunity: not subject to attack or infection by a specified pest or pathogen.
• Resistance: the ability of a plant variety to restrict the growth and development of a specified pest or pathogen and/or the damage they cause when compared to susceptible plant varieties under similar environmental conditions and pest or pathogen pressure. Resistant varieties may exhibit some disease symptoms or damage under heavy pest or pathogen pressure. 
  •     Two levels of resistance are defined:  
    • High resistance (HR): plant varieties that highly restrict the growth and development of the specified pest or pathogen under normal pest or pathogen pressure when compared to susceptible varieties. These plant varieties may, however, exhibit some symptoms or damage under heavy pest or pathogen pressure.
    • Intermediate resistance (IR): plant varieties that restrict the growth and development of the specified pest or pathogen, but may exhibit a greater range of symptoms or damage compared to highly resistant varieties. Intermediate resistant plant varieties will still show less severe symptoms or damage than susceptible plant varieties when grown under similar environmental conditions and/or pest or pathogen pressure.
• Susceptibility: the inability of a plant variety to restrict the growth and development of a specified pest or pathogen.
• Tolerance: the ability of a plant variety to endure abiotic stress without serious consequences for growth, appearance and yield. Vegetable companies will continue to use tolerance for abiotic stress.

3. Coding

Resistances in varieties of our crops will be coded (please, see our coding list for explanation), unless indicated otherwise.

To separate pest organisms, species codes and strain codes, the following separators will be used:

• In case a variety is resistant to more than one pathogen, the individual resistance codes will be separated by the symbol “/ ” (solidus).
• Species codes will be separated from the strain code(s) by the symbol “:” (colon).
• Strain codes will be separated by the symbol “,” (comma).
• In case that there are more than two strain codes in a logical order, the notation will be abbreviated in a from-to mode by the symbol “-” (hyphen-minus).

Varieties claiming the same level of resistance against a specific pest or pathogen may exhibit a different resistance response due to a different genetic makeup of a variety.
It is to be noted that if a resistance is claimed in a plant variety it is limited to the specified biotypes, pathotypes, races or strains of the pest or pathogen.
If no biotypes, pathotypes, races or strains are specified in the resistance claim for the variety, it is because no generally accepted classification of the cited pest by biotype, pathotype, race or strain exists. In this case resistance is only claimed against certain not further specified isolates of that pathogen. New biotypes, pathotypes, races or strains that may emerge are not covered by the original resistance claim.