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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, June 2010).  

1. Preamble

Differing degrees of specificity exist in the relations between plants and pests or pathogens. Identification of such specificity generally requires the use of highly elaborate analytical methods. Recognizing whether a plant is subject to a pest or pathogen or not may depend on the analytical method employed. It is important, in general, to stress that the specificity of pests or pathogens may vary over time and space, depends on environmental factors, and that new pest biotypes or new pathogen races capable of overcoming resistance may emerge.  

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).

If in a resistance code of a certain variety reference is made to certain strains for which the resistance is claimed this means that no resistance is claimed to other strains of the same pathogen.

If in a resistance code no reference is made to strains of the pathogen for which the resistance is claimed this means that resistance is claimed only to certain non-specified strains of the pathogen and herewith disclaiming any guarantee that the variety will not be infected by the said pathogen.