Resistance explained

Disease resistance is one of the major goals when breeding new varieties. It plays a key role in vegetable crop production and integrated pest management practices.

Plant diseases and resistance

The ability of a pest to cause disease in a plant depends on cropping environmental conditions, the properties of the organism itself and the capacity of the plant to defend itself. Even within the same crop, varieties can differ in their ability to defend themselves. Under different climatic conditions and cropping techniques, interaction between the same plant and pest may have different outcomes.

Pathogens are known to develop and form new biotypes, also known as pathotypes, which are races or strains that can cause damage to plants which remain unaffected by the original form of the pest.

Resistance level definition:

  1. Susceptibility: is the inability of a variety to react and limit the growth and/or development of a specified pest.
  2. Resistance: is the ability of a variety to limit the growth and development of a specified pathogen, or the damage they cause, when compared with susceptible plant varieties under similar environmental conditions and pathogen pressure.

Resistance: is the ability of a variety to limit the growth and development of a specified pathogen, or the damage they cause, when compared with susceptible plant varieties under similar environmental conditions and pathogen pressure.

  1. High resistance (HR): varieties that highly restrict the growth and development of the specified pest and the damage it causes, under normal pest pressure, when compared to susceptible varieties. These plant varieties may, however, exhibit some symptoms or damage under heavy pest pressure.
  2. Intermediate resistance (IR): varieties that restrict the growth and development of the specified pest and the damage it causes but may exhibit a greater range of symptoms or damage compared to highly resistant varieties. Intermediate resistant varieties will still show less severe symptoms or damage than susceptible plant varieties when grown under similar environmental conditions and pest pressure.

Varieties claiming the same level of resistance against a specific pest may exhibit a different resistance response due to a variety’s different genetic background.

It is to be noted that, if resistance is claimed in a variety, it is limited to the specified biotypes, pathotypes, races or strains of the pest in question.

If no biotypes, pathotypes, races or strains are specified in the resistance claim for a variety, it is because no generally accepted classification exists for the cited pest by biotype, pathotype, race or strain. In this case, resistance is only claimed against certain otherwise unspecified types of that pathogen. New biotypes, pathotypes, races or strains that may emerge are not covered by the original resistance claim.