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

Plant diseases and resistance

The relationship between a plant and a pest1 is very complex. The ability of a pest to cause disease in a plant depends on environmental conditions, the properties of the organism and the capacity of the plant to defend itself. Varieties within a plant species can differ in their ability to defend themselves. Under different climatic conditions the interaction between the same plant and pest may have different outcomes.

Resistance is the ability of a plant variety to restrict the growth and development of a specified pest or the damage they cause when compared to susceptible plant varieties under similar environmental conditions and pathogen pressure. Resistant varieties may exhibit some disease symptoms or damage under heavy pest pressure.  Pests are known to adapt genotypically and form sub-species, formae speciales, biotypes, pathotypes or races that can cause disease in plants previously unaffected by, or resistant to the original form of the pathogen.

Resistance genes may be effective against all or some sub-species, formae speciales, biotypes, pathotypes or races of a pest, and the emergence of new sub-species, formae speciales, biotypes, pathotypes or races is not uncommon. To identify and distinguish different isolates within a species, sub-species, forma specialis, biotype, pathotype or race, plant pathologists use different methods including ‘differential hosts’ – see Differential Hosts.   After characterization, isolates that do not match an already described sub-species, forma specialis, biotype, pathotype or race of the pest, are designated as a new sub-species, forma specialis, biotype, pathotype or race after verifying it has substantially established in nature.  Verification of establishment occurs through documentation of repeated occurrence in multiple locations over several seasons, adopted by the ISF Disease Resistance Terminology Working Group and are reported in a peer reviewed scientific publication.

Consistent terminology

To promote consistency in the terminology used to describe the reaction of a plant to a pest, the ISF Vegetable and Ornamental Crops Section has adopted a set of agreed definitions. The paper defines two levels of resistance: Company claims on the level of resistance are based primarily on tests carried out with well-characterized isolates of a pest under controlled environmental conditions. In some cases, claims of resistance are based on field tests carried out under carefully monitored natural conditions. A collected isolate reflects a population of microorganisms, viruses(/viroids), phytoplasms, nematodes or insects that represent commercially important field pests.  The level of pathogenicity of an isolate can be further characterized using differentiating hosts and represents a particular sub-species, forma specialis, biotype, pathotype or race biotype, of the targeted pest.  All collected isolates of the targeted pest should be similar in morphology and pathogenicity to other isolates of the same sub-species, forma specialis, biotype, pathotype or race. The resistance terminology paper is also available in the following languages: Chinese, Dutch, FrenchGerman, Italian, JapaneseKorean, Spanish, Turkish

Seed companies are also encouraged to use uniform codes for pests affecting vegetable and cereal crops in catalogues and other communication with customers – see Pest Codes. ISF recommends using the listed pest codes for viruses, bacteria, fungi, insects and nematodes.

1 The FAO defines a pest as: “Any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant products.” Pathogens (microorganisms such as bacteria, viruses and fungi that cause a disease) are therefore included in the term ‘pest’.