Plant Resistance to Pests and Diseases

Plant Resistance to Pests and Diseases

Variety Resistance

Various living organisms can cause damage to crops, such as pathogens that cause diseases, pests or weeds. This leads to:

  • yield loss from disease or weed competition,
  • loss of visual or technological qualities of the product,
  • conservation problems,
  • toxicity risks (mycotoxin, alkaloids (ergot of rye, seeds of numerous weeds such as Datura stramonium, groundsel),
  • harvesting difficulties (machine blocking, lodging),
  • and in the case of weeds : increasing the seed stock which may be detrimental to future crops.
Symptoms of Fusarium oxysporum f. sp. lactucae on susceptible varieties (left) and resistant varieties (right) of lettuce

Chemical control (synthetic phytosanitary products, copper, …) has been an effective way to fight against pests, but it is reaching its limits.  Active ingredients (selection of resistant pests) are losing effectiveness, or have reached dead ends for certain pathogens, such as soil pathogens (aphanomyces for peas, beet rhizomania, wheat mosaics). For these pathogens, varietal resistance is the only effective control method. Chemical control has previously been used, because its effectiveness has an impact on all living components. It is therefore essential to limit the use of chemical control to preserve biodiversity and the sustainability of our environment.

To reduce pest damage, prophylactic measures are the first step. These measures are intended to limit the development conditions of these pests: succession of crops, diversification of species and planting period, tillage, management of plots, plants used in association and use of varieties providing a certain level of resistance, etc. It is recommended to use a combination of these measures.

Choosing resistant varieties or varieties offering a good level of pest resistance is another option which is easy to implement and can be widely used by farmers to reduce the use of plant protection products. Varietal resistance is an important focus of breeders’ work. Significant genetic progress has been made in recent years: tolerance to JNO in barley, resistance to Phoma rapeseed … although sometimes progress is a race against pathogens such Bremia in lettuce and yellow rust in cereals.

Incentives for the use of pest- and disease-resistant varieties

In order to foster the dissemination of plant protection product saving practices by plant protection product distributors, a plant protection product saving certificate scheme (CEPP) has been implemented as part of the Ecophyto plan. The CEPP scheme proposes standardised actions to save plant protection products. Several actions concern the use of varieties providing resistance to pests or lodging. There are also actions on varieties used in association. The invoice of the sale of seeds is used as proof of implementation of the action.

Actions “to reduce the number of treatments using fairly resistant or unsusceptible varieties” are developed with agricultural technical institutes that contribute their expertise to translate resistance levels to diseases and pests into potential savings of phytopharmaceutical products. The CTPS proposes lists of eligible varieties based on resistance scores given at the time of registration, or based on additional post-registration information provided mainly by agricultural technical institutes. These lists are updated every year to incorporate new products, remove varieties that are no longer on the market, and consider possible changes in the behaviour of varieties. A simple list of eligible varieties is not enough to reduce the Indicator of Frequency of Treatment (IFT); it is essential to have more detailed information on the level of resistance of varieties for each pathogen, in order to plant the most resistant varieties in light of risks in the region or plot.

CEPP Actions related to varieties and seed purchase on 1 June 2019 (source: EcophytoPIC portal):

Species Action no. Name of CEPP action Further details on level of variety resistance

Actions: reduce the number of treatments by using varieties that are partially resistant or with low susceptibility

Reduce the number of fungicide applications for potato varieties with low susceptibility to late blight.
Soft wheat
Reduce the number of treatments for soft wheat varieties which are partially resistant to pests and lodging.
Sugar beet
Reduce the number of treatments for various diseases using partially resistant sugar beet varieties.
Fight turnip yellowing virus on rapeseed by choosing a partially resistant variety

Action: Varietal blend

Soft wheat
Reduce treatments with mixtures of partially resistant soft wheat varieties.

Actions: Association of companion plantes

Replace autumn herbicide and insecticide treatments with a combination of winter canola and legumes.
Avoid insecticide treatment against meligèthes by combining a very early-flowering rapeseed variety with the main variety.
Reducing the use of plant protection products by combining a canopy of frost-susceptible and frost-resistant legumes between rows of rapeseed.

Resistance to pests and pathogens for registration in the Catalogue

Resistance to pests and pathogens has always been a component of registration in the Catalogue, especially disease resistance for which there is a fairly wide genetic diversity. The consideration of resistance differs from species to species, depending on the impact of the diseases on the culture and the quality of the products, and whether there is a test for Agronomic Technological and Environmental Value.

For agricultural plants (field and forage crops), the variety’s yield and use value of the product are evaluated, with a description of its behaviour with regards to the main diseases . This is part of the registration decision rules for many species (cereals, oilseed rape, beetroot …). The registration of varieties with good behaviour is favoured, and very susceptible varieties are penalised or refused (these rules are specified in the technical regulations for registration). Yield tests also include disease behaviour: for cereals, yield information comes from both protected and unprotected disease tests; for sugar beet fungicidal protection is tested in comparison with varieties that are not susceptible to disease. As soon as the varieties are registered, all results are made available (documents are accessible by searching the name of the variety in the Catalogue).

Variety resistance evaluation is mainly carried out in the field in multi-location trials, or with pathogens and pest resistance tests using artificial inoculation of predominant strains or breeds in France. In these tests, reference varieties covering a scale of susceptibility to the studied disease can describe the behaviour of varieties being studied. Biotests in the laboratory are also used.

Variety resistance to yellow rust

The table opposite lists the pests characterised during registration tests, mainly for VCU but also for DUS description.

  • 41 field-specific resistance tests,
  • 27 laboratory tests,
  • other studies are carried out by natural contamination in “yield” tests and depend on the pest or pathogen pressure for that year.

For vegetable plants, disease resistance is part of the phenotypic characteristics used to describe varieties (Distinctness Homogeneity and Stability studies). Current resistance is mostly qualitative, of the yes / no type, with a description for each pathogen race. This type of resistance, known as monogenic resistance, can be les durable. Currently, quantitative resistances are also used.
For vegetable plants, evaluation mainly consists of bio-tests, carried out in a controlled environment (greenhouse or climatic module), on young plants, with a response time after rapid inoculation (two to four weeks). Some types of resistance are evaluated in the field in contaminated conditions, such as Alternaria carrot.

For vegetable species: 50 host / pest pairs (viruses, bacteria, fungi, nematodes, aphids) and nearly 100 host / race / pest pairs (DUS) and thousands of official tests are carried out to evaluate the reference collection and new varieties.
GEVES has created a network with its research and seed partners to maintain and share these reference varieties (susceptible, resistant, intermediate resistant and differential host) and reference strains for resistance tests. The MATREF network provides this reference material.

Symptoms of TSWV (Tomato spotted wilt virus) on resistant varieties (left) and susceptible varieties (right) of pepper
Behaviour of 2 varieties of soft wheat (one on the left and the other on the right) in relation to weeds, in this case ‘wild mustard’

Weed pests

For the moment, variety behaviour with regard to weeds is not closely examined in registration testing. There is no “resistant” variety to weeds (there are varieties that are resistant to orobanche, a parasitic plant), but there are more competitive varieties which are taller, with more cover, wider leaves, more ramifications or tillers, with stronger starting vigour, etc.

Behaviour with regard to weeds is very important in organic farming, as weed control is one of the main limiting factors in organic agriculture. Organic farming looks for varieties that compete with weeds, as well as varieties that facilitate the passage of mechanical weeding tools.

In current registration tests, covering power is described for protein peas and for varieties of soft wheat classed as suitable for organic farming. Certain morphological traits used to describe varieties for DUS can be used to describing the architecture of the plant. For example, a soft wheat leaf can be upright or spreading. GEVES is working to better disseminate this information.

Expertise and research at GEVES for the seed and variety sector

Methodological and research work is continuously conducted by GEVES in national and international partnerships to:

  • develop and improve methods for assessing variety resistance to pests and pathogens,
  • study new pathosystems and develop methods,
  • participate in updating knowledge on strains and races, to combat resistance breakage,
  • harmonise methods with counterpart examination offices and breeders to achieve consistency of results and define reference material.

These research programs are presented on the research and development pages of this website.

GEVES’s expertise in evaluating resistance is carried out in support of the seed sector: for registration in the catalogue, national or EU plant variety protection, research programs, and also for private services.