Resistance news

Resistance News

To read more about the Torrado Virus, please click here for the complete article in a pdf file.

Capsicum-Tomato Spotted Wilt Virus (TSWV) 

Resistance to Tomato Spotted Wilt Virus (TSWV) is governed by a single gene, based on hypersensitivity, which causes isolation of virus particles immediate after infection. In such cases varieties usually have either a high resistance or they are susceptible.

Reports from Spain (P. Magaria, Plant Pathology 53) proved already the occurrence of a new TSWV strain that is able to overcome the resistance gene. This new strain is now widespread throughout Spain and means that the resistance to TSWV has limited effect in Spain.

For this reason we claim only Intermediate Resistance against TSWV based on the present resistance genes. However, a variety with the old TSWV resistance gene is usually still more effective against TSWV than a variety without. There are many areas in the world for which this gene is still effective.

The Resistance Codes for Potato Virus Y are different to the Scientific coding. Scientific literature is complicated in that it takes into account the background of the pathogen. The use of this coding makes communication to the grower complicated, and so this has been simplified. The table underneath is an explanation for persons wishing to bridge the gap between the Scientific and Seed World.

example: PVY: 0-2  if resistance is present against strains 0, 0.1 and 1.2.

Following older conventions, the Tobamovirus group have a separate Resistance Code based on the ability of a gene to resist virus strains of the Tobamovirus group.  In this case the single code gives Resistance Claim against a number of viruses.

In some countries the name of the gene is still used as an abbreviation of the resistance code.

Cucumbers

In cucumbers it has long been common to claim Powdery Mildew as a "group" resistance and not have specific claims against the different fungi in this "group". In the codes of De Ruiter Seeds this is still the case today.

The basis of pathogen resistance concerns a highly specific interaction between the pathogen and the resistant plant. Specific proteins, released by the pathogen in order to infect the plant, are recognised by the resistant plant after which the resistant reaction is switched on. The most commonly used resistance mechanism, especially for viruses and nematodes, is based on hypersensitivity which means that a plant cell will die immediate after infection. In this way spread of the pathogen is restricted, as the pathogen requires living tissue.

Mutant forms of pathogens, lacking the ability of releasing one or more proteins, do exist in nature. In the case this mutant form doesn't release the proteins which are recognised by the resistant plant this mutant is able to infect the resistant plant. This mutant form is then called a resistance-breaking strain.

In general these mutants show a decreased fitness and will disappear. However in the case of a resistant crop the mutant doesn't have to compete with non-mutant forms and will be able to establish on the crop.
For a large number of resistances there are isolates which have been identified which break through this resistance, e.g. for ToMV resistance, TSWV resistance or resistance to M. incognita in tomatoes. The speed if which a resistance-breaking isolate will spread is highly dependent on the mobility of the pathogen.

Mutants are not the only reason why a resistance gene is less effective. Especially hot and stressful circumstances can be the cause that a resistance gene is switched off temporarily. Also under extremely high infection pressure a plant may also lose its ability to resist the disease.

New strains may appear, not all analytical methods give good results

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. Recognising 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.

For a large number of resistances there are isolates which have been identified which break through this resistance, e.g. for ToMV resistance, TSWV resistance or resistance to M. incognita in tomatoes. In most cases these are isolated occurences which have no, or limited impact on commercial crops.

In those cases that the breakage of resistance does occur and that this poses a concern for commercial growing we will try to adjust our coding as quickly as possible.

In general, resistances work less effectively above 28C. This might mean that under especially hot and stressful circumstances the plants may show symptoms. Also under extremely high infection pressure a plant may also loose its ability to resist the disease.