Resistance news
Resistance News
On this page the Plant pathologists at De Ruiter Seeds will regularly place articles concerning resistance and disease.
Click here to download all information on this page in a pdf-file
- General
- Aubergine
- Capsicum (sweet and hot pepper)
| Glossary of terms and abbreviations used in text | 21/04/05 | pdf file |
| Occurence of new strains | 27/05/05 | |
| New strains may appear, not all analytical methods give good results | 21/04/05 |
| Capsicum - Tomato Spotted Wilt Virus (TSWV) | 21/04/05 |
Capsicum - Tobamovirus Group (Tm) - Tobacco Mosaic Virus (TMV) Tomato Mosaic Virus (ToMV) and Pepper Mild Mottle Virus (PMMV) | 21/04/05 |
| Potato Virus Y Resistance Coding | 21/04/05 |
- Cucumber
- Melon
- Tomato
Tomato Torrado Virus, 21 March 2007
To read more about the Torrado Virus, please click here for the complete article in a pdf file.
Verticillium Wilt Incited by Verticillium dahliae in Eggplant Grafted on Solanum torvum in Italy - August 2005
Eggplant cultivars grafted on rootstocks resistant to root-knot nematodes (_Meloidogyne_ spp.) are increasingly grown in Italy to reduce nematode infection.
During the winter of 2003-2004, eggplants (cv Black Bell and Mirabell) grafted on the nematode-resistant rootstock _Solanum torvum_ were observed with symptoms of a wilt disease in several greenhouses in Sicily (southern Italy). The vascular tissue in stems of affected plants appeared brown. These plants were stunted and developed yellow leaves with brown or black streaks in the vascular tissue.
The wilt appeared in several greenhouses at a very low incidence (0.01 to 0.05 percent). Later, during the fall of 2004, disease incidence was approximately 10 times greater in the same greenhouses on new crops.
_Verticillium dahliae_ was consistently and readily isolated from symptomatic vascular tissue of the rootstock (_S. torvum_) and the scion (cv Black bell) when cultured on potato dextrose agar (PDA) (1).
To read more, please click here for the complete article in a pdf file.
Occurrence of new strains - 27 May 2005
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 - 21 April 2005
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.
Capsicum-Tomato Spotted Wilt Virus (TSWV) - 21 April 2005
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. Hopefully we will find a new source of resistance and be able to claim a High Resistance again in the not too distant future.
Capsicum - Tobamovirus Group (Tm) - Tobacco Mosaic Virus (TMV), Tomato Mosaic Virus (ToMV) and Pepper Mild Mottle Virus (PMMoV) - 21 April 2005
About 120 years ago Tobacco Mosaic Virus, member of The Tobamovirus Group, was the first virus ever discovered. This group of viruses proved to be present worldwide and turned out to be the cause of a lot of formerly unexplained diseases.
The Tobamovirus group proved to be highly variable and the nomenclature is still under debate in the scientific world.
For Capsicum the UPOV-guidelines are used as the official nomenclature.
| Tobamovirus pathotype | Pepper Virus | |
| P0 | TMV ToMV TMGMV | Tobacco Mosaic Virus, Tomato Mosaic Tobacco Mild Green Mosaic Virus Tobacco Mild Green Mosaic Virus |
| P1 | ToMV TMGMV | |
| P1.2 | ToMV PMMoV | |
| P1.2.3 | PMMoV | Pepper Mild Mottle Virus |
Instead of writing down the long list of individual viruses the virus codes of the Tobamovirus group have been listed under a single code with various numbers denoted the level of resistance:
| New codes | Gene present | Resistance to |
| Tm:0 | L1 | TMV/ToMV/TMGMV |
| Tm:0,1 | L2 | TMV/ToMV/TMGMV |
| Tm:0,1,2 | L3 | TMV/ToMV/TMGMV/PMMoV:1.2 |
| Tm:0,1,2,3 = Tm:0-3 | L4 | TMV/ToMV/TMGMV/PMMoV:1.2, 1.2.3 |
The name of the gene is often used in Hispanic countries as an abbreviation of the resistance code: L4 is now Tm:0-3.
Potato Virus Y Resistance Coding - 21 April 2005
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.
| New code | Scientific Coding |
| PVY:0 | Pvy 0 |
| PVY:1 | Pvy 0.1 |
| PVY:2 | Pvy 1.2 |
| PVY:3 | Pvy 1.3 |
example: PVY: 0-2 if resistance is present against strains 0, 0.1 and 1.2.
