TopRes® clubroot resistance in Brassica's
Clubroot is a widespread disease that causes serious problems in many brassica growing areas. The disease is caused by Plasmodiophora brassicae , a one-cellular organism. Symptoms of the disease include root malformations with hard swellings (clubs), that eventually rot. The disease also causes stunting through reduced growth, and wilting of leaves is observed under water stress.
Clubroot is considered to be the most important disease in brassica crops. This soil-borne disease is present in many growing areas, and can locally prevent brassica culturing. Chemical control of the disease is not effective. Therefore, good genetic resistance of the crop is important for its protection against the disease. For brassica crops the trait potential is significant as the disease is among the most important economic loss causing disease in brassica's.
The genus of the Brassicas comprises several species of commercial interest, such as B. rapa (Chinese cabbage, pak choi, turnip), B. napus (oil seed, swede), B. juncea (mustard), B. nigra (black mustard) and B. oleracea (cauliflower, broccoli, cabbage, Brussels sprouts, savoy cabbage, kohl rabi, borecole and others). While sub-species within a species of the Brassica genus are usually sexually compatible, this is not necessarily the case between different species of the Brassica genus. For example, B. rapa and B. oleracea do not have the same number of chromosomes (10 chromosomes versus 9 chromosomes) and are therefore not sexually compatible. This renders the transfer of a trait from one brassica species to another particularly difficult.
Several sources of resistance to clubroot have been described within the Brassica genus. Some resistances are monogenic, some polygenic, some are dominant, some recessive. Monogenic dominant resistances have been described in B. rapa and B. napus such as, for example, a monogenic dominant resistance in the B. rapa Chinese cabbage. Chinese cabbage F1-hybrids with this resistance have been shown to have good protection against clubroot, although a small number of strains (`races`) of clubroot have been able to break through this resistance. Such races seem more prevalent in Asia than in Europe.
By contrast, only polygenic, recessive sources of resistance have been described in the brassica species B. oleracea . Such sources have proven not only to be insufficiently resistant to clubroot, but they are also very difficult to transfer between commercial B. oleracea lines. This renders the breeding of the resistance a difficult and time consuming task.
The clubroot parasite is a highly variable organism. In the past attempts have been made to characterize this variation with so called differentials: these are host plants that show resistance to some variants of the parasite, but not to others. A much used differential set is the European Clubroot Differential (ECD) from 1975 in which 5 B. rapa (mostly turnips), 5 old oilseed rape landraces (B. napus) and 5 primitive B. oleracea cabbage landraces were present. Each set of 5 has one susceptible control and the other varieties each show a varying spectrum of resistance against different strains of the pathogen. An investigation carried out in 1985 with 299 strains originating from mainly European countries, showed 128 different resistance spectra. Many interactions were not clear cut and difficult to reproduce. Although this system has been widely used in academic literature, for commercial purposes it had little value.
Several organizations involved with brassicas identified a need to establish a new differential system to distinguish commercially available resistances in relation to prevalent races. The Dutch registration office NAKT, Bejo and Syngenta worked together to define a new differential set for the B. oleracea crops. The outcome was a discriminating set of 3 cabbage and 1 cauliflower varieties, each with its own resistance spectrum and 4 defined races abbreviated as Pb:0, Pb:1, Pb:2 and Pb:3. The 4 host varieties are: Bartolo (white cabbage-Bejo) as universal susceptible standard, Bejo 051632 (a resistant breeding line), Clapton (cauliflower-Syngenta) and Lodero (red cabbage-Bejo), each with its own resistance spectrum as shown in the table below. Syngenta has developed a broccoli line with a similar spectrum as Lodero (SYT-CR2). This new source of resistance is based upon 2 genes on chromosomes C3 and C8. The 2 genes are partially dominant and interact in an epistatic way with each other and therefore the resistant alleles of both genes need to be present in both parental lines of a hybrid to achieve a maximum level of resistance.
Bartolo is susceptible to all races, Bejo-051632 is resistant against races Pb:0 and Pb:3, Clapton is resistant against races Pb:0, Pb:1 and Pb:3 and Lodero/SYT-CR2 is resistant against races Pb:0, Pb:1 and Pb:2.
Differential set of varieties to identify clubroot races.
+ susceptible; - resistant; Pb: race
At this moment there are no varieties available that are resistant to all races. Races Pb:0 and Pb:1 are the most predominant types in Europe, Pb:2 and Pb:3 are more rare. Syngenta TopRes® varieties in cabbage, cauliflower and B. sprouts introduced since 2005 have performed well in most fields to date. In a small number of fields patches were detected where plants were infected. Subsequent lab research tests confirmed that Pb:2 was present.
The trait can be introduced into Brassica parental lines for use in producing clubroot resistant commercial hybrid varieties. The commercial varieties containing this trait provide higher yield under disease pressure.
BOREKALE: examples commercial varieties: not yet available
BROCCOLI: examples commercial varieties: Monclano
BRUSSELS SPROUTS: examples commercial varieties: Crispus, Cryptus
CAULIFLOWER: examples commercial varieties: Clarify, Clapton, Clabiny, Cleobis, Clarina, Claforsa
KOHLRABI: examples commercial varieties: not yet available
SAVOY CABBAGE: examples commercial varieties: Coradi, Cordesa, Cordoba, Corripa
WHITE CABBAGE: examples commercial varieties: Tekila, Kilazol, Kilaton, Kilajack, Kilaherb, Kilagreg, SG3443
The Syngenta resistance against clubroot is effective against the predominant races Pb:0 and Pb:1 and against the less frequent race Pb:3 but not against the infrequent race Pb:2 that may occur in some fields.
Genetic resistance is only one of the tools to manage clubroot. Culture measures such as liming, use of fertilizers with high percentage of calcium, proper drainage, good crop hygiene management are several of important components of an integrated approach to manage the disease.
We always recommend to first execute small variety trials before starting commercial production of a new variety.
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Based on FRAND license terms the licensee pays a fixed fee / ks for the use of this resistance locus in commercial varieties. This license includes the TopRes® trademark license
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Access to trait know-how and molecular markers to increase the efficiency with which the trait can be introduced into the market will be negotiated as a lump-sum or an additional royalty rate.