Clubroot Brassicas

Clubroot is a widespread disease that causes serious problems in many brassica growing areas. The disease is caused by Plasmodiophora brassicae , a singelecelled 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. For all brassicas, clubroot represents a significant economic value, t herefore good genetic resistance of the crop is important for its protection against the disease.

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. 

This 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.

Patent

Syngenta has not filed a patent application on the YT-CR2 trait.