Germany
January 2018

Kohlhernie (Clubroot disease) - Symptoms

The International JKI/UFOP-Workshop “Clubroot disease in oilseed rape - status quo and research demand” was held in Berlin/Germany from January 15 to 16, 2018.

Clubroot of oilseed rape (Brassica napus), caused by Plasmodiophora brassicae, is one of the most destructive diseases worldwide. The disease incidence and severity has increased significantly in recent years and it has now become one of the most serious diseases on oilseed rape and other crucifer cultivations. The importance of the disease has led us to organize the workshop here in Germany. It was sponsored by the Union for the Promotion of Oil and Protein Plants (UFOP).

Aims of the workshop were:

• sharing knowledge,
• informing participants about the current situation of clubroot disease in different countries,
• informing about integrated management techniques,
• identifying gaps in knowledge and research needs,
• avoiding unnecessary duplication of works.

With respect to registrations, 82 delegates from nine countries (Austria, Canada, Czech Republic, Denmark, France, Germany, Poland, Sweden and United Kingdom) registered and participated in the meeting. Eighteen experts from different universities, research centres and breeding companies could be welcomed as keynote and impulse speakers (in total 24 presentations). By the plenary lectures and discussion sessions, the workshop was intended to address the following issues:

(1) Pathogenicity, life cycle, symptoms and damages caused by P. brassicae

• Stephen Strelkov (University of Alberta Edmonton/Canada): Overview of the occurrence and distribution of the pathogen in Canada
• Elke Diederichsen (Free University of Berlin/Germany): Introduction into Clubroot and Plasmodiophora brassicae

 (2)  Epidemiology, overview of the occurrence and distribution of the P. brassicae-pathotypes and risk factors

• Nazanin Zamani-Noor (Julius Kühn Institute/Germany) and Wolfgang Lüders (Limagrain GmbH/Germany): Epidemiology, risk factors and overview of the occurrence and distribution of the pathogen in Germany
• Geoffrey Orgeur (GEVES/France): Epidemiology, overview of the occurrences and distribution of the pathogen and related damage in France
• Małgorzata Jedryczka (Institute of plant genetics/Poland): The occurrence, severity and pathotypes of Plasmodiophora brassicae on WOSR in Poland
• Ann-Charlotte Wallenhammar (Agricultural Society/Sweden): Epidemiology, overview of the occurrence and distribution of the pathogen and related damages in Sweden
• Francois Dussart (Scotland's Rural College/United Kingdom): Epidemiology, overview of the occurrence and distribution of the pathogen and related damages in United Kingdom and Scotland
• Veronika Konradyová (University of Life Sciences/Czech Republic): Epidemiology, overview of the occurrence and distribution of the pathogen in Czech Republic

 (3) Integrated disease management and further research demands

(3-1) Cultivar resistance (Field/Greenhouse resistance test, process of crop approval and registration, resistance source)

• Martin Frauen (NPZ/Germany): Potential and limitation of breeding clubroot resistant WOSR varieties
• Wolfgang Lüders (Limagrain GmbH/Germany): Evaluation of clubroot resistance in the field and greenhouse – a comparison
• Elke Diederichsen (Free University of Berlin/Germany): Clubroot resistance in Raphanus and its use for crucifer breeding
• Christian Möllers (University of Göttingen/Germany) and Nazanin Zamani Noor (Julius Kühn Institute/Germany): New sources of resistance in oilseed rape and its diploid ancestor species
• Nazanin Zamani-Noor (Julius Kühn Institute/Germany): Screening and evaluation of resistance to clubroot in new clubroot resistant oilseed rape varieties during registration process in Germany

(3-2) Field hygiene, crop rotation and farming methods to control the disease

• Gunilla Berg (Swedish Board of Agriculture Alnarp/Sweden): Long history of oilseed cropping in Sweden
• Christine Struck (University of Rostock/Germany): Clubroot monitoring and infestation influencing parameters
• Nazanin Zamani-Noor (Julius Kühn Institute/Germany): Suppression of clubroot of oilseed rape by soil amendments with fertilizers
• Dania Bornhöft (Norddeutsche Pflanzenzucht/Germany): Field trials for the control of clubroot with different sowing dates and liming
• Stephen Strelkov (University of Alberta Edmonton/Canada): Field hygiene, crop rotation and farming methods to control disease in Canada
• Georg Mevenkamp (AMG Agrarmanagement GmbH/Germany): Managing clubroot in vegetable brassica crops

(3-3) New Approaches (Occurrence of new pathotypes, Remote sensing, Soil ecology, Microorganisms, Bait crops)

• Xavier Pinochet (Terres Inovia /France): Clubroot - questions and hopes
• Elke Diederichsen (Free University of Berlin/Germany): Managing clubroot with bait crops – a survey
• Stefan Dobers (University of Applied Science, Brandenburg/Germany): Possibilities for the detection of clubroot suspicious areas on large production fields by fusion of different geo-data sources
• Susann Auer (University of Dresden/Germany): Alternative methods to fight clubroot in oilseed rape – what can be achieved with beneficial microbes?
• Stephen Strelkov (University of Alberta Edmonton/Canada): Management of clubroot disease with soil amendments

Briefly, the causal pathogen of clubroot disease, Plasmodiophora brassicae, promotes to spread in most of European countries and in some Canadian provinces. The detection of new P. brassicae-infested fields during previous years suggests that the disease is more widespread in oilseed rape fields than previously known. Variation in virulence and physiologic specialization was observed in all isolated populations of P. brassicae across Europe and Canada. Whilst most of the Brassica rapa genotypes were completely resistant, B. napus cultivars showed a wide range of reactions from completely resistant, intermediately susceptible, to highly susceptible, and most of the B. oleracea genotypes were highly susceptible. Conversely, the fact that „Mendel” resistance-overcoming isolates were unable to be differentiated into separate pathotypes or physiological races emphasizes the importance of specific host differentials to identify new pathotypes. Additional host plant cultivars or a new set of differential genotypes may allow all worldwide isolates/populations to be grouped into separate pathotypes. Besides optimizing the differential sets, standardization and harmonization of disease assessment methods and scales in defining pathotypes in European countries is necessary.

Field observations showed that farmers in affected regions increasingly have relied on resistant oilseed rape cultivars, often growing them in very short rotations (mostly in a 2- or 3-years rotation) or in heavily infested fields. This has placed enormous selection pressure on pathogen populations and several isolates were found to be moderately or highly virulent on resistant cultivars. These virulent populations were not restricted to a small geographical area in different countries.

Soil-borne and obligate biotrophic nature of the pathogen as well as long-term viability of resting spores makes big challenges in controlling clubroot disease. Cultivar resistance is the most widely used clubroot management strategy but reliance on a single option method is not likely to be sustainable. Therefore, other strategies such as field hygiene, the sanitization of field equipments, wide crop rotation, high grade agronomical practices, weeds and volunteers control, liming and using of bait or catch crops should be taken into consideration to support the resistant cultivars.

Studies under controlled conditions showed that the first symptoms of clubroot could be visible as early as 7 days past inoculation. Early termination of infected plants at 7dpi resulted in a significant reduction in the development of clubroot symptoms, root fresh weight and the number of resting spores per root mass. As the concentration of resting spores inside the soil is an essential factor in the subsequent development of a clubroot epidemic, the timing of postharvest management is an important determinant of the incidence and severity of clubroot disease in volunteers as well as resting spore propagation.

Field studies demonstrated that soil amendments with calcium cyanamide or burnt lime could give variable control between locations with different disease pressure and also between years but showed some potential as part of an effective clubroot management strategy.

Breeding for clubroot resistance mostly is a cumulative process. Since the discovery of clubroot, intensive researches in pathogen population structure and virulence and also genetics and breeding for resistance have been conducted by different research groups and breeders. From experimental results and comparing the reaction of Mendel resistance-overcoming isolates with the isolates which could overcome the resistance in one of ECD1 to ECD4 plant genotypes, it can be concluded that there are several major and minor clubroot-resistance genes in the host plants that affect the level of infection. Alternatively, the genetic heterogeneity of the P. brassicae populations may cause these differences in infection and other virulence factors might affect the pathogenicity of P. brassicae isolates. To date, most resistant cultivars are based on a single gene and are generally race or pathotype-specific. In various sources of clubroot resistance in brassica species, European turnips are the best ones so far, which can be used for breeding clubroot resistant oilseed rape cultivars. Identification of novel clubroot resistance genes and pyramiding of these genes is regarded to be the most effective approach to develop varieties with durable and broad spectrum resistance.

Authors: Nazanin Zamani Noor (JKI Braunschweig), Manuela Specht (UFOP Berlin), Wolfgang Friedt (Universität Gießen)

Website: http://www.jki.bund.de/

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