Bacterial canker of tomato

Phytosanitary control of Clavibacter michiganensis subsp. michiganensis, the causal agent of bacterial canker of tomato

The phytopathogenic bacterium Clavibacter michiganensis subsp. michiganensis (Davis et al., 1984) is the most serious bacterial pathogen of tomato worldwide, causing bacterial canker and is considered as A2 quarantine organism by the European Plant Protection Organisation (EPPO). The pathogen survives in seeds, plant debris and on greenhouse structures (Ftayeh, et al. 2004; Strider 1969).

Increased outbreaks of bacterial canker were observed recently in Germany (non- reported personal observations) and in several European countries, including Austria, Belgium, Czech Republic, France, Netherlands, Serbia, Slovakia, Slovenia and Spain (CABI/EPPO, 2009).

Because commercial tomato cultivars resistant against Cmm are unknown and effective bactericides are not available, hygienic and phytosanitary measures remain the best way for avoiding further infections by Cmm. Thus, most important is the use of pathogen-free seeds, transplants and planting material for preventing any further distribution of the disease into new locations (Strider, 1969). Since this pathogen is a quarantine organism, the export or import of tomato seeds and plants and all movements of seeds or transplants must be attendant with issues of Seed Health Certifications ensuring that seeds or plants are sufficiently healthy and labelled. Seeds-marketing is regulated so that in most cases exclusively certified seed lots will be traded.

Certification of commercial tomato seeds as pathogen-free can be fulfilled according to the European Commission Directives 2000/29/EC (Anonymous 2000) when: Tomato seeds are gained from healthy-appearing plants, which didn’t show any disease symptoms until the date of seed extraction and if one of the following conditions is fulfilled: 1) The seeds have been extracted by diluted acids, or 2) Seeds have been tested according to internationally accepted laboratory methods.

However, these directives were insufficient to prevent further spread of the disease in Europe recently because

  • Healthy appearing plants may be latently infected and the incubation time may extend more than 5 months (Ftayeh et al., 2008a),
  • An internationally accepted standard seed extraction method by acids is not available although this method has been recommended by EPPO (Petter, 2009, personal communications)
  • Seed extractions by acids do not ensure an effective and absolute eradication of the pathogen whlich is required by the international quarantine regulations for Cmm. These restrict import and export to zero tolerance for Canada, the USA, the EU, and some other countries in order to prevent the outbreak of tomato bacterial canker (European Union, 1995; Bach et al., 2003), and even one contaminated seed in 10.000 must be detectable. Thus, Anwar et al. (2004) and Gitaitis (Gitaitis and Walcott, 2007) proved the presence of Cmm in certified commercial tomato seeds indicating the need for more sensitive detection methods.

Some recent reports on further spread of Cmm, such as in West Germany during 2006, in Syria (Ftayeh, et al., 2008b), in Austria during 2007 and again during 2009, in Baden-Württemberg in Germany (Moltmann, personal communication) indicated that infested seeds are the most important source of the primary infection with Cmm. Thus, many questions arose on the reliability of the present diagnostic protocol applied for detection of Cmm (Anonymous 2007; Olivier et al., 2009) confirming our previous observations and the aims of this study, i.e. the search for highly sensitive detection methods to allow an effective control of the pathogen by indexing latent infestations in order to avoid further transmissions via contaminated seeds and young plants (Ftayeh, et al., 2008a; Ftayeh, et al., 2009).

In summary, the aim of this project is searching for new ways and strategies for an effective elimination of Cmm by improving the recommended detection protocol of EPPO (Anonymous, 2005), i.e. development of a new semi-selective medium (Ftayeh et al., 2008c) in combination with highly specific PCR primers (Bio-PCR) (Ftayeh, et al., 2009). Parallely, the effectivity of seed treatments by chemical and physical methods has also been investigated (Ftayeh, et al. 2008d).


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