Potato blackleg caused by pectinolytic Pectobacterium atrosepticum (syn. Erwinia carotovora subsp. atroseptica), P. carotovorum subsp. carotovororum (syn. E. carotovora subsp. carotovora) and Dickeya spp. (syn. E. chrysanthemi) gives increasing damage in seed potato production in Europe. In the past, the blackleg pathogens contributed equally to the occurrence of blackleg, but in the last five years Dickeya spp. was responsible for 50-100% of the incidences in France and The Netherlands. In this paper, the diversity and some ecological aspects of Dickeya spp. are discussed, which may explain the increasing significance of this pathogen. Dickeya spp. has been recently divided among six species, largely coinciding with seven biochemically distinct groups (biovars). In potato in Europe, before 2000 D. dianthicola (biovar 1 and 7) was almost exclusively found. This species is more adapted to temperate climates. Since 2000, a biovar 3 Dickeya spp. was isolated from potatoes grown in Israel, Finland, Poland and the Netherlands, which could not be classified in any of the six new species. Results from dnaX and 16S rDNA sequence analysis, rep-PCR and biochemical assays indicate that strains belonging to this biovar 3 variant are clonal. This variant has a higher temperature maximum than D. dianthicola. Possibly due to the increasing average temperature during the growing season, the Dickeya biovar 3 variant is taking over from D. dianthicola. In contrast to a biovar 7 D. dianthicola strain, the biovar 3 variant efficiently colonizes plant material. Soil infestation with a GFP-tagged strain resulted in a systemic colonization of potato plants within 30 days after inoculation. The biovar 3 variant was also able to colonize roots, stolons and progeny tubers from infected stems. Spread within a crop may also occur during crop production if bacterial cells of Dickeya spp. are disseminated via free water in soil from rotten tubers to tubers of neighbouring plants. We showed that plants adjacent to blackleg diseased plants both within a row and between rows became contaminated after heavy irrigation. Dickeya spp. was able to cause disease symptoms even when present in seed at low densities. In field experiments with vacuum-inoculated tubers, a level of 40 cells per gram of potato peel was sufficient to end up with 30 and 15% diseased plants in 2005 and 2006, respectively. Such low levels of infection easily remain unnoticed during seed testing, even if sensitive detection methods are used. As for Pectobacterium spp., spread of contamination within and between seed stocks often occurs during harvesting and grading. In an experimental field, contamination with Dickeya spp. was spread by mechanical harvesting up to a distance of 80 m behind a zone with rotten tubers, with an average of 12 meter. Hand-harvested tubers from a disease-free crop remained clean. Dickeya spp. seems to act like a biotrophic organism, which needs the host for long-term survival. D. dianthicola and the biovar 3 variant survived maximally for only three months in soil. Soil type, temperature and humidity only had a minor effect on survival. In conclusion, a Dickeya spp. has become the dominant blackleg pathogen, probably due to its higher optimal growth temperature and its ability to colonize plant tissue more efficiently compared to Pectobacterium spp. The increasing importance of Dickeya spp. may be related to the increasing average temperature during the growth season due to global warming.
|Publication status||Published - 2009|
|Event||Pests and climate change - |
Duration: 3 Dec 2008 → 3 Dec 2008
|Conference||Pests and climate change|
|Period||3/12/08 → 3/12/08|