Potato phytopatogens of genus Dickeya - a mini review of systematics and etiology of diseases

In recent years, plant growers in Russian Federation have met significant changes in species of bacterial pathogens causing economically harmful diseases of potatoes that is associated with the import of infected planting material, recent climatic changes favorable for bacterial disease development, over-wintering of the pathogens and their vectors (insects, mites and nematodes), and with lack of bactericidal pesticides for integrated plant protection. Damage of potato plants ( Solanum tuberosum L.) by Enterobacteriaceae family is one of the greatest problems in production of seed and food potatoes. The bacteria cause a black leg, wet rotting of the stem in the field, and soft rot of potato tubers in storage. In temperate climate, the bacterial diseases of potatoes was usually caused by two species of genus Pectobacterium, Pectobacterium atrosepticum as a pathogen of black leg of potato, and Pectobacterium carotovorum causing a soft rot of potato and different vegetable crops (A.N. Ignatov et al., 2015). However, recently, many countries have faced the spreading on potato fields of new enterobacteria of genus Dickeya, which has been normally considered as pathogen of ornamentals and vegetables, particularly in countries of tropical and subtropical climate. A detailed study of genus Dickeya has shown that this diverse group of bacteria affects a number of plant species, including many economically important crops (I.K. Toth et al., 2011). The strains differed in attacked host plants, and phenotypic properties. It was found that strains isolated from European potato fields in years 1979-1994 were mainly related to D. dianthicola, the species well-adapted to temperate climatic regions. However, since 2005, the variants of Dickeya 's biotype III, referred to the new species D. solani were detected on potato in Europe, and soon became one of the most aggressive pathogens of this crop. Clarification of the taxonomic position and diversity within species of the genus Dickeya ( D. chrysanthemi, D. dadantii, D. dianthicola, D. dieffenbachiae, D. paradisiaca, D. zeae, D. solani ) gives a chance for development of new methods of diagnostics and control measures against these pathogens (L. Tsror et al., 2011). Except for D. dieffenbachiae, all the species of this genus can affect potatoes. Infection of D. dianthicola and D. solani has been already reported in some regions of the European part of the Russian Federation (A.N. Karlov et al., 2010, 2011; A.M. Lazarev, 2013), and genome sequencing of the isolated bacteria showed their identity with strains of this genus isolated in Western Europe and Latin America (S.V. Vinogradova et al., 2014). The spreading of these pathogens abroad and in Russia, data on taxonomic position and description of their biological properties, and sources of infection, created ground for development of control measures against them. It is believed that D. dianthicola and D. solani have aroused as potato pathogens moving from vegetable crops in the early 1990s. Now they are striking plants in European countries, USA, South America, Africa and Asia. D. dianthicola and D. solani, first described at the territory of the Russian Federation in 2009, cause serious potato losses in Russia in recent years. In 2009-2013, the annual two-fold increase of contamination of seed potatoes by these pathogens was documented. Thus, in just 4 years, the prevalence of pathogens of the genus Dickeya in potato seed lots in Russia increased from 3 % to 26-28 % (A.N. Ignatov et al., 2015). Control of these pathogens on potato is based on the rejection of contaminated material and prevention of contamination at all stages of the technological cycle of seed potatoes. Potato varieties resistant to these pathogens have not been yet discovered.