Risk factors

The main favourable factors for the development of blackleg and tuber soft rot bacteria are relative humidity and anaerobic conditions. Both factors enhance the multiplication of bacterial populations to a critical level that leads to the appearance of the disease.

The development of one or the other of the different bacteria, Pectobacterium spp. or Dickeya spp., depends on climatic conditions, temperature probably being highly determinant.

• Pectobacterium  atrosepticum, (Eca) whose optimal temperatures range from 15 to 25°C, is usually described as being associated with temperate regions.

• Dickeya spp. (Ech) thrives in temperatures ranging from 25 to 40°C. Initially described in tropical and subtropical regions, there is evidence that the pathogen is now present in Europe, often occuring on crops after warm periods.

• Pectobacterium  carotovorum subsp. carotovorum, formerly Erwinia carotovora subsp. carotovora (or Ecc), can also induce blackleg symptoms. Its optimal temperatures range between 20°C and 40°C. Identified in the 70’s as being responsible for blackleg in limited areas (Arizona), this subspecies is now frequently identified from blackleg symptoms in temperate climates. The proportion  of pathogenic strains able to cause symptoms on stems still remains to be determined among the subspecies Pectobacterium  carotovorum subsp. carotovorum which also includes a large part of saprophytic strains

Tubers can be the primary source of inoculum but pathogens can also be present in the environment: thus, Pectobacterium and Dickeya have been evidenced in the water (rivers, reservoirs) but poorly survive in bare soil. Pectobacterium colonizes roots of many plant species and has also been detected on and in insects. The host specificity varies according to the Pectobacterium species: very wide for P. carotovorum subsp. carotovorum  but limited in the case of P. atrosepticum. The host range of the newly identified species  P. carotovorum subsp.. brasiliense or P. wasabiae is little known so far. The emergence of D. solani on potato is probably related to its passage on this host from cultures of ornamental plants in the Netherlands. Field contamination of tubers originated from bacteria-free material suggests the presence in the environment of plants likely to host, without expressing them, bacteria and then, constituting reservoirs for the pathogens.