Causal agent(s) and transmission
The bacteria associated to blackleg and soft rot on potato, belong to the genera Pectobacterium et Dickeya (formerly Erwinia).
Blackleg on potato stems is historically associated in Europe to Pectobacterium atrosepticum (formerly Erwinia carotovora subsp. atroseptica). However, other species belonging to the genera Pectobacterium and Dickeya can also cause blacklef and soft rots on potato.
Recent studies have shown that the complex ‘Pectobacterium carotovorum subsp. carotovorum’, formerly Erwinia carotovora subsp. carotovora, included species or sub-species such as P. parmentieri and P. carotovorum subsp. brasiliense, which are now clearly identified as distinct taxa.
Among the genus Dickeya (formerly Erwinia chrysanthemi or Ech), the two main species which have beeen identified on potato blackleg symptoms are D. dianthicola and D. solani, this latter classified as a new species following its emergence in Europe, in the years following 2000.
Tuber soft rot, which can develop during storage or in the field, is usually associated with Pectobacterium carotovorum subsp. carotovorum and Pectobacterium atrosepticum but bacteria belonging to the genus Dickeya spp. can also cause soft rot on tubers.
The primary blackleg inoculum source appears to be the tuber. The other sources and vectors of infections are water, soil, weeds and cultivated plants. Additionally, manure and insects have been identified as vectors of these bacteria
Blackleg disease occurs widely in potato growing areas. The incidence of the disease is very variable from sporadic presence to high losses (poor emergence, wilting, tuber rot) in favourable conditions and on susceptible potato cultivars.
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.