Blackleg and soft rot

Pectobacterium spp. and Dickeya spp.

• Causal agents 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

• Significance 

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.