Blackleg and Soft rot

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.

Symptoms on foliage

Early bacterial infections induce rotting of mother tubers and may be responsible for poor or non-emergence.

Blackleg is the most typical symptom (photo 1). It ranges from a dark brown wet rot of the lower part of the plant to dry necroses depending on weather conditions (photo 2). In some cases, only internal necrosis develops, sometimes associated with hollowing of the stem due to pith degradation (photo 3). Chlorosis and/or leaf wilting symptoms associated with blackleg (photos 4 and 5) enable the identification of diseased plants in infected fields.

Although humid conditions are favourable to the development of rot, warm dry conditions are more likely to cause wilting.

Symptoms on tubers

Diseased tubers show soft rot, often extending from the stolon end into the tuber (photo 6). The wet rot presenting a granular consistency has a creamy-white colour or a brown to black colour near the margins of the diseased tissue (photo 7). Secondary saprophytic organisms developing in infected tissue cause nauseating smells and the formation of mucus. Lenticel rots are observed in the case of field conditions that are too wet or in high humidity storage conditions (photos 8 and 9).

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.

Control

Bacteria control is essentially prophylactic and includes the following measures in particular:

• avoid planting in humid fields and compacted areas;
• use certified seed potatoes and discard rotted tubers before planting;
• eliminate (roguing) diseased plants from the field (for seed production);
• avoid excessive nitrogen supply and over-irrigation;
• minimise mechanical damage to tubers during harvesting and handling as induced wounds can provide entry points for the bacteria;
• dry the tubers just after harvest and store under dry, ventilated conditions at low temperature to avoid water films on tuber surfaces;
• avoid soaking or cutting the seed potatoes. Dry the tubers thoroughly following treatment against dry rot and gangrene diseases, or against black scurf;
• maintain a high degree of farm hygiene (waste destruction, frequent washing and disinfection of storage areas and equipment).