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Soft rot, blackleg and stem rot is a bacterial disease complex caused by a group of closely related pathogens, namely the pectinolytic enterobacteria. The disease was recently renamed from Erwinia to Pectobacterium.
The disease complex is regarded as a tuber-borne as well as a soil- and waterborne disease. The disease develops and is most likely to occur in waterlogged conditions combined with high temperatures.
Soft rot can affect seed potatoes, daughter tubers, and harvested tubers. Soft rot can occur at any stage, and spreads if a source of contamination is present and conditions are favourable for the development of the disease. There is therefore a risk of soft rot blackleg complex throughout the growing season (on the land, during harvesting, during post-harvest handling, and during storage).
Blackleg develops when the bacteria from infected potatoes spread to the haulms. In other words, blackleg always occurs in conjunction with soft rot in seed potatoes. Wilting sets in after a dark brown discolouring of the vascular tissue and pith necrosis develop at the bottom of the haulms.
Stem rot is a secondary soft rot of haulms and leaf stems when the haulm is damaged by external factors such as hail and wind, among others.
Wounds that consequently develop are infected by the soft rot bacteria. Wilting in the early stages of infection can be characterised by the youngest leaf margins rolling upwards.
In time, this symptom spreads to the lower leaves. Wilting symptoms can be limited to only one haulm of the plant.
Disease development Pathogen complex
- Opportunistic pathogens.
- Tuber-borne pathogens.
- Several species can cause soft rot blackleg.
- Facultative anaerobic (able to cause disease under oxygen-poor conditions).
- Makes use of a quorum system – quorum sensing occurs when conditions are ideal for infestation.
Environment
- Free water is critically important for survival, multiplication, and disease development.
- High temperature. Multiplication of the bacteria and disease progression can be extremely rapid at relatively high temperatures (25 to 35°C).
- Anaerobic conditions such as de-oxygenation promote the development of the disease since the pathogens are able to flourish under oxygen poor conditions.
- Poor ventilation due to anaerobic and wet conditions.
Plant
- Host tissue under stress. A plant’s natural immune system is unable to function effectively under stress.
- Mechanical damage of the tissue is necessary for pathogens to enter the plant.
- Wide host plant range, among others the Solanaceae family.
The pathogen complex
Soft rot, blackleg, stem rot, and wilting are caused by a group of bacteria known as pectinolytic enterobacteria. In South Africa, soft rot blackleg is mainly caused by Pectobacterium carotovorum subsp. Brasiliensis (Pbc), which is a highly aggressive pathogen. Pbc subsp. carotovorum is the second most important, while Dickeya dadantii and P. wasabiae have been identified in a small percentage of isolates. Pbc subsp. atrosepticum, which commonly occurs in Europe, has not yet been isolated in South Africa.
In South Africa and many other potato producing countries, tests on soft rot enterobacteria for certification purposes are conducted by way of conventional pathological methods. In Scotland and the Netherlands the polymerase chain reaction (PCR) method is used. Important characteristics of pectinolytic enterobacteria associated with Quorum sensing are:
- Pectolytic enzymes are secreted that digest the cell walls.
- The enzymes are not able to digest the cuticles, meaning that a wound must be present for the enzymes to invade the plant tissue. The enzymes need free water to survive and multiply but can survive either with or without oxygen (are facultative anaerobes).
- As opportunistic pathogens, it attacks tissue when the plant’s natural defence mechanisms are weakened. • Has a wide host range.
- Can survive pathogenically on living tissue or as saprophytes on dead host tissue.
- Multiplies quickly at relatively high temperatures of 25 to 35°C.
Spread of Pectobacterium and Dickeya
Potato tubers: The soft rot blackleg complex is considered a tuber-borne disease rather than a soil-borne disease. Symptomless latent contamination of tubers plays a very important role in the spread of the disease complex in potato fields and during post-harvest handling.
Seed potatoes: The South African Seed Potato Certification Scheme has zero tolerance for soft rot enterobacteria in in vitro plantlets and mini-tubers (generation 0). Later-generation seed potatoes are not tested for the presence of the pathogens, but are subjected to a visual inspection. Seed can be tested at one’s own discretion for the presence of soft rot-causing bacteria.
Soil: A good rotation system will support the breakdown of the pathogen in soil. Once a rotting tuber has decayed, the cells can enter the groundwater and go on to contaminate other tubers, as well as roots and stems.
Water is an effective medium for the spread of the disease. Surface water from rivers and storage dams where water is reused poses a particular risk. Recirculating the washing water used in the washing of table potatoes is one of the major ways in which soft rot pathogens are spread.
Implements spread the pathogen during harvesting, planting and processing, provided there is sufficient moisture present. Animals walking across contaminated fields can also spread Pectobacterium through mud clinging to their hooves. Wind can spread the bacteria suspended in water droplets.
Dust: Research has shown that soft rot pathogens suspended in water droplets can survive for approximately ten minutes after the water dried up. There is little likelihood, therefore, of the pathogens being spread through dust.
Insects feeding on contaminated plants can spread the pathogen from one plant to another. Within the plant, pathogenic cells move through the vascular tissue from one organ to another, for example from a contaminated tuber, through stolons, to daughter tubers.
Pectobacterium has a wide host range, namely maize, the Solanaceae family (such as tomato, tobacco, pepper, paprika), onions, the crucifers (such as cabbage, cauliflower, broccoli, mustard, canola), cucurbits (such as pumpkin, squash, watermelon, cantaloupe), other vegetables such as beet and carrot, ornamental plants such as arum lily and flower bulbs, chicory, weeds, especially those belonging to the Solanaceae family.
Disease development
The higher the population present on seed potatoes and table potatoes, the quicker the disease will develop and the more intense it will be.
Research in Scotland found that the risk of soft rot blackleg is low where seed potatoes with a low inoculum level are planted under unfavourable growing conditions. The risk of disease development increases as the inoculum level rises, and seed potatoes with high inoculum levels are at risk of soft rot, even where the conditions are conducive to production.
Disease distinguishing
The first symptom of bacterial wilt disease (caused by Ralstonia solanacearum) and soft rot blackleg is green wilting of one or more leaves on a plant. If seed tubers of such plants are cut, however, it is relatively easy to distinguish between the two diseases. Tubers infected with R. solanacearum often show ring symptoms, as the bacterial cells occur in vascular tissue. R. solanacearum does not cause wet rot of stems.
Symptoms
Leaves: Young leaves that wilt during the hottest time of the day are often the first sign of the presence of soft rot blackleg. Wilted leaves are found on one or more haulms of the plant. Initially the leaves are able to recover overnight or during cooler times of the day. Leaves wilt when the vascular tissue (xylem vessels) becomes blocked by bacteria originating from rotting tubers and is thus deprived of water.
If the disease develops further, the older leaves also start to wilt and the entire stem dries out. Closer inspection will reveal rotting at the base of the affected stems.
Stems/haulms: Close inspection of the stems on which wilted leaves occur will reveal rotting at the base of the infected stems. If the stems are dissected lengthwise, vascular discolouration and medullary pith necrosis are often visible. Typical symptoms of blackleg are associated with wilting and die-back (necrosis) of leaves.
Black wet rot starts at the base of one or more stems of a seed potato and is associated with vascular discolouration. As the rot spreads, the symptoms of blackleg can be seen ever higher on the stem. The cortex of the stalk decays and becomes hollow and dark in colour. Blackleg occurs when masses of bacterial cells spread from rotting tubers to haulms.
Since soft rot can affect tubers at any stage of the growing season, the blackleg that is preceded by soft rot can also occur at any stage. If the disease occurs early in the season, plants are normally stunted, and a low yield can be expected. Where the disease occurs late in the season, the damage is less severe. Tubers from infected plants that are harvested will probably be infected.
Tubers: There are often no symptoms of latent contamination of the tubers. The bacterial cells are found in lenticels, vascular tissue or wounds. If conditions are unfavourable for the development of the disease, the cells will survive without causing soft rot.
Where lenticels are contaminated and the conditions are favourable for soft rot, small watery spots form around the lenticels – the first sign of soft rot. The affected areas are usually visible as sunken brown spots. As the rot spreads, these spots melt together and form a slimy wet rot that is cream to brown in colour. The border between rotting and healthy tissue often appears dark brown or black.
Should conditions become unfavourable for the development of disease, the rotting tissue can dry out and form light coloured, dry spots. Contamination can occur in deeper-lying wounds or vascular tissue, from where the rot spreads.
In such cases, the skin may be unaffected while the deeper-lying tissue rots, which causes the rotten areas to become sunken. When an affected tuber is handled, it breaks up and releases a watery, slimy mass.
Soft rot is initially odourless, but due to secondary contamination, it is usually associated with an unpleasant odour. This unpleasant odour is usually an accurate indication of soft rot in seed potatoes under storage. Where vascular tissue is contaminated but conditions are not really optimal for soft rot, discolouration of the vascular tissue at the stalk end of tubers is usually the only symptom of contamination.
Soft rot in tubers often leads to leaf wilting and blackleg, as the bacteria spread from the tuber to the haulms – initially only through the vascular tissue, but later to the rest of the haulm tissue. Where tubers rot completely at a very early stage, there is often no possibility of blackleg.
Control
Unlike fungal diseases, few crop protection remedies are registered against soft rot. Bacillus amyloliquefaciens FZB42 has been approved under Act 36 of 1947 as a biological remedy to prevent this disease. This is applied as a soil drench at four-weekly intervals. Calcium strengthens the cell walls, thus improving the plant’s resistance to soft rot. Optimise the fertilisation programme to ensure early availability of calcium. – Compiled and published by Potatoes SA, department of research and development
For more information, visit www.potatoes.co.za.