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Potatoes - black leg and soft rot | AG0312 |
Roger Osborn, Knoxfield
June, 1995 |  |
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The disease complex comprising black leg and soft rot occurs commonly in almost all the potato growing areas of the world. While black leg causes considerable reduction in yields in the field, soft rot causes losses in transit and in storage. Both diseases result from the breakdown of the softer tissues of the plant.
Causal organisms
The black leg-soft rot complex is caused by a group of bacteria known as the soft rot erwinias. This group consists of Erwinia carotovora ssp. carotovora, Erwinia carotovora ssp. atroseptica and Erwinia chrysanthemi. All of them, either singly or in combination, could cause either the black leg symptom or soft rotting or both, depending on climatic conditions. In Victoria, where cool and moist conditions prevail, the principal causal organisms are Erwinia carotovora ssp. carotovora and Erwinia carotovora ssp. atroseptica.
Economic importance
In the past the black leg-soft rot complex had been a serious threat to the potato industry in Victoria. However, with the introduction of the pathogen-tested scheme this has been averted, though it still, occasionally, causes serious losses in seed and ware crops.
Symptoms
The most obvious symptom of the disease complex in the growing crop is the blackening of stems, extending upwards and downwards from soil level as a result of the soft-rotting and host tissue reactions caused by the bacterium; hence the name black leg. Diseased plants are stunted and have pale green or yellow foliage. Upper leaves usually remain erect and stiff with their tips often red in colour and margins rolling upwards. However, the most damaging symptom is the soft rotting of tubers and seed pieces. Tubers may become infected either through stolons or through wounds and lenticels.
Those infected through stolons may show a soft, discoloured area spreading out over the tuber surface from stolon attachment. Internally this region will show a rotted and partly hollow area. In advanced stages, the flesh often turns brown to black and becomes wet and rotten. Tubers that become infected through wounds and lenticels turn brown to black around these points. Such tubers break down completely by harvest time if warm and wet conditions prevail. If conditions remain relatively dry, these tubers may appear to be healthy at harvest time, but would rot badly in storage.
Infection may also occur after harvest. Such tubers would show small water-soaked lesions that enlarge rapidly and become soft and mushy.
Figure 1. Typical blackening of potato stems caused by black leg
Disease cycle
The causal bacteria are carried over from one growing season to the next principally in infected tubers. While the most common vehicle of carry over is the infected seed, others include infected tubers in storage, and those left behind in the field, bins, potato machinery, etc. The bacteria are readily spread by cutting and handling operations.They do not survive in the soil for more than a few months except in association with diseased potato tissue. Within a crop the black leg-soft rot bacteria are spread to adjacent plants by soil water and by insects.
Control
Use of pathogen-tested seed and good hygiene at all stages of production are of prime importance in the control of black leg-soft rot disease. Good hygiene combined with from two to three-year rotations with other crops considerably minimise infection. All potato refuse and selfsown potatoes should be destroyed. Storage areas, potato machinery, bins, etc. should be regularly cleaned out and disinfected with an appropriate disinfectant. Tubers should be handled with care and should be dry prior to storage. They should be cooled to 10°C or lower as soon as possible after harvest and stored at temperatures of 2-5°C. Planting in wet soil and over irrigation should be avoided.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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