Bacterial soft rot is the most serious causes of potato losses in storages. Soft rot bacteria infect potato tubers which have been damaged by mechanical injury or the presence of other diseases. Bacterial soft rot develops much faster when potatoes are wet. Tubers may be wet when put into storage, may become wet by the excessive application of a storage fungicide or may become wet because of excessive respiration and water loss in storage. Wet conditions in storage allow the soft rot to spread from one tuber to another. Soft rot can be kept to a minimum if potatoes are kept dry
Symptoms:It causes characteristic soft decay of the fleshy tissues of several crops, e.g., potato tubers, carrot roots, onion bulbs and cabbage that generally have highcarbohydrate and nitrogen contents. When soft rot affects, the tissue softens. It becomes watery or slimy and as the rot progresses, the water extrudes, or in a dry atmosphere the water is lost rapidly by evaporation. This is a most common and destructive vegetable disease in storage and transit. Some diseased tissues give an offensive sulfurous odour, which may be due in part to the invasion of secondary bacteria. When the seed tuber of potato are affected in the soil the same types of decay follows. The shoots arising from the seed tuber may be similarly affected if the soil is moist for a long period. However, if the soil moisture is reduced as the disease progresses, the shoot develops a blackened, shriveled carto and its growth is stunted, but is not promptly killed. The leaflets become reddish in colour at the tips and branches more upright than normal and the entire shoot gradually becomes pale and yellowish. This phase of disease is commonly known as balckleg.
Causal organism: Erwinia carotovora
Disease Cycle:Wound caused by harvest bruises, freezing injury and insect activity is the most common avenue of infection; the organism lives over in soil and plant refuse. Abundant moisture at the surface of the tissue, where wounds are present, is essential for invasion; Penetration through lenticels of freshly harvested potatoes has been observed but is less common. Fairly high humidity is essential for the progress of the disease. Progress of disease is checked in a dry atmosphere.
Several maggot fly carry the organism. Adult flies lay their eggs on decayed vegetable material and as the eggs hatch, the larvae feed upon decayed debris, ingesting the bacteria including the soft rot organism. The adults developed from then are internally contaminated. The eggs laid by them are smeared with soft rot organism. The mouthparts and the intestines of the larvae hatched from these eggs become contaminated. As they feed or potato seeds pieces, or fleshy organs of vegetable, they carry the bacteria and infect the plant tissue. The larvae can penetrate any cork par the plant may form. Thus the insect becomes a very important factor of the causal organism.
The bacteria produce an enzyme, which is capable of dissolving the middle lamella. The continuation of this process accounts for the watery condition and for the loss of consistency of the decayed tissue.
Epidimiology:The presence of wounds and high humidity are most essential for the progress of this disease. The disease development takes place between a temperature range of 7-35 Degree C.
Control:
- Bruising of plant parts should be avoided.
- Provision should be made for healing of wounds and for drying of surfaces.
- Storage should be at the lowest temperature at which the particular product can be expected to retain its culinary quality.
- Potatoes should not be washed but may be passed through a current of warm air before transportation.
- Plant refuse should be destroyed especially for the control of black leg
- Careful selection of seed pieces.
- Treating seed pieces with streptomycin sulphate or streptomycin sulphate + tetramycin.
