Black leg
Erwinia carotovora subsp. atroseptica) et Erwinia carotovora subsp. atroseptica et subsp. carotovora,
Erwinia chrysanthemi
(Blackleg = Soft Rot)
Causative agent and transmission
In temperate zones, the appearance of blackleg is caused by the bacterium Erwinia carotovora subsp.atroseptica, which develops more easily in a cool and damp climate.
In tropical and sub-tropical zones, this disease can also be caused by the bacteria Erwinia carotovora subsp. carotovora and Erwinia chrysanthemi.
The development of soft rot on tubers is due to the bacteria Erwinia carotovora subsp. carotovora and Erwinia carotovora subsp atroseptica.
The tubers seem to cope with the basic elements of contamination, but the development of the disease depends largely on weather factors.
Description of symptoms on crops
(clic on photos to enlarge)
Early attacks by the parasite can rot the mother tubers and cause blanking [shortfalls at emergence].
The most typical symptom is the appearance of the black leg phenomenon, i.e., a more or less damp black rot at the base of the stems (and sometimes the roots), due to bacterial development. The tissues soften and a poor supply of water to the plants can lead to the wilting of foliage. The yellowing and coiling of the leaves make the symptoms very clear on plants that are strongly affected. Although the bacteria prefer damp and cool conditions, the symptoms are more commonly seen following a drought.
In hotter conditions, Erwinia chrysanthemi causes wilting and brown to black rot inside the stems.
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Description of symptoms on tubers
The symptoms on tubers are characterised by internal soft rot often starting from the stolon. The bacteria deteriorate the tuber tissues which become spongy and the rot, initially in a light colour, then darken. Other saprophyte organisms are rapidly superfluously added and lead to foul-smelling odours and mucus.
When there is too much water in the soil or storage, lenticle rot may be noted (photo on the right).
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Control
- Eliminate all the plants in a crop showing symptoms (roguing);
- Avoid damaging the tubers when handling, since this provides entry points for the bacteria;
- Store in well-aired and dry conditions at a low temperature;
- Prohibit soaking and cutting the seed potatoes. Dry thoroughly after treating against dry rot or black scurf.
