BLACK LEG (vine) and BACTERIAL SOFT ROT (tuber)

Erwinia carotovora causes black leg, a stem rot, on vines resulting in wilt and bacterial soft rot, a wet rot, on tubers resulting in potentially severe decay in storage.

I. On vines, the disease is called black leg. It is usually an early season wilt but can occur late in the season under certain weather conditions.

a) Early symptoms are unlike that of stem canker described above. Black leg is caused by a bacteria not a fungus. This bacteria eats out the center of the stem (pith) at the base near ground level. It decays from the inside out, first hollowing the stem. Unlike the others, the stem becomes black and slimy, and oozes. Stunting and delaying emergence is common.

b) Later symptoms show the stem to be destroyed and the plant wilts. At this point, it becomes difficult to distinguish it from the fungal diseases unless the hard vascular tissue is still somewhat intact and the hollow pith can still be seen.

II. On tubers, the disease is called Bacterial Soft Rot.

Symptoms range from vascular discoloration to a soft rot, both starting at the stem-end of tubers. The soft rot goes from the stem-end through the center of the tuber extending further in, hollowing out the tuber much like what happens to the stem as black leg. The rot is wet and creamy in color with sometimes black margins. I find that the smell of the rot at early stages, before other organisms invade, is the slight odor of frozen meat. Pythium leak, a fungal disease with similar symptoms, has a strong nasty smell of rotting fish. Pythium leak has no vine symptoms, and is a harvest and storage disease. Both soft rot and leak are promoted by the same harvest and storage conditions -- heat, wet and lack of air.

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