Sprouts that do emerge may show curled upper leaves, compact foliage and fading from green to yellow green. The plants later assume a distinct yellow color. The plant gradually dies as the base of the stem is rotted away by the bacteria. When pulled, affected plants will show slimy rotted, dark or inky black, mushy stems. These black stems give the disease its name.
Soft rot occurs as a slimy rot of the tuber without the dark color.
Spread Blackleg is caused by the bacterium Erwinia atroseptica and soft rot is caused by E. carotovora bacteria. These can live in the soil, in decaying plant debris and in the seed tubers. Bacteria either enter the seed potatoes and lower stems through wounds and injuries, or move directly from contaminated seed pieces to lower stems. Abundant moisture at the surface of the wounded tissue is needed for infection and continued high humidity after infection favors spread of the disease in the plant. Several kinds of insects may be involved by carrying the bacteria from decayed tubers to new seed pieces or shoots.