Blackleg, aerial stem rot, and tuber soft rot of potato are all similar diseases caused by several types of soft-rot bacteria (Latin name - Erwinia carotovora).

Blackleg has developed in the vegetable garden after the potato plant is well up. The stem bases of the diseased plants show an inky-black to light-brown decay that originates from the seed piece and extends up the stem to more than two feet.

Leaves of infected plants tend to roll upward at the margins, become yellow, wilt, and often die.

There are also a few potato tubers in the garden with soft rot, which have tissues that are very soft and watery, and have a slightly granular consistency. The diseased tissue is cream- to tan-colored, and often has a black border separating diseased from healthy areas.

 

Black leg enters the potato from the stem, so appears at the end of the potato.

 

 

The soft rot then develops from the point of blackleg infection, as seen in this section of a potato:

 

 

In the early stages, soft-rot decay is generally odorless, but later a foul odor and a stringy or slimy decay usually develops as secondary decay bacteria invade infected tissues.

Blackleg originates from the seed, which is why it is important to by certified seed, as certified seed will have been checked for blackleg.

Blackleg, aerial stem rot, and tuber soft rot of potatoes are all caused by similar bacteria which grow more rapidly in we, hot conditions.