The stem canker spreads down to the roots, which are eventually killed, stunting the plant. With high rainfall, the disease can spread regardless of temperature, although it is more active at coo ler temperatures. Blackleg can also infect the crop in storage. Blackleg will not survive on seed that is soaked in 122 degrees F water for 25 minutes. Infected transplants are also often sources of blackleg.
Sheep manure can be a source of blackleg inoculum if the sheep have grazed on infected plants.
Blackleg symptoms are worse on plants also damaged by herbicide. Black rot is caused by a bacteria to which all brassicas are susceptible. This bacteria is seed-borne, but can also overwinter on weeds and crop debris. Worldwide, black rot is considered to be the most important disease of crucifers. Infected leaves develop a wedge shaped yellowing on the margins, often followed by blackening of the veins and dying of V-shaped sections of the leaves.