Blackleg, a bacterial soft-rot of potatoes caused byErwinia atroseptica, has been recognized since the turn of the century as a disease capable of causing tremendous losses to the crop in the field or in storage. The pathogen is both seed- and soil-borne, and generally attacks the seed piece through wounds caused by insect larvae, fertilizer burns, frost injury, bruises, or lesions caused by other pathogenic bacteria and fungi. Infection is increased by low temperature and high moisture, especially in the soil. These conditons are less than optimum for the pathogen, but are less favorable for the infected plant, as they slow normal growth and prevent formation of protective cork tissue which would restrict the spread of the infection.