Potato tubers are usually contaminated by more than one species or pathovar of soft rot erwinia and, because blackleg incidence is related to the contamination level of seed tubers, the disease potential of seed stocks may be assessed by determining seed‐tuber contamination level. A method is described for identifying and quantifying directly from tubers the three soft rot erwinias commonly associated with potatoes. Replicate lots of 10–15 tubers are peeled by dry abrasion in a commercial potato peeler and the peel weight determined by weighing the tubers before and after peeling. Sap is expressed from the peel, an antioxidant (0.075% dithiothreitol) added, and the sap is dilution‐plated on a diagnostic selective medium (crystal violet pectate [CVP]). After incubating for 24 h at 20°C, the plates are velvet‐replicated onto fresh plates of CVP with or without 35 μg ml‐1erythromycin and incubated for 48 h at 27°C and 24 h at 33.5 or 37°C. Soft‐rot erwinias typically form deep cup‐like cavities on CVP and they can be identified and enumerated according to the pattern of cavity formation. Cavities are formed byErwinia carotovorapv.atrosepticaonly at 27°C, byE. carotovorapv.carotovoraat 27 and 33.5°C but not at 37°C, whereasE. chrysanthemiforms cavities at all temperatures but fails to grow in the presence of erythromycin. Contamination levels can be expressed as the number of different erwinias per