Reports of Blackleg in Potatoes

Some growers in Michigan are reporting 40 – 50% of plants with stem blackleg and seed piece decay caused by Erwinia carotovora ssp atroseptica (Eca). That amount of seed piece decay and post-emergence blackleg usually indicates that the seed was frosted at some point, which can happen during transportation. All potato tubers carry the pathogen on their surfaces but normally problems develop only after infection occurs through wounding, frosting or other physical damage. The bacterium is a facultative anaerobe which means it thrives both with and without oxygen. Eca gets into the sprout and develops rapidly after emergence resulting in a lower stem and root rot. The stem is left with no anchoring and the stem pulls easily out of the ground. Typically, plants affected by Eca are stunted, with yellow necrotic leaves and black stems. The stems can also have a characteristic acidic odor after other saprophytic bacteria invade the stem. Some cultivars are more susceptible than others and usually early maturing cultivars such as Red Norland, Onaway and other colored cultivars are prone. Other sprouts on the tuber seed piece can compensate but if the temperature reduction was at the surface of the pile of seed in transit then the damage can be extensive and kill all the sprouts on the seed piece. This disease can be particularly important in seed crops as there is a tolerance of 0.5% incidence in the growing crop. Generally, in lighter infestations the crop can compensate but when fields have up to 15% plant loss, compensation is inadequate. There are no recommendations for controlling blackleg other than rouging out affected plants or post-harvest disinfestations. In severe cases and when wet conditions prevail there is risk of movement of inoculum form plant to plant in the soil.