Bacterial ring rot (Clavibacter michiganensis subsp. sepedonicus)

Bacterial ring rot: biology

The pathogen is generally tuber-borne, but can survive on harvest machinery and storage equipment. It is spread from infected (including symptomless) tubers to healthy tubers by knives used to cut seed. Little spread occurs in the field. Spread in storage results from direct contact between tubers.

In the field symptoms develop slowly, especially early in the season when plants are growing rapidly. Plants show progressive wilting and leaf yellowing, and eventually die. Several adjacent plants in a row may be affected. Vascular tissue in stems is brown and a milky juice can be squeezed out near the mother tuber. Infected tubers may be symptomless or show a cheezy decay of the vascular tissue and, in severe cases, surface cracking.