Symptoms and control methods of potato ring rot

This disease is a bacterial vascular disease, characterized by stunted and wilted aboveground stems and leaves, and ring-shaped rot of underground tubers along the vascular ring. Early and severe infection leads to dead seedlings; most diseased plants grow slowly, are stunted and thin with few branches and small leaves, and the leaves are wrinkled and do not expand; when infection occurs late and is mild, plant height, leaves, and branching grow normally, but the top leaves become smaller, and later one or two branches or the whole plant wilt. Symptoms are most obvious after flowering, with the lower leaves first wilting, drooping, and dying; the upper leaves roll inward from the edges, appear dehydrated, turn grayish green, and then gradually turn yellow from the leaf margin toward the center or along the veins, and finally the yellowed areas die. Near harvest, diseased plants die prematurely, but the dead leaves mostly do not fall off and the stems remain green. When the stem is cut open, the vascular bundles at the stem base turn yellowish brown, and when squeezed by hand, milky white sticky bacterial ooze can overflow. Symptoms are not easily seen in tubers at harvest, with only slight reddish brown at the tail end. After storage, the tuber skin color darkens, and the tail end becomes distinctly reddish brown. When the tuber is cut open, the subcutaneous vascular ring can be seen turning light brown. When a severely diseased tuber is squeezed by hand, yellow bacterial ooze can overflow. In severe cases, the cortex and pith separate, and the skin bursts. The pathogen is Clavibacter michiganensis subsp. sepedonicus. The bacterial cells are short rod-shaped, without flagella, single or occasionally in pairs, and do not form capsules or spores. Gram stain reaction is positive. On culture medium, colonies are white, thin and transparent, and glossy.