Pathogen. The ring rot bacterium is extremely infectious. It overwinters mainly in seed tubers, but can survive for up to 2 years as dried slime on cutting knives, storage bins, planting and harvesting equipment, burlap bags, paper and plastic. The bacterium is not killed by freezing.
Disease development. Ring rot spreads primarily from infected seed pieces to healthy tubers during seed cutting and planting. Contaminated seed-cutter knives and pick planters are excellent disseminators of bacterial ring rot because fresh wounds provide a means of entry for the bacteria into the tuber. Ring rot may be spread in the field by direct contact between diseased and healthy plants. The bacteria may also be carried by farm equip- ment or by insects such as the Colorado potato beetle, the potato flea beetle or the green peach aphid.
Symptoms. One of the characteristics of this disease is that the expression of foliar symptoms and the type of symptoms expressed vary depending on the variety. In In hot, dry seasons, ring rot develops rapidly. The first symptoms in the field usually appear on the lower leaves about mid-season.
Zero tolerance control measures are in place against this disease. The seed potato regula- tions in Canada indicate that upon finding bacterial ring rot, all lots under cultivation by the grower are rejected for certification.