Bacterial Ring Rot (BRR) is an especial major concern in potato seed tuber production. There is a ZERO tolerance for it in seed lots by all certification agencies. This means that, when a single plant with BRR is discovered in a field during a certification inspection, the field is de-certified for seed production. It also means that, when a single potato tuber with BRR is identified in storage during a bin inspection, the entire seed lot is de-certified. De-certification means that none of the tubers in that field or in that storage can be used for seed. This disease is caused by a bacterium called Clavibacter michiganensis (formerly called Corynebacterium sepedonicum). It does not survive well in soil but can survive for long periods even during severe winters in exposed potato sacks and other debris. It can be transported from field to field and from storage to storage on clothing, shoes, car tires, farm tractors, and potato equipment. It can spread through a field rapidly and cause complete devastation in a short time. It can survive on walls, floors, and ceilings. This disease is the number one reason for sanitizing equipment and facilities. Once BRR is detected in storage, that facility, even after thoroughly sanitized, should not be used for potato seed for at least three years during which no BRR has been detected ("flushing-out"). This disease is responsible for many potato seed producers going out of business. BRR can reduce yield drastically; the potato tubers simply rot to nothing in the ground. It can render a crop totally unmarketable by harvest. Costs for grading and processing increase.

Symptoms - Vine

The earliest symptom is a pale-yellow discoloration of the small veins on leaflets. Top leaves on the vines will show this most. Lealets may have a slight twist to them. Petioles will be shorter and their density may give a rosette-like appearance. Lower leaflets may be limp. Stems are usually stunted. Later in the season, marginal burning of leaflets may be observed. Stems appear dark green. Leaves wilt and die. Ring rot can be diagnosed preliminarily by an excretion of a creamy or white ooze emanating when the petiole or stem is squeezed. This also occurs when underground stem is cut and squeezed. Not all stems on a plant may show symptoms.


Ring rot on tuber Ring rot tuber cracks
Ring rot on tuber Ring rot tuber cracks

Symptoms - Tuber

Usually only after wilting occurs can tuber symptoms be detected. Infection begins at the stem end and progresses through the vascular ring. A rot with a creamy yellow or light brown develops. A creamy ooze will be released when tubers are squeezed. The skin will form cracks or breaks due to internal pressure (pictures). Secondary infections will occur causing a complete breakdown of the tuber.


Laboratory test are necessary to verify ring rot infection. The quickest (minutes) and easiest method is gram staining. The ring rot bacteria is unusual in potato-infecting bacteria in that it stains blue (gram-positive) while others such as those causing soft rot stain red (gram-negative). Several confirmation tests that are a little more involved can be conducted. The most common is ELISA; see Diseases / Viruses and More / ELISA Testing.


The only way to control BRR is not to have it, therefore, store and plant only Certified seed! Sanitizing facility and equipment is an absolute MUST. Not all infected seed tubers will show symptoms as not all cultivars will show vine symptoms. Seed cutters are notorious for spreading BRR from tuber to tuber; knives should be sanitized repeatedly and often, at minimum between seed lots. Volunteer potatoes growing in fields that previously had BRR must be destroyed.


  1. Plant Certified Seed.
  2. Sanitize Equipment.
  3. Sanitize Storage Facilities.
  4. Destroy Old Potato Sacks.
  5. Destroy Volunteer Potato Plants in Previously Infected Fields.
  6. Separate Seed Lots.
  7. Keep to a Minimum of a Three-Year Rotation.

Suggested Reference:

"Compendium of Potato Diseases" 2nd Edition. 2001. Eds. Stevenson, W.R., Loria, R., Franc, G.D., and Weingartner, D.P., Publ. Amer. Phypathological Soc. Press, St. Paul, MN.