PI-009: Seed Potato Tuber Inspection
Appendix 13: Bacterial Ring Rot

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Bacterial Ring Rot (BRR) is caused by the pathogen Clavibacter michiganensis subsp. sepedonicus and is one of the most serious regulated non-quarantine diseases of potatoes in Canada. The BRR pathogen is highly infectious and is readily spread by potato cutters, planters, harvesters and even containers. It is one of the condition defects where a zero tolerance is required at all times.

Tubers expressing BRR symptoms sometimes have darkened areas under the skin and may show surface cracks. However, depending on the timing of the infection, growing conditions, and the variety, infected tubers may appear symptomless. When the tuber is cut across the stem-end, a creamy-yellow to light-brown rot may be visible in the vascular ring; the rot is crumbly to cheesy and odourless. When a cut tuber is squeezed between the thumb and fingers, a cheesy substance sometimes oozes from the affected part of the ring. Often infected tubers are affected by a secondary invasion of soft rot organisms. In some cases, the soft rot organisms may cause complete tuber flesh disintegration, leaving only the outer skin of the tuber. Such hollow tubers are most often found in the field. Most of the severely affected tubers do not reach storage and infections would be identified in the field, but there are occasions when infected tubers are found in storage. If conditions and temperature are properly maintained during storage, little change takes place in the development of the disease; BRR can advance rapidly if storage conditions change and become more ideal for disease development.


Since there is a zero tolerance for BRR, if presence of this pathogen is suspected on a seed producing farm unit, growers must contact their local CFIA office to allow proper follow-up procedures. CFIA inspectors will submit suspect samples to the Charlottetown Laboratory – Potato Diseases, Charlottetown, PEI for confirmation. All lots on the seed farm unit must be held under detention until results from the lab confirm or deny the presence of the pathogen. The confirmation of the presence of this pathogen will lead to the revocation of certification for all the seed lots produced on the farm unit and a full investigation as detailed in the policy directive D-95-18 Seed Potato Certification Program - Investigation Procedure after Clavibacter Michiganensis Subsp. Sepedonicus Has Been Detected on a Seed Potato Farming Unit will be carried out.

Symptoms may include:

97 | Creamy yellow to light brown decay in vascular ring
97 | Creamy yellow to light. Description follows.
Description for 97

This image of a potato demonstrates bacterial ring rot. This image is of a potato cut in half and held up by a hand. It is a creamy yellow colour with a light and dark brown ring that occurs around 0.5mm from the edge of the potato.

98 | Cavities surrounded by corke tissue
98 | Cavities surrounded by cork. Description follows.
Description for 98

This image of a potato demonstrates bacterial ring rot. This image is of a slice of a potato against a black background. The potato is a creamy-yellow colour with several spots of decay appearing as an orange-brown colour. The rot appears as several cavities both on the edge of the top right side of the potato, and some towards the centre. The rot takes the form of cavities with its edges appearing as orange-brown cork-like tissue.

99 | External Tuber Cracks
99 | External Tuber Cracks. Description follows.
Description for 99

This image of a potato demonstrates bacterial ring rot. In this image the rot is shown through external tuber cracks that have occurred due to the ring rot inside the potato. On the top left side of the potato there is a modest crack which takes up 5% of the surface area that is being viewed, is a grey-brown colour along its edge, with an orange-brown colour inside. On the bottom left corner of the potato there is a small thin crack which takes up 5% of the viewed surface area, and is a dark-brown colour. The potato overall is an almost golden yellow colour, and is covered in different sizes of grey-brown spots.

Photographs courtesy of Dr. Solke H DeBoer, Canadian Food Inspection Agency

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