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New UI Extension bulletin focuses on devastating potato disease, bacterial ring rot

By John O’Connell

University of Idaho

MOSCOW, Idaho – University of Idaho Extension has published a new bulletin to help potato farmers recognize the symptoms of bacterial rot and protect their operations from the devastating crop disease.  

Idaho hasn’t had a ring rot flareup in a few years and the state’s last major outbreak of the disease was in 2012.

UI Extension Seed Potato Specialist Kasia Duellman hopes the bulletin will remind commercial and seed potato farmers to maintain good sanitation practices and remain vigilant for symptoms.

She warns ring rot is highly destructive and tough to eliminate once it gets a foothold on a farm. 

“It can be an existential crisis for a seed potato grower’s business if it is found on their farm,” Duellman said.  

The new UI ring rot bulletin can be found online at:  

Ring rot, which is not a human health issue, is a tuber-borne bacterial disease that can be present in seed while remaining asymptomatic in seed lots throughout several generations of replanting before symptoms surface.

The bacterium produces a protective biofilm that allows it to survive on surfaces in a dormant state for several years, often contaminating crevices in handling equipment, seed cutters, truck beds, machine belts and storage walls.  

The new UI Extension bulletin provides a synopsis of research detailing how 25 different potato varieties exhibit ring rot symptoms in two different growing environments.  

The data comes from a study published in June 2019. The study was led by Jonathan Whitworth, a research plant pathologist with USDA’s Agricultural Research Service, and included Duellman and other scientists from U of I, Idaho Crop Improvement Association and North Dakota State University.  

During two growing seasons, the researchers inoculated seed of each variety with ring rot and planted it in both Idaho and North Dakota.  

“I took the data from that publication and adapted it into a user-friendly table in this bulletin,” Duellman said. “The take-home message is that different potato varieties respond differently to ring rot in different environments.” 

In North Dakota, dwarfing of plant internodes was more common and occurred early in development. The dwarfing symptom was noted in only two varieties grown in Idaho.

Marginal necrosis occurred consistently across varieties in both locations. Yellowing between the veins was also observed at both locations, as was flagging – leaves wilting and drooping from branches.  

The classic tuber symptom of ring rot is a deteriorated vascular ring that secretes a creamy bacterial ooze when squeezed. Tubers may also exhibit growth cracks.  

Ring rot may remain latent for a long time.  

“The reason we worry about it so much is it can go for several generations of increasing in seed without knowing it’s there,” Duellman said. “Symptoms often take several generations of increasing potato to develop. Fortunately, our current testing assays are much more sensitive than a decade ago and they can detect latent infections.” 

Ring rot outbreaks tend to be cyclical, occurring roughly every nine years. Farmers tend to implement strict sanitation practices and follow the proper protocols shortly after an outbreak, thereby controlling the problem.

Too often, however, they may gradually let their guard down, relaxing their sanitation practices in later years to provide an opening for the disease to return. 

Duellman suspects that some outbreaks across the country over the years may be related to re-contaminating seed potatoes with the ring rot pathogen by exposing healthy seed to contaminated surfaces such as handling equipment, storages, seed cutters and trucks. 

“The defense is two-fold: planting seed free of the pathogen and practicing meticulous sanitation while remaining diligent,” Duellman said.  

The bulletin advises commercial growers to use certified seed potatoes and avoid cutting or planting infected seed. If a seed lot with suspected infected seed has already been planted, monitor the lot for erratic emergence and carefully collect samples for testing.

Scout fields for foliar and tuber symptoms and submit any symptomatic plants or tubers for diagnostic laboratory testing.

If ring rot is confirmed, the tubers shouldn’t be used for seed but should rather be sold commercially as early market potatoes. Avoid storing the harvest from a tainted field, as decomposing spuds with ring rot may create ideal conditions for further losses through soft rot.  

When handling a commercial crop that is already planted where ring rot has been confirmed, there are a couple of options for harvesting.

In one scenario, it may be optimal to harvest fields with ring rot last to avoid spreading the disease on tainted equipment.

Delaying harvest also allows any infected potatoes to decompose in the field. In this scenario, infected tubers should be picked out before piling into storage.

A second option may be to harvest early, for commercial growers who have access to an early market. This option avoids the risk of storing potatoes with bacterial ring rot and consequently contaminating storage facilities.

A third option is to destroy plants in an infected field with a disc plow, enabling a seed potato grower to avoid contaminating other equipment that is far more difficult to clean and disinfect.

Idaho’s potato seed certification program requires intensive testing of lots for ring rot. Growers must submit samples comprising at least 400 tubers for DNA-based testing, and there’s zero tolerance for any ring rot detection.

A positive test essentially results in the flushing of all seed on a farm. The disease causes tremendous economic loss to a seed potato grower because the seed is no longer eligible for certification. 

The bulletin includes instructions for cleaning and disinfecting potato equipment and storage facilities and links to a 2011 Extension bulletin authored by UI Extension Potato Storage Specialist Nora Olsen and Phil Nolte, Duellman’s predecessor.

The bulletin advises farmers to thoroughly wash equipment to remove dirt. They should then apply a disinfectant, allowing it to remain in contact with surfaces for at least 10 minutes.