The University of Idaho is warning potato growers across the Pacific Northwest to be on the lookout for ring rot.  UI Extension Seed Potato Specialist Kasia Duellman said the tuber-born bacterial disease attacks crop yields, in both commercial and seed potatoes.


A common symptom of ring rot is a deteriorated vascular ring that secretes a creamy bacterial ooze when squeezed, with growth cracks commonly present in tubers.  The bacterium produces a protective biofilm that allows it to survive on surfaces in a dormant state for several years, and can contaminate handling equipment, seed cutters, truck beds, machine belts and storage walls.


She noted the last major ring rot flareup in Idaho occurred in 2012, with outbreaks occurring on average every nine years.  Duellman theorizes that outbreaks occur when contaminated surfaces expose healthy seeds, whether that be through handling equipment, storages, seed cutters and trucks.


“The reason this bulletin is helpful is it helps remind commercial growers and seed growers alike to maintain their vigilant sanitation practices,” Duellman explained. “It also has some new information compiled together that talks about what the symptoms look like in the field. We’re also looking at the symptoms on the tubers after harvest [and] this bulletin highlights some of the things you can see in the field.”


Back in 2019, Research Plant Pathologist with USDA’s Agricultural Research Service, Jonathan Whitworth, led a study with Duellman and other scientists from U of I, Idaho Crop Improvement Association and North Dakota State University.  Research from this study contributed to the work done in this most recent bulletin, which recommends commercial growers to use certified seed potatoes and avoid cutting or planting an infected seed.


Photo: University of Idaho
Photo: University of Idaho


This bulletin lists 25 different potato varieties that exhibited ring rot symptoms in two different growing environments for two seasons, one in Idaho and another in North Dakota.  Both locations saw necrosis, yellowing between the veins, and wilting leaves on “flagging” or upright branches. Meanwhile, seeds in North Dakota repeatedly showed signs of dwarfing, a symptom that was seen in only two seed varieties in Idaho. If a seed lot was already planted with a suspected infected seed, producers should monitor and sample their lots for diagnostic lab testing.


"The seed potato grower, if they get this disease in their seed, it makes the seed ineligible to be certified,” Duellman said. “So they lose out on that investment, especially if they’re an early generation grower, they lose out on that tremendous investment because they cannot sell the tubers as seed.”


If your crop is confirmed for ring rot infection, you can harvest that specific lot last to avoid cross-contaminating equipment, delaying the harvest and letting the infected crops rot; harvesting crops early if possible, removing the risk of contaminating storage facilities; or destroy the crops with a disc plow, which is easier to clean.


For testing, growers would need to submit 400 tubers for DNA testing.  A positive test could flush the rest of their seed, which is stripped of its certification.  The bulletin also instructs farmers to thoroughly wash equipment to remove dirt, apply a disinfectant, and then leave it alone for at least 10 minutes.


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