A Resistant Russet
June 1, 2009 
Columbia root-knot nematode (CRN) can send shivers up the spine of potato growers in the West. This pest causes tuber injury, generally rendering the potato unmarketable.
When a potato is damaged by CRN, the tuber receives a double whammy, so to speak. The outer skin becomes bumpy, and on the inside, brown spots appear. The outcome, as you can imagine, is not good for the grower.
How do these pests damage potatoes? According to Chuck Brown, a scientist with the Agricultural Research Service Vegetable and Forage Crops Research Unit in Prosser, WA, the nematodes begin by reproducing on the roots.
“When the eggs hatch, the nematodes, called juveniles, penetrate the tubers,” explains Brown. “The females find feeding sites in the tubers and develop an egg mass in the tuber. This makes the tuber look rough on the outside, like it has little warts. When you peel the potato, there are egg masses on the inside. This spoils the potato for both fresh market and processing.”
According to Brown, the biggest problem area for CRN in the U.S. is in the warmer areas of Northwest production, which include western Idaho, the Columbia Basin, Washington, Oregon, and the San Luis Valley of Colorado. Because CRN has different hosts and has a good reproductive quality, it can build up over the course of the season, explains Brown.
Treatment Plans
How do growers treat infested fields? According to Brown, it has generally been with fumigants that typically cost about $300 per acre. “The price to fumigate is getting on up to about 20% of the cost of production,” he says. “As a result, people are interested in having options to not fumigate or fumigate less often.”
Two fumigants currently used in Washington’s Columbia Basin are Vapam (sodium methyldithiocarbamate, Amvac Chemical) and Telone (1,3 dichloropropene, Dow AgroSciences). The use of fumigants gets more complicated if the potato also has corky ring spot, says Brown. If corky ring spot is present, then Telone must be added to the arsenal.
In addition, continues Brown, Vapam is effective against CRN and Verticillum wilt. “So if you also have Verticillum wilt, you may not have the option of omitting Vapam,” he explains. “To reduce Verticillium wilt, growers must have a four-year rotation out of potatoes.”
The Good News
After years of research — 21 to be exact — Agricultural Research Service scientists and others have developed a russet potato, PA99N82-4, that has resistance to CRN. Developed from wild potatoes from Mexico, the process involved protoplast fusion, a procedure in which two protoplasts are fused together so they can grow into a hybrid organism. The development process involved taking cells from the leaves of wild potato plants and fusing them together with cells of leaves of cultivated potatoes, explains Brown.
“After we made the fusion, we had to do something called ‘backcrossing’ to eliminate the traits we didn’t want,” he says. “We had to do that five times. Each time we did backcrossing, we were able to get rid of more of the wild potato genes. Our first goal was to create a potato for the processing industry, so we were backcrossing with potatoes of that type.”
Brown also has hopes for the potato in the fresh market arena. “It is delicious,” he says. “That makes me think that it will have a place in the fresh market. A lot of time what you wind up with is a bitter taste when you are working with wild species. This potato, however, doesn’t have any bitter flavor.”
In addition to its resistance against CRN, Brown says the potato has moderate resistance to black dot, powdery scab, and corky ring spot. It also has some resistance to pink rot.
This year marks the fourth year of regional trials for PA99N82-4. “I think we know a lot about it at this point,” says Brown. “It tends to yield well early on. Last year, it was ranked first and second in various trials. It does have a tendency to be more round than oblong and the average size of the tuber tends to be low.”
Some of the drawbacks include growth cracking in certain locations, but it isn’t too bad, says Brown. “Plenty of fries can be made from those potatoes,” he adds.
The potato also stores well and doesn’t dehydrate or sprout too early. In commercial practice, however, he says growers will still need to use a sprout inhibitor.
When will growers be able to plant this variety? Brown says that is still a couple of years away. Right now, he says the line may be released in 2010. “If it continues to look like it is processing well, then a lot of seed growers will get into the act,” adds Brown.
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