Organic potatoes require special field preparations

Controlling nematodes in organic potatoes is challenging, according to Dan Chin, owner of Wong Potato and an organic potato grower in the Klamath Basin.

The main option for controlling nematodes for organic growers is not to plant in a field that has them, Chin said.

"Obviously, one of the best strategies is to avoid planting potatoes in fields with root knot nematode," said Rob Wilson, center director and farm advisor for the Intermountain Research and Extension Center in Tulelake, adding this is recommended, in particular, for organic producers.

"I think for most cases with the organic producers, they just try to avoid those fields, but obviously in a year like this year, where there's water challenges and water shortages, that may not be an option," Wilson said.

Crop rotation is another method for reducing the number of nematodes in the soil, he said.

There are also some organically approved products on the market that have been shown to suppress nematodes, Wilson said.

"We've done some testing with those," he said, "but the results have been inconsistent."

"In the research trials, we haven't really seen anything that's shown itself to be a viable product," Wilson said, adding some growers have seen benefits and some haven't.

The field testing has been inconclusive and not nearly as effective as the regular nematicides used in conventional production, he said.

Chin is using some of the organic products.

"MeloCon, we've tried it. Majestene seems like it works pretty decent. And there's a new one out with Marrone Bio that I'm going to try," Chin said.

Biofumigation is another option. This practice suppresses soil-borne pests and diseases through the use of plants like mustard. Mustard is from the brassica family, and it produces compounds called glucosinolates.

How it works is, the mustard is planted, then chopped and incorporated into the soil to potentially suppress and kill nematodes in the soil.

Another option is nematode trap crops. Basically, the crop would trigger the nematode and trick it into being a host, then prevent the nematode from completing its life cycle, Wilson said.

There are some nematode trap crops and cover crops that can definitely influence the nematode populations the following year when potatoes are planted, Wilson said.

"We try to do some trap crops like radishes and turnips, and we try to do some crops like rye," Chin said.

Radishes are a trap crop, Chin explained, so the nematodes will go into the radish root and then they won't reproduce. The goal is to get the percentage of nematodes down, he added.

"This year, we actually have a field of winter wheat that froze. We're going to cut it for hay, and then we're going to put radish and peas in it," Chin said, adding the crop will be incorporated back into the soil and hopefully reduce the nematode populations of that field.

"You won't ever kill them all, but you want to try to get your populations down," Chin said.

Planting a cover crop means a loss of revenue from that cash crop, plus the cost of growing the cover crop, Wilson said.

"Those are significant things that producers have to look at," he said.

"Obviously there's the cost of growing the cover crops, so that's always a consideration, and then for many producers, especially in our short growing season, the other question is, what's the alternative crop that could be grown there?" Wilson said.

A producer may consider growing wheat or barley, but the economics currently on those crops aren't making very much money per acre, he said.

One option growers have looked at is growing a cover crop in combination with something that produces nitrogen, so that they could get a fertilizer benefit, Wilson said.

And this has become more attractive with the rising cost of organic fertilizers like chicken manure, Wilson said.

The grower may decide he's better off growing a cover crop and getting the nitrogen benefit from that cover crop, he said.

Root knot nematodes will cause blemishes on potatoes, making them unmarketable in the fresh market and reducing processing quality for chips and fries.

The potatoes will often have bumps, according to Wilson.

"Those bumps are from the nematodes laying eggs below the skin of the potato," he explained, "and that really reduces the quality of the potato."

"Root knot nematode has been a common nematode pest that's prevalent in a lot of soils, especially soils where potatoes have been grown historically," Wilson said.

Wilson has been looking at doing research on nematodes, but he hasn't done so because he doesn't have Columbia root knot nematodes at high enough populations to evaluate how well they work.

A couple factors make it difficult to conduct nematode research. First, it's difficult to find fields that have high enough nematode populations to do good quality research, and those fields that typically have high nematode populations have a lot of damage, Wilson said.

"I've been reluctant to infest a field here at the research center because we do a lot of variety work," he said, "so it's challenging from a research perspective."

Another factor is water availability. Wilson planned to do a nematode trial this year, but couldn't do the trial because the field didn't have irrigation.

"It can be very difficult to do research projects when we don't know whether or not we will have water," Wilson said.

"In conventional potatoes, the major way that we manage nematodes is with fumigants or nematicides that are put on at the time of planting," Wilson said, "and this has worked pretty well."

The fumigants have become more expensive, and they are more limited in availability, but there have been quite a few nematicides come on the market, and some newer ones more recently that are effective, Wilson said.

Most growers test fields before planting to know what the nematode level is in the soil, Wilson said.

(Kathy Coatney is a reporter in Bend, Ore. She may be contacted at kacoatney@gmail.com.)