Potatoes to the Rescue

(Steve is busy this week planting potatoes, and Jeanne’s off to the dentist for a root canal, so we’re rerunning this article on potatoes from 2009.)

Among the hundreds of pests and diseases that make organic farmers regularly consider changing careers, perhaps the worst of the worst is the garden symphylan. These soil dwelling, root-feeding critters are no more than ¼ of an inch long and have the appearance of an albino centipede. What makes them such a vile pest is that the classic practices of good organic soil stewardship—cover-cropping, reducing tillage, and adding compost—create the ideal conditions for them. Symphylans love loose soils, rich in organic matter. They feed on decaying plant matter and the roots of nearly every type of crop we grow.

Stories abound of massive crop losses and fields so badly infested that ultimately they had to be abandoned. Infested fields commonly have localized circular areas where crops are so stunted that they never grow out of the seedling stage. These areas increase in size and number year after year. And while conventional growers fumigate or apply chemical soil drenches to obtain control, organic growers have few treatment options.

It was quite a blow when I first discovered symphylans in one of our fields at the Redman House site. I had attributed weak spots in the strawberry patch that had occupied the same field the season prior to verticilium wilt, a fungal disease that attacks a plants roots and vascular system. When broccoli and cauliflower transplants in those same spots failed to grow out of the transplant stage and eventually withered and died, I knew something else was wrong. When I pulled up the root ball of one of these stunted plants and shook it out on my palm, several tiny white centipede-like critters scurried about. I didn’t need to look them up in a book—I knew exactly what they were.

One person who has seen a lot more symphylans scurry across his palm than I have is Jim Leap, the Farm Manager at the Center for Agroecology and Sustainable Food Systems, where they train apprentices from around the globe and carry out numerous research projects. After decades of cover-crops and heavy compost applications, the soils at the center were very high in organic matter and symphylans became a serious problem. After trials with numerous organic control options proved totally inconclusive, Jim began to notice that in the fields that had been planted to potatoes during the previous season the symphylan populations were greatly reduced. In time, by monitoring and charting symphylans throughout their farm, they were able to document that a potato rotation does in fact reliably take symphylan populations way down.

Now as it turns out, we grow potatoes. Nearly for as long as I have been farming, potatoes have been one of my favorite crops. If I time my cultivation and hilling passes correctly, they require little if any hand labor. And in the deep silty-clay soils at the Redman site, they literally grow like weeds. They are also one of my favorite foods and I’ve always known that a well grown new potato of some great heirloom variety is something that our CSA customers could find in few other places.

Potatoes haven’t been grown on a commercial scale here in the Pajaro Valley for a long time, and as small as we are, the 4,000 lbs or so of seed potato that we planted this year probably makes us one of the biggest potato growers in the county. So when my friend Joji Muramoto, who is a researcher who also works out of CASFS, mentioned Jim’s successes with potato rotations, it wasn’t a hard decision to give it a try.

The following spring we planted nearly an acre of five different types of potatoes in the worst hit part of the field. The crop was spectacular—tall, lush and uniform. After we finished harvesting the amazing bounty of potatoes the field produced in mid-July, we worked the field up into beds to plant a fall vegetable crop. In the days after we transplanted out lettuce, cauliflower, cabbage and fennel plants, I watched the field nervously. The last vegetable crop we had planted in that field prior to the potatoes was a disaster—nearly 40% of the plants were stunted and eventually died. To my amazement, after 10 days or so, all of the transplants appeared to be growing uniformly. And in the end we harvested beautiful crops, completely unaffected by symphylans.

This is just another case where growing a diverse range of crops serves us well. If I were a dedicated berry, or lettuce, or artichoke grower who had a symphylan problem, I wouldn’t have had this as an option. Figuring out how to grow potatoes would be one thing, figuring out how to sell them would be a larger problem still. (Commercial potatoes are marketed on a large scale in places where land is cheaper.)

After that initial success, we developed a plan to rotate potatoes through all of the affected areas. We are currently in the second year of that plan and after next year, most of the worst areas will have received the “potato treatment”. According to Jim, the symphylans will eventually re-enter so we will probably rotate the potatoes through every 5th or 6th year on an ongoing basis.

But, how does it work? Well it just so happens that Jim cooperated with a researcher who secured a sizable grant to answer just that question. And their conclusions? Well, they didn’t really have any. They suspect that the potato plant roots exude some sort of a phytotoxin that kills symphylans, but they haven’t been able to isolate it. I guess despite all of our abilities to dissect and analyze, some things in nature will always remain mysterious.