‘Sweet’ (potato) science

  • Laurel Smith / The Garden Island Master gardener James Keach stands inside the the University of Hawai‘i College of Tropical Agriculture and Human Resources Wailua substation greenhouse. He is part of a team conducting sweet-potato research to identify pest-resistant varietals that will do well in Kaua‘i growing condition.

  • Laurel Smith / The Garden Island Entomologist Roshan Manandhar at the University of Hawai‘i College of Tropical Agriculture and Human Resources Wailua substation describes the different varieties of sweet-potato seedlings, ranging from historic canoe crops to ornamentals. The seedlings are grown to preserve cuttings that will be later transferred to the field.

  • Laurel Smith / The Garden Island Master gardener James Keach unearths a small, sweet-potato tuber. Under current conditions, the tubers will remain small. The plants are kept in the greenhouse to preserve cuttings that will be later transferred to the field.

  • Laurel Smith / The Garden Island Three weevils pulled from a trap at the University of Hawai‘i College of Tropical Agriculture and Human Resources Wailua substation sweet-potato research field are seen last week. When the potatoes reach peak season researchers expect hundreds of these destructive pests to fill the trap each week.

  • Laurel Smith / The Garden Island The University of Hawai‘i College of Tropical Agriculture and Human Resources Wailua substation is growing and monitoring 20 varieties of sweet potato in hopes of identifying pest-resistant varietals that could help farmers improve sweet-potato production on Kaua‘i.

WAILUA — A handful of experts are growing sweet potatoes on the Eastside to identify pest-resistant varietals that could improve production of the classic Hawaiian crop.

The years-long project is conducted by University of Hawai‘i College of Tropical Agriculture and Human Resources (CTAHR) personnel at a 260-acre substation on Kuamo‘o Road in Wailua Homesteads, near the Wailua Reservoir.

Sweet potatoes grown in Hawai‘i, they explained last week, are plagued by a trifecta of pestilential insects: the sweet potato weevil (Cylas formicarius [Fabricus]); the rough sweet-potato weevil (Blosyrus asellus [Oliver]); and the reniform nematode (Rotylenchus reniformis), a microscopic worm.

“The main problem in sweet potatoes is the reniform nematode,” Roshan Manandhar, an entomologist, said. “When you grow for the first time in virgin land, there aren’t many problems. In the second year, you’ll get some problems, maybe 30% symptoms. In the third year, when you plant sweet potato in the same field, it’s like 70% reniform nematode damage.”

Infected sweet potatoes become deformed, splitting or developing clefts. These irregularities cause the market value of infected yields — called “slipper sweet potatoes” — to drop, even though the crop remains edible.

Rough weevils produce aesthetic damage as well, leaving fingernail-like scratches on tubers’ skin, while regular weevils render sweet potatoes inedible.

“It tastes terrible, really, really bitter and bad,” James Keach, who is the coordinator of the master gardener program, said.

These insects are not unique to Hawai‘i. But the islands have an especially strong relationship with the crop they damage, due to their history and geography.

The first sweet potatoes were canoe crops introduced by Polynesian settlers. Up to 100 Native Hawaiian varietals, at least one of which is sacred, have since been documented, according to Keach.

Far more recently, the white-skinned, purple-fleshed Okinawan sweet potato has claimed ascendancy in Hawai‘i.

“The reason that Okinawan is so important is we’re one of the few places on Earth that can grow it really well and get good quality,” Keach said.

“Talking with commercial sweet-potato growers, they’ve tried sometimes on different islands and the US mainland. They just can’t get the right seasons and conditions to get the quality that we get here.”

Indeed, the majority of Hawai‘i’s Okinawan sweet potatoes are sold to the mainland. But farmers have to jump through hoops to make those sales: all sweet potatoes must be irradiated before leaving the state to kill stowaway weevils.

Costs associated with the irradiation process compound the other financial losses brought on by pests. That’s why Manandhar, Keach and colleague Emilie Kirk have cultivated, harvested and analyzed 48 varieties of sweet potato since 2019. They want to identify tubers that produce high yields in the face of weevil and nematode infestations without sacrificing good looks and tastiness.

Each sweet potato is graded using a state-designated system based on specimens’ size and weight, and CTAHR staff’s judgement of shape.

“Shape is really subjective. If you think of a stereotypical sweet potato in your mind, that’s what we want it to look like,” Keach said. “We’ll end up, otherwise, with ones that are kind of lumpy, really weird. We had one that looked like the Venus of Willendorf.”

The CTAHR team is now down to 20 varietals, planted two months ago. The crops will be harvested and graded four to six months from now.

Eventually, Keach will engage in old-fashioned plant-breeding, cross-pollinating the highest-rated varietals by hand. He will search for something that looks and tastes like an Okinawan sweet potato without that varietal’s susceptibility to pests.

This sought-after plant would reduce sweet-potato farmers’ need of pesticides and herbicides.

“If we have a sweet-potato cultivar that, on its own, can put up defenses to resist a pest or insect, then we don’t have to apply it, for one thing,” Keach explained. “So it’s less labor. There’s no chemical runoff, so it’s not affecting the ecosystem. And it’s something that anyone can use. It’s a kind of parity: anyone who has access to the cultivar has access to that resistance.”

Only one big farm here

“Everybody has a little bit of sweet potato here,” Manandhar said of Kaua‘i and its farmers.

But the island has only one commercial sweet-potato farm, operated by Taiwan and Lu Gu in Kapaia. There, they have grown 100 to 200 acres of Okinawan sweet potatoes per year since 2015, when they moved from their farm on Hawai‘i Island.

“We supply local first,” Lu Gu said in a recent interview. “Whatever is leftover (about 80%) goes to the mainland.”

Taiwan Gu testified to the destructive power of weevils and the nematode. The latter’s is exacerbated by sweet-potatoes’ nature as a long-duration crop: a nematode population can increase rapidly by completing multiple lifecycles during infected tubers’ several-months-long growing season. The nematode-ridden soil is then primed for further population growth when new sweet-potato cuttings are planted in the same space.

“It’s a very real problem for us,” Taiwan said.

The Gus believe a new varietal developed by the Kaua‘i CTAHR team could greatly benefit their farm.

“This is what we’ve been looking for, I think,” Lu Gu said, noting the hoped-for plant must taste like an Okinawan and be of high quality overall in addition to pest-resistant. “It would protect the environment, using as few chemicals as possible.”

But the Gus also claimed a hardy Okinawan lookalike, while valuable, is not a comprehensive solution to sustainable pest control. They are searching for more-arable land to practice crop rotation on a greater scale. This time-honored method allows some fields to rejuvenate by lying fallow while others are farmed.

“The best time for the land to get rest is about three to five years,” Lu Gu said. “It’s good for both the environment and for the potatoes — for anything you grow.”

This article was updated at 2:40 p.m. on Monday, Nov. 22, 2021, to reflect that James Keach is the coordinator of the master gardener program, not a master gardener.

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Scott Yunker, reporter, can be reached at 245-0437 or syunker@thegardenisland.com.

3 Comments
  1. mark November 22, 2021 5:05 am Reply

    How are they developing the sweet potato- using a hybrid option or are they using GMO options? Article fails to mention this……


    1. Reed Gud November 22, 2021 3:39 pm Reply

      Do people read the articles before asking questions?

      “Eventually, Keach will engage in old-fashioned plant-breeding, cross-pollinating the highest-rated varietals by hand. He will search for something that looks and tastes like an Okinawan sweet potato without that varietal’s susceptibility to pests.”

      Or do you just not know anything about GMO?


  2. Rev Dr Malama November 22, 2021 2:20 pm Reply

    Glad to read about crop rotation and may I mention the companion plants that rid the soil of nematodes and other pests? Mustard is a NATURAL fumigation for soils and we love to eat the greens, the greens of the sweet potato is superior to kalo and spinach because it doesn’t contain the chemistry that causes kidney problems and sweet potato greens are great for Laulau or as a spinach substitute with the hint of the tubers taste.
    Hopefully we can create more organic, sustainable solutions to the ever present challenges of farming in Hawai’i and the world.


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