MERCED, Calif. — Planting 25 percent of their acreage each year with new, virus-tested seed means sweet potato growers can have fewer culls and realize higher yield potential, especially for the California Beauregard variety, says a Merced County farm advisor.

Kent Stoddard made that recommendation and revealed results of his 2004 collaborators trials during a gathering of sweet potato growers here recently.

His continuing test plots, measuring varieties typically grown in California with those from elsewhere in the U.S., are part of the annual National Sweet Potato Collaborators Trials.

Beauregard, developed and released by the Louisiana Agricultural Experiment Station, has been grown in California since 1987. Although it produces well and has good shape and other traits, it is vulnerable to rootknot nematodes and russet-crack virus.

Stoddard has been searching for new varieties that will have the yield and appearance of Beauregard, plus resistance to the pests.

Until a replacement is found, he is urging growers to use new seed that has been tested and found free of the virus. As documented by research in California and other producing states, more culls are created on seed from successive, infected generations.

G-0 seed, for example, is a plant coming directly from virus testing and having never been planted in the field, and G-1 seed is from plants after one season in the field.

Stoddard said if G-2 seed has an 8 percent loss one year, the loss can escalate to 18 percent the following year. Later, somewhere between the third and sixth seasons, the yield decline tends to flatten out, and at that point growers may not realize they are still sacrificing the true yield potential.

Seed turnover

Growers, he said, if they are not already doing it, should incorporate a 25 percent turnover of new seed every year for Beauregard plantings.

"Without virus-free seed, you lose yield potential rapidly in the first three years after a virus-testing program. You might be getting by with seed you’ve saved for years, but if you plant some G-1 seed with it for a side-by-side comparison, you will see fewer culls and more production with the G-1."

In his 2004 trials with 13 lines or varieties, Stoddard said California Beauregard was the foremost yielder with more than 1,200, 40-pound boxes per acre, including more than 700 boxes of US No. 1s, about 200 boxes of canners, and about 300 boxes of jumbos. He credited the high yields to a good stand with G-1 seed.

The second highest yield was about 900 boxes from G-2 seed of original Beauregard. Other Beauregard selections planted with G-2 and G-3 seed and entrants from South Carolina and Mississippi failed to yield 800 boxes.

He also evaluated characteristics of a half-dozen other numbered varieties for which he did not have yield statistics, due to problems with replicating plants and establishing trials. Notable were the Beauregard types, NC-98-608 and L-99-35.

NC-98-608, a patented variety expected to be released as "Covington" to North Carolina growers, showed good flavor and sweetness, plus Fusarium resistance and moderate resistance to rootknot nematode and pox.

Excellent baker

L-99-35 from Louisiana has resistances similar to NC-98-608 and has excellent baking qualities.

Stoddard said another Louisiana variety likely to be of interest to California growers is L-01-29, a so-called "Japanese" type having red skin and white flesh. It has resistance to Fusarium and rootknot nematodes and moderate resistance to pox.

"L-01-29 looks almost like a Beauregard and may be of interest to growers who want something with eating qualities like Koto Buki but with better disease and nematode resistance," he said.

Stoddard also reported good results from his other trials with the contact herbicide Scythe, a pelargaonic acid product from Dow AgroScience labeled for use in California.

He found that a 3 percent concentration of Scythe, plus a 0.4 percent concentration of Roundup Ultra Max, which, he noted, is also registered in California for sweet potatoes, worked well on crabgrass and other weeds in a Garnet plot near Stevinson.

The spray was made at vining in a band on bed middles over the drip tape. Although some crop phytotoxicity occurred, it was temporary.

"Scythe," he said, "burns weeds back - results are seen in just a few hours – but it has no residual control. When combined with a little Roundup, weed control was excellent, giving season-long control from only one application."

Stoddard reminded growers to follow labels for both herbicides.

Fumigants ‘crunch’

Tom Trout, USDA-ARS agricultural engineer at Parlier, said the sweet potato industry has come to a "crunch time" for fumigants with the phase-out of methyl bromide and as Telone’s local use limitations or "township caps" are reached. Meanwhile, chloropicrin and metam sodium are in re-registration.

One new material in the California registration process being watched, he said, is iodomethane, marketed by Arvesta as Midas and typically used in combination with chloropicrin.

"It’s the closest surrogate to methyl bromide we have, but it is expensive to manufacture. So it will likely be too expensive for sweet potato growing fields, although it might be an alternative for hotbeds."

Some other compounds under evaluation by Trout and his USDA colleagues, including sodium azide, propylene oxide, furfural, and DMDS, may eventually find niche uses, but none is expected to fit broad purposes.

"The reality is," he said, "we started looking for alternatives to methyl bromide 10 years ago and today we have the same compounds we started with, with the exception of iodomethane."

Trout’s research continues with application of fumigants to sweet potatoes by drip irrigation. "Some of the materials are either sufficiently soluble or can be mixed with an emulsifier to put them through drip systems. For bedded crops like strawberries, melons, and peppers, this can be a very cost-effective method, and it is catching on with those crops."

Wetting limit

However, for sweet potatoes on sandy-textured soils in Merced County, Trout said more than one drip line per bed would be needed to sufficiently wet the soil for fumigant application. "Even on sandy loam soils we can only move water about 12 inches laterally, and for sandy soils it is even less."

Surface drip applications would have to be tarped with plastic, and that would be an added expense of $300 to $600 an acre for sweet potatoes. Buried drip is not considered practical for sweet potatoes because of root intrusion into the lines.