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Intensified nationwide testing under consideration

Potato growers in Idaho are in for “a lot more of the same kind of activities” as they enter the 2007 growing season. But growers across the United States may get a greater share of the scrutiny as APHIS both “homes in” and “zooms out” in its pursuit of the potato cyst nematode (PCN).

That assessment was made by Wayne Hoffman, assistant to the director of the Idaho State Department of Agriculture, who has been working on the state side of its cooperative program with USDA’s Animal and Plant Health Inspection Service (APHIS).
Larry Hawkins, the APHIS public information officer working with PCN, said last summer APHIS proposed a program to intensify surveying and expand it beyond Idaho to other states and potato production areas. APHIS proposed that 100 percent of the seed potato fields and 10 percent of the production fields across the United States be surveyed over the next two years, and asked the American potato industry and state departments of agriculture to participate in the national survey.

John Keeling, executive director of the National Potato Council, said growers are currently weighing the value of participating in such a national survey. Decisions will likely be made on a state-by-state basis, since participation is voluntary.

“We think, all in all, it would be of tremendous value to the whole industry if we had the data,” Keeling said.

But, he added, there is risk involved to growers. If either pale or golden cyst nematodes are found, the procedure that follows is quarantine, restriction on movement of people, potatoes and machines, and the loss of fields to future potato production. There is currently no indemnity program.

APHIS conducts, with states, Cooperative Agricultural Pest Surveys looking for evidence of exotic pests that have slipped into the country. It was that routine sampling that found the pale cyst nematode in the area around Shelley, Idaho, last April, Hawkins said.

Finding the pale cyst nematode there caused APHIS to home in on the area with a quarantine and intensified sampling, he said, but it also provided a warning that similar areas might exist elsewhere. That is why APHIS proposed intensifying beyond its routine sampling to see if additional pockets occur elsewhere.

When a regulated pest like potato cyst nematode is found, it is normal for trading partners to assume the worst, Hawkins said. Canada, Mexico and Japan all responded to the finding last April in the small area of Idaho by stopping imports from that area – and from a much bigger surrounding area as well. Canada and Mexico stopped imports from the entire state of Idaho, and Japan cut off imports from the entire United States.

It’s not just foreign countries that react that way, he said. Growers and processors in other states also become concerned about accepting Idaho potatoes for processing or planting in their area.

The way out is testing to prove you aren’t contaminated, he said.

“Testing shrinks the regulated area,” Hawkins said. “It tells our trading partners that they don’t need to quarantine the entire United States or the entire state of Idaho. Our goals are to determine the exact distribution of PCN and confine it where it is now.”

Since last summer, sign-up sheets have been available to growers who want to volunteer to have their farms tested.

“Growers should be assured that there is a long-term benefit for them to participate in the sampling process,” Hawkins said. “Every negative test serves to restrict the size of the area that might be suspect.”

Traceback, unfortunately, has found nothing. If the source of the nematodes in the Shelley area could be found, that would be a valuable clue telling searchers where else to look for them.

“It’s very seldom that we find the source,” Hawkins said.

The nematode may have come into Idaho with machinery imported from Europe or on the roots of a plant, perhaps not even a host plant. The time lag between infestation and discovery can be years.

The finding in the Shelley area is somewhat like an unlucky lightning strike and is not predictive of anything. Growers shouldn’t assume PCN won’t be found anywhere else, but they should not fear it will be found everywhere else, either. Testing goes on all the time.

George Bird, a Michigan State University nematologist, said the Diagnostic Services laboratory at MSU annually processes 30 to 50 soil samples from Michigan seed potato fields for the Michigan Department of Agriculture. In addition, soil samples from approximately 10,000 to 12,000 acres of Michigan potato land are processed annually for potato farmers and consultants seeking recommendations related to the potato early die disease complex.

“Based on the cyst expertise of the MSU nematode diagnosticians, PCN would be immediately recognized if it was present in these samples,” he said. “The samples represent a significant portion of the Michigan seed potato industry and about 20 percent of the Michigan potato tablestock and processing industry.”

On the other hand, being sure of finding nematodes requires sophisticated sampling. These are not the usual soil samples.

The protocol APHIS recommends is to choose fields randomly, and then choose a 1-acre parcel in each field. On that chosen acre, a mechanical wheel sampler would be used to pull many small cores – 112 cores per sample, three samples per acre, about five pounds of soil per sample.

That procedure, APHIS says, will detect PCN with 95 percent confidence in a field where there are 200,000 cysts per acre.

The Two-Cyst Discovery

The potato cyst nematode adventure started April 13, when employees of the Idaho State Department of Agriculture (ISDA), doing routine survey work, got the results from a soil sample taken from tare soil in a grading plant in Blackfoot, Idaho. Two cysts of the pale cyst nematode, never before found in the United States, were discovered. The knitting unraveled from there.

Information describing the whole process can be found at http://www.aphis.usda.gov/ppq/ispm/potato/pcn.html
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Following the usual protocol, trading partners were informed, and they responded. The most extreme response came from Japan, a country where exports had only begun two months earlier.

APHIS and ISDA began work to trace the cysts and the tare soil to their field of origin. By Nov. 12, a total of 33,042 soil samples had been collected from 777 fields and 56 storage and packaging facilities.

Samples from seven fields have tested positive. All of the potato fields within a five-mile radius, about 10,000 acres, have been placed under a regulatory protocol restricting movement of potatoes and equipment. Tightly controlled arrangements have been made for processing of potatoes grown in the area.

Soil samples are forwarded to the ISDA Food Quality Assurance Laboratory in Twin Falls or the University of Idaho Nematology Laboratory in Parma for the initial screening. Presumptive PCN positive samples are forwarded to the ARS Nematology Laboratory in Beltsville, Md., for confirmation.

Except for samples from the seven positive fields within the regulated area, all other soil samples have been determined not to contain PCN.

The University of Idaho Nematology Laboratory in Parma has also processed an additional 3,500 samples collected through an expanded survey program extending across Idaho. Except for the initial positive sample from the ISDA grading facility, all other samples have been found to be free of PCN.

Traceback investigations to determine the source of PCN in the confirmed positive fields continue. Farming practices are being evaluated, seed sources tracked, tillage and harvesting equipment and their movement checked. All fields directly associated with the confirmed positive fields have been identified.

During November, 105 on-site personnel (21 ISDA and 83 APHIS) played various roles within the Incident Command (ICS) structure. Unified Command (UC) continued to implement the delimiting survey associated with the confirmed positive field and associated fields and continues to implement the federal order and state rule restricting the movement of regulated articles from the regulated area.

One field in the regulated area is currently under a Federal Emergency Action Notice (EAN) where no potatoes are to be planted, no soil is to leave and equipment must be cleaned before it is moved away. The current potato crop was moved under a Limited Permit at harvest and tare soil had to be buried according to approved processes. Also under EAN are two storage cellars receiving potatoes from untested fields in the regulated area.

A total of 380 limited permits were issued by mid-November for movement of potatoes from or within the regulated area. A total of 277 certificates were issued for movement of cleaned farm equipment. Steam treatment and pressure washing of farm equipment from infested fields continued.

Eighty compliance agreements were signed by potato growers, shippers, processors or handlers involved with potatoes in the regulated area.

Impact on Growers

None of the growers in the regulated area or those who own the infested fields have been identified by either federal or state agencies. But their futures as growers will be affected.

Some of their land will no longer be available to plant to potatoes, and growers with infested land will have be extremely watchful so that soil from infested fields does not move into production areas. Keith Esplin, the executive secretary of the Potato Growers of Idaho, said that the seven fields found to contain PCN are owned and operated by several growers, are rather small fields and four of the them were rented, not owned, by growers.

In the area, which is irrigated by sprinkler systems – mostly center pivot – growers plant potatoes once every two or three years, usually rotating with wheat but sometimes alfalfa or barley.

The nematode itself is virtually immobile in soil, moving only a few millimeters unless carried in soil or on potato tissue. The nematode does not burrow into potato tubers, but it can be carried on soil on tubers or in plant roots. The microscopic worms may number 100,000 or more in a pound of soil, and each cyst is a sack containing from 200 to 600 eggs ready to hatch at a chemical signal from a growing host plant. Without hosts, the nematodes gradually decline in number, but cysts can persist for 30 years awaiting a host, according to information on the APHIS Web site.

Because the nematode doesn’t move unless moved, good places to look for it are at entry points in fields, in headlands and places where tillage or harvest by equipment would logically start.

The APHIS protocols in regulated areas include:

Treatment of infected fields with nematicides.

No planting of potatoes or host plants in infected fields.

Movement of potatoes from infected fields is regulated and compliance includes washing of potatoes for processing, burial of tare soil and proper disposal of wash water.

Movement of equipment is restricted. Tillage and harvest equipment must be cleaned before leaving the area.
Non-host crops may be similarly restricted, Hawkins said. Hay, for example, should not be handled in ways that transfer dirt or mud. It should be stacked on tarps or grassed areas.

Fields in the regulated area that are currently growing non-host crops remain suspect until tested. When potatoes are grown there nematodes, if present, would begin to build in numbers to where they would become detectable.

Even non-host plants can be carriers of nematodes or cysts in soil. Movement of trees and nursery stock, for example, will be restricted in the regulated area.

A Serious Pest

The potato cyst nematode is a major pest of potato crops in cool-temperate areas, and is recognized as one of the most difficult potato pests to control, according to APHIS.

The potato cyst nematode is thought to have originated in Peru and is now widely distributed in many potato-growing regions of the world. The pale cyst nematode, the one found in Idaho, infests Western Europe. The golden cyst nematode infests Eastern Europe, and both have been found elsewhere. Growers in those countries must manage around the pest and have lost their export markets.

In Europe, potato growers work in an environment where potato cyst nematodes put constant pressure on their crop – and the world may ban their potatoes for fear of spreading nematode problems.

Growers in nematode-infested areas must use crop rotations of seven or more years between potato crops and use nematicides and resistant cultivars. Resistant cultivars keep levels of golden cyst nematode below economic thresholds, but few potato varieties have resistance to pale cyst nematode.

In the United States and Canada, both golden and pale cyst nematodes are quarantine species. In North America, pockets of these nematodes were confirmed in Long Island, Newfoundland, Vancouver Island and western New York in 1942, 1962, 1965 and 1967, respectively. Pale cyst nematode was found in Idaho in 2006.

The potato cyst nematode has a narrow host range that includes potatoes, tomatoes, eggplant and some weeds, especially nightshade.

The symptoms that potatoes show when attacked by potato cyst nematodes are not specific. Patches of poor growth generally occur in the crop, sometimes with yellowing, wilting or death of foliage. Even with minor symptoms on the foliage, tuber size can be affected.

PCN control is accomplished by planting non-host crops for several years. Nematicides can help reduce nematode populations below economically damaging levels. Planting non-host crops is recommended on land known to be PCN infested.

Economic damage caused by the potato cyst nematode can be severe. If left uncontrolled, these nematodes can cause up to 80 percent yield loss. The introduction of this pest could potentially result in a loss of domestic or foreign markets for U.S.-grown potatoes and other commodities, according to APHIS’ Web site.

Originally posted Saturday, Apr. 7, 2007

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