PCN Update

Strategies pursued by USDA-ARS

Published in the January 2012 Issue Published online: Jan 19, 2012 Roy Navarre, Mike Thornton, Louise-Marie Dandurand
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Recent Potato Cyst Nematode (PCN) developments are worrisome for the U.S. potato industry. In 2006, PCN (Globodera pallida) was found in southern Idaho and the fields were quarantined. In November 2007, another type of PCN was identified in a seed potato shipment from Alberta, Canada, and in 2008 a previously unknown species of Globodera was found in Oregon.

While PCN may be most problematic because of its potential impact on exports, it can also devastate potato crops in untreated fields, causing up to 80 percent yield losses. Infested fields may face restrictions for over 30 years, because cysts can remain dormant and viable in the soil for decades in the absence of any host. The pest can be managed by fumigation, but there is a strong need for alternatives to methyl bromide, which is likely to be phased out in coming years. Because PCN has only been found in a small number of fields, eradication is the goal of the scientific team tackling the issue. The eradication effort is being directed by APHIS and includes scientists from the University of Idaho and the USDA-ARS.

Fumigation has been the primary method used for the eradication effort, but the team is aggressively pursuing additional strategies that if successful would complement the fumigation.

Trap Crop Option

A major strategy being pursued utilizes the concept of a "suicide hatch." The typical new cyst has a few hundred eggs, and while inside a cyst, the eggs are relatively protected. The eggs are stimulated to hatch by "hatching factors" that are secreted into the soil by potato roots. Most plants do not stimulate hatching of PCN; only potatoes and some other Solanaceous plants produce hatching factors. Once the eggs hatch, the emerged juveniles have about a week to find a plant and start feeding, otherwise they will die. If one can make the eggs hatch in the absence of a host, then this is in essence a "suicide hatch" and the nematodes will die.

One way to cause a suicide hatch is to plant a trap crop that stimulates the nematodes to hatch but does not allow them to reproduce. Potatoes can't be used as a trap crop, because no immune varieties are known. We have been evaluating sticky nightshade as a potential trap crop. Sticky nightshade causes the Idaho population of PCN to hatch and no new cysts have been observed in plants grown in the greenhouse and inoculated with Idaho cysts.

Sticky nightshade is a thorny plant sold commercially as a PCN trap crop in Europe. It has no real commercial value, though it can be used as a green manure, and some people make a jam from its berries. We are trying to make sticky nightshade an even more effective trap crop and USDA-ARS scientists in Prosser, Wash., have generated plants that have fewer thorns or more vigorous root growth. Theoretically, a plant with more vigorous root growth would cover more soil, produce more exudate and be a superior trap crop. It is not known how well these plants will grow under Idaho production conditions. A field trial in 2012 is planned near Parma, Idaho, to evaluate these plants.

UI researcher Mike Thornton has evaluated a number of crops under the climatic conditions of western and eastern Idaho. Sticky nightshade tended to produce the most biomass, followed by quinoa and lupine. Of the green manure crops, oilseed radish and mustard/radish blends appeared to be best adapted to Idaho growing conditions.

Green manures may reduce nematode populations by acting as a non-host or causing mortality of cysts after incorporation of plant material into soil. In order to be suitable as green manures, these crops must produce large amounts of plant biomass under local growing conditions, should not pose problems as volunteers, cause odor problems during production or incorporation, or act as hosts to insect pests of local crops.

Ideally, a PCN trap crop would provide an economic return to the grower, but no such crop has been identified that has significant economic value but also stimulates hatching and is immune to PCN. We are screening a large number of non-Solanaceous plants for hatching activity, searching for additional options to sticky nightshade. None of the over-80-such plants evaluated so far have stimulated significant hatching.

Hatching Factor

Another major "suicide hatch" strategy being pursued by USDA-ARS scientists is to identify the compounds that stimulate hatching. Identifying these compounds would create new opportunities to eradicate PCN and understand how potatoes make these compounds. Hatching factors could potentially be produced in large amounts and used to treat fields. For example, the purified material could be chemigated onto a field of alfalfa or corn grown in PCN infested soil, thereby generating a return for the grower, while reducing cyst numbers at the same time. In this scenario one would no longer be restricted to growing a trap crop.

Finally, bio-control options are being explored by UI scientist, Louise-Marie Dandurand. The researchers are trying to identify beneficial microbes including certain bacteria, fungi and predatory nematodes that attack PCN. Some fungi can directly penetrate the nematode and digest its internal contents, others form special "nematode-trapping" structures, while others parasitize cysts and eggs. The stage in the nematode life cycle that is attacked can determine how much crop damage is prevented, and the level of pest control. One such fungus, a species of Paecilomyces, was isolated by UI post-doctoral researcher Jeerapun Worapong, and attacks PCN eggs and larvae. The research group is looking at ways to grow and formulate the fungus and apply it to seed tubers.