2013 UI Snake River Pest Management Tour

Published in the September 2013 Issue Published online: Sep 23, 2013
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Pale cyst nematode populations in infested eastern Idaho potato fields highlight the importance of work being done by University of Idaho Research and Extension scientists.Dr. Pamela Hutchinson, potato cropping systems weed scientist at the UI Aberdeen Research and Extension Center, spoke about ongoing research trials with visitors attending the Snake River Pest Management Research Tour on June 26.

The Animal and Plant Health Inspection Service (APHIS) added 343 acres to the PCN-regulated area in Bingham and Bonneville counties, Idaho, in May.

"The good news is that approximately 355 acres of farmland were deregulated after completing several surveys with negative laboratory results for PCN," Hutchinson said. This brings the total infestation to 21 quarantined fields covering 2,300 acres. The total area in Idaho under regulation is 13,053 acres.

"It can ruin potato crops for any area it gets into," Hutchinson said. "In regulated fields where PCN has not been detected, you can plant potatoes, but these fields are under watch because they have commonalities with a quarantined, infested field."

Tools to combat the nematodes are expensive and labor intensive. Methyl bromide costs $3,700 per acre to apply, yet its future availability is in question. The EPA is expected to issue new buffer zone requirements for the chemical that could block its use for PCN control.

Telone II is another fumigant that kills nematodes. Tarping after application is not needed as it is when methyl bromide is applied.

"It can also be very expensive," Hutchinson said.

Growers sometimes fumigate with methyl bromide in the spring and Telone II in the fall.

Hutchinson said that research shows the cysts can be somewhat resistant to fumigation, however. All this puts the onus on finding another control option for growers quickly.


Looking for a Trap Crop

Litchi tomato might be the answer as a trap crop to help eradicate PCN populations.

Researchers in the Netherlands found that this crop, also called Fire-and-Ice Plant and sticky nightshade (see our cover feature in the April issue, page 14), stimulates PCN eggs to hatch but the nematodes die. The plant has been used extensively in Northern Europe to control nematodes and can cause up to 80 percent of cysts to "suicide hatch."

Hutchinson is conducting one trial to screen how potato herbicides affect Litchi tomato.

"My job is to find which herbicides would kill Litchi tomato if it comes up in a crop the following years after it's planted as a trap crop.

"We also want herbicides safe to the Litchi tomato but which can control weeds while growing it," she added.

The Idaho State Department of Agriculture is regulating the study and other UI PCN research. Other cooperators besides the potato growers include APHIS, USDA, Idaho Potato Commission and University of Idaho Ag Experiment Station.

Hutchinson said 100 seeds per square meter of Litchi tomato are planted in the study at Aberdeen.

Hutchinson also is conducting a second-year root depth study off the station to determine how deep Litchi tomato roots will go in gravelly soils such as the ones in infested fields.

So far the trials are promising.

"Our desiccation product we sprayed on the Litchi tomato at the end of last season worked, so we didn't have any regrowth at the station this year," Hutchinson said. "If the Litchi tomato proves successful, growers can plant it at the same density as we have here and grow it for one summer. They may be able to use it in conjunction with biocontrol agents being developed and/or fumigation. Eventually the field would be cleaned of PCN and removed from quarantine and regulation to again grow potatoes.

"We want to get the herbicides worked out so that we control weeds in Litchi tomato, such as hairy nightshade, which can host PCN, and also find herbicides to control Litchi tomato if it germinates in crops planted after the trap crop," Hutchinson said.

Scientists might find a Litchi tomato that doesn't produce any seed.

"A sterile Litchi tomato would help prevent the plant from ever becoming weedy," Hutchinson said.


Entomology Trials

UI Extension entomologist Erik Wenninger is conducting a potato psyllid insecticide trial at the Kimberly Research and Extension Center.

Three potato psyllids have tested positive for carrying the Liberibacter bacterium that causes the crop disease zebra chip in potato fields.

Wenninger's study compares the efficacy of various insecticide programs for management of potato psyllids and zebra chip. It includes more than 60 entries from 10 sponsors. Foliar sprays were initiated June 7 and continued on 10-12 day intervals until vine kill. Adults and nymphs were sampled in between spray dates.

Another study examines aphid and Potato Leafroll Virus response to different potato varieties. It compares aphid response and PLRV infection among three different potato varieties: Russet Burbank, Russet Norkotah and Ranger Russet. Data on aphid numbers (green peach aphids and potato aphids) as well as level of virus infection are recorded throughout the season.

Aphids can carry virus particles and spread them between plants. Potato Virus Y mostly infects plants in the family Solanaceae. These include the nightshades.

When PVY infects the potato plant, it replicates by assuming control of some of the plant's proteins and enzymes to make more PVY viruses. This disrupts the normal functioning of the plant.

Symptoms of PVY infection in potato include mosaic patterns on leaves, leaf distortion, necrotic line patterns often on veins or shoots, stunted growth, death of growing points and tuber necrosis or potato tuber necrotic ringspot disease.

Hutchinson spoke about PVY and the interaction between the virus and green peach aphid and how hairy nightshade affects the spread of PVY.

"Green peach aphids actually prefer hairy nightshade over potatoes and if they had a choice they would go and live on hairy nightshade," she said. "Hairy nightshade can get affected with the virus. It can more effectively carry the virus from a hairy nightshade to a potato plant than it can carry from a potato plant to a potato plant. We know hairy nightshade has impact on that disease out there."

From her research, Hutchinson knows that the presence of one hairy nightshade per meter row in Russet Norkotah resulted in loss of U.S. No. 1s and total tuber yields up to 20 percent.

"The critical time where you cannot have any hairy nightshade or you lose yield more than 5 percent is between five and 21 days after potato emergence," Hutchinson said. "We need to put the competition- work together with the green peach aphid and virus-work, because the grower has to manage all those things out in the field-not just one or the other."

Scientists are studying different densities of hairy nightshade to see if that affects the spread. They also transplant hairy nightshade at different times after potato emergence and before green peach aphids are there.

"We put our own green peach aphids on one plant, because we know it's infected," Hutchinson said. "And then we monitor after that."


Glyphosate Drift

Hutchinson said another area of research is glyphosate carryover in seed potato.

Her research has shown it can carry over in the daughter tubers if the mother crop encounters glyphosate drift or as contamination in an inadequately cleaned sprayer tank. Now growers want to know if glyphosate can carry over all the way through into the second generation sometimes called "granddaughter" tubers.

One of the studies shown during the tour on June 26 began with a Russet Burbank mother crop sprayed with various glyphosate rates at different timings in 2011. Most injury occurred that first year when glyphosate was applied to plants four to six inches tall.

After determining the yields in 2011, which were less from that early treatment compared with yields from glyphosate sprayed at tuber initiation, mid bulking or vine-kill time, daughter tubers were kept from each plot to be planted at Aberdeen in 2012. Ten with symptoms like rough skin and folds/cracks on the bud end and 10 with no symptoms.

In 2012, regardless of daughter tuber symptoms, plant emergence was worse from those tubers kept from the mother crop which was sprayed at tuber initiation, mid-bulking or timings even though damage to the vegetation in 2011 was not noticeable after those treatments. Many daughter tubers kept from those 2011 timings also had multiple sprouting underground in 2012.

At the end of 2012, the granddaughter tubers from each plot were kept to be planted in 2013. Seed pieces from 10 granddaughter tubers per plot were planted. So far, no emergence problems or multiple sprouting has been observed in any of the plots.

"Glyphosate drift can get on the mother crop foliage and translocate into the daughter tuber," she said. "As our research has shown, glyphosate can remain in tuber and affect germination the following year. It's very troublesome for a seed potato grower, especially if the glyphosate didn't cause any symptoms on the mother crop.

"There may not be above-ground symptoms depending on what time of year the drift occurs," Hutchinson said. But if the drift goes right into the daughter tuber, it can create a problem next year.

"We know it can carry over in the seed," she said. "However, it seems to be metabolized in the daughter tuber during the first year after that mother crop is exposed to it." As mentioned, she does not think it carries over to the next crop-the granddaughter crop.

"The only sure-fire way to eliminate effects on daughter tuber seed is to not spray glyphosate around your seed potato field."