“Education and Innovation” was the theme of the 45th Annual University of Idaho Potato Conference and 34th Ag Expo in Pocatello in January.
Highlights of the conference included a new back-to-the-basics “Potato Science Course,” updates on expanding potato markets, the current potato cyst nematode program, input on the values of potatoes for livestock feed, a cultivar update workshop and several in-depth presentations on zebra chip and potato psyllids.
Wednesday kicked off with a series of presentations on Candidatus Liberibacter solanacearum, or Lso, the bacterium that causes zebra chip, as well as the potato psyllid, the vector that carries it.
Researchers have some answers but there are still many more questions to be answered.
UI Entomologist Erik Wenninger explained that in 2012, most zebra chip in Idaho was found in the Magic Valley and south-central part of the state. While the average incidence was 1 percent in most fields, some fields were at 3–12 percent and a couple fields were rejected for having more than 15 percent ZC.
“We’ve got hot spots among our different sampling sites, and hot spots within sites,” Wenninger says.
Nora Olsen, UI extension potato specialist, says that classic symptoms include pink discoloration, crinkly leaves and aerial tubers; however, it’s difficult to distinguish between zebra chip and another disease without testing for Lso.
“Not all tubers under an infected plant will show symptoms,” she says.
One question they need to answer is, “Do mild symptoms turn into severe symptoms out in the field with time or over time in storage?”
Wenninger says that this coming year, researchers would like to monitor more fields because there’s a lot of variation among sites in terms of the number of trap factors.
Neil Gudmestad, North Dakota State University distinguished professor, explained what we know about the biology of the disease. The bacterium is a phloem-restricted pathogen, meaning it disrupts the carbohydrate metabolism.
He says that the bacterium wants to be where the potato psyllids feed—on the leaves where sugars are made before travelling down to the tubers and the roots.
“It’s very primitive—it’s got to have food ready for it to assimilate,” Gudmestad says. “It introduces very efficiently the bacterium where the bacterium wants to go.”
One thing researchers are suggesting is that Lso is a secondary endosymbiont of the psyllid—e.g., it helps the insect digest proteins into assimilatable amino acids.
One of the questions he says that is unanswered is, “When do psyllids pick up Lso?”
Andy Jensen, regional research director for Idaho Washington and Oregon, spoke about the over-wintering patterns of potato psyllids in the Pacific Northwest. Researchers have found that Solanum dulcamara, or bittersweet nightshade, is a good overwintering host for potato psyllids. Scientists at this time don’t know what habitats are important for psyllids, or when and why psyllids leave bittersweet nightshade and go to potatoes.
“It’s a fabulous host—why would psyllids leave that and go find potatoes?” he asks.
Wenninger suggests some tips on managing potato psyllids. He says that an at-plant neonicotinoid can last for about 50 days; however, do not use foliar neonics if an at-plant neonic was used. Also, he warns that growers should not use pyrethroids, which tend to flair psyllid populations.
“It may kill the adults, but what it seems to do is cause the females to start laying more eggs. They’ll die, but they’re going to lay more eggs before they die. There may be a bit of a lag, and then you’ll see a flare in the psyllid populations. That’s what we’ve been told and that’s what we’re continuing to relay.”
Finally, more than 20 large nymphs per 100 leaves is a sign that the adult population will explode.
Gudmestad says that the key to psyllid management—and the key to zebra chip management—is not permitting the psyllid to establish residency in your fields. He warns that our dependency on insecticides to control psyllids isn’t sustainable, but says, “Once [they] establish residency, your choices of chemistries that are going to get to rid of that problem become very, very narrow.”