Threat Assessment
When zebra chip burst onto the scene in the U.S. in the 2000s, it came as a nasty surprise for many growers and cost producers millions of dollars. A little over a decade later, is zebra chip still as serious of a threat as it was then, and do researchers have a better handle on how the disease works and how to fight it?
The answer to the first of those questions, is an emphatic yes according to Charlie Rush who directs the research program in plant pathology at Texas A&M AgriLife Research in Amarillo.
“There is no question it is still potentially devastating,” he said. “They (growers) get where they have a low-pressure year and they stop worrying about it or they use their chemicals wisely and have good control for a few years and they start thinking it’s not really out there anymore, it’s not a problem, I can back off, and that’s when they get hurt.”
To answer the second question, researchers have made advances in knowing more about how the disease is spread by its vector, the potato psyllid, and how the disease manifests. Rush said growers should be aware that a plant can be infected any time during the growing season (counter to some early indications that pointed to sprouting being the only vulnerable time). Also once a plant is infected, the characteristic discoloration brought on by the disease can take around three weeks to set in.
That means newly-infected potatoes that are processed right away could be fine. Those same potatoes left in storage for a period of months would show symptoms.
Since zebra chip was first reported in the Texas area around 2005, the main avenue of fighting the disease has been chemical control of psyllids. While not the only weapon in a farmers’ arsenal, neonicotinoids have been an important part in controlling the pests because of their wide availability and relatively lower cost.
Now there is research showing that, at least in some parts of the country, psyllids have developed resistance to these chemicals. Ada Szczepaniec, an entomologist with Texas A&M, has looked at psyllid resistance to neonicotinoids.
She built on previous research that showed resistance to neonicotinoids, including imidacloprid, in the lower Rio Grande Valley region of Texas. She said there was also anecdotal reports of similar resistance across Texas. She looked at populations from across the state as well as one from Colorado and New Mexico.
The experiment exposed the psyllids to the field dose of the insecticide as well as twice and four times the field dose. All of the populations showed resistance; in fact, the survival rate of the psyllids on the treated plants was no different from untreated plants.
“We’d have an easier time killing the plant than the psyllids with this insecticide,” she said.
Szczepaniec said while the results with thiamethoxam were not as definitive as with imidacloprid, they were not positive for its use to control psyllids.
There are anecdotal reports of psyllids possibly developing resistance to other insecticides, but so far, her team has not secured funding to look at those other chemicals.
She added new ways to fight psyllids could include trap plants and predator insects, which would require further study, though some operations in Texas are already having some success by using predator insects.
Resistance is a concern for other parts of the country too, like the Northwest. However, while researchers are on the lookout for it, said Tim Waters, Washington State University Extension educator and entomologist, they have not seen it so far in his area.
He said the key to effective control of psyllids as well as managing resistance is adhering to the principles of integrated pest management.
For example, he said to look at 2016 and 2017. 2017 was a light psyllid pressure year, which meant those using a calendar application approach probably over applied. However, a year earlier when psyllid numbers were much higher in 2016, those using a calendar approach probably didn’t apply enough insecticide to stay ahead of the buildup in the population.
“It really reinforces how important scouting is when you look at those two years,” Waters said.
WHAT’S ON THE HORIZON?
Ongoing work is starting to have success in tracking the life cycle of psyllids when they are not in potato fields. This important information would give researchers an idea of where to look for indication of the numbers of psyllids early on in a growing season. This is also important because tracking psyllids could lead researchers to determine which plant harbors the liberibacter bacteria that actually causes zebra chip.
USDA entomologist Rodney Cooper based in Wapato, Washington, is looking at those questions. Cooper said that a new tool, gut content analysis, is showing promise in letting researchers do just that. Using this tool, he can look at the genetic makeup of the plant material in a psyllid’s gut to determine what species they had been feeding on.
His work has already identified a couple of plants of interest, bittersweet nightshade and matrimony vine, also called lyceum or goji berry. Matrimony vine in particular seems to be habitat for the insects, but may not be the source of zebra chip pathogen.
The next step, he said, is to conduct a large study to see if the psyllids are coming from matrimony vine into potato fields, as well as seeing what other plants they are migrating from and which of those could carry liberibacter.
“We want (to see) that direct evidence from this gut content analysis and also use gut content analysis to see what else they are coming from and if those other weeds are the sources of the ones carrying the zebra chip pathogen,” Cooper said. “We have been developing these methods for a while now but we are ready to move forward with this and answer some of these big questions.”
The research will take a couple of years, Cooper said, though he may have some initial results in a year’s time or so. He said his main goal is to eliminate the uncertainty that currently exists.
“My hope is that in a couple of years we will be able to predict psyllid outbreaks later in the year and predict the extent of those, whether we are going to have tens of thousands or a couple of hundred, for example,” Cooper said. “And be able to better predict the threat of a zebra chip outbreak and then based on that information growers can make informed management decisions.”
