This article appears in the June 2017 issue of Potato Grower.
Zebra chip is an emerging disease of potato in which tubers are produced with striped necrotic patterns that make them unmarketable. Zebra chip is associated with the bacterium Candidatus Liberibacter solanacearum (Lso), which is transmitted by an insect called the potato psyllid. Although potato psyllids had been known to occur in the Pacific Northwest for decades, their presence was of little concern to potato growers in this area until zebra chip was discovered at the end of the 2011 growing season.
In 2012, we initiated a monitoring program in order to clarify the extent and severity of the threat posed by zebra chip in Idaho. After the first year, the monitoring program grew substantively and covered 90 to 100 commercial potato fields across Idaho’s potato-producing region during each of the last four seasons (2013 through 2016).
Our monitoring program over the last five years has revealed some interesting patterns that can help us in our efforts to manage this pest. Each season, abundance of potato psyllids on yellow sticky traps initially increased gradually, then showed a sharp late-season rise and a sharp decline as most fields were being harvested. Thus, the threat of zebra chip is low during the early part of the growing season in Idaho because few or no psyllids may be present. This is in contrast to some other potato-producing states, where psyllids may be abundant at crop emergence.
Another pattern that has been largely consistent is the observation of a greater abundance of psyllids in western Idaho relative to the eastern part of the state, which appears to reflect the elevation and temperature gradient across the state. This may seem like an obvious pattern to expect, but the conventional wisdom had been that potato psyllids could not overwinter in Idaho and only occurred here as a result of annual migrations from southern climes. If this were the case, we might expect psyllids to occur more uniformly across the state. Lower psyllid abundance in eastern Idaho suggests that psyllids may not overwinter as well in that part of the state.
Despite scrutinizing thousands of potato leaves in search of eggs and nymphs, we have observed immature psyllids in our samples only late in the season and only during years with a higher abundance of adults. We recorded our highest counts of psyllids on sticky traps in 2012 and 2016, so it is perhaps not surprising that these were the years with the highest number of eggs and nymphs (especially 2012). Moreover, 2014 featured by far the fewest captures on sticky traps and was the only year in which we found no eggs or nymphs in leaf samples. The low incidence of immature potato psyllids in our samples overall suggests that potato growers were effectively managing this pest in their crop and limiting psyllids colonization.
Although psyllid phenology patterns were consistent each year, the total abundance of psyllids varied greatly from one season to the next. The incidence of Lso also can vary considerably among years. About 28 percent of the psyllids we collected in 2012 harbored the zebra chip pathogen, whereas incidence ranged from 2.4 to 3.5 percent during the next three years and was even lower (under 2 percent) in 2016. The year with the highest incidence of Lso (2012) was also the year in which zebra chip was recorded at high levels in multiple fields, whereas the disease has been less prevalent in subsequent years. We collected many psyllids in 2016, but Lso incidence was lower than we have ever recorded. This underscores the importance of the presence of both the vector and the pathogen for disease development.
The potato psyllid monitoring program has been an enormous undertaking and would not have been possible without generous support and cooperation from numerous industry stakeholders and funding provided by the Idaho Potato Commission, the Idaho State Department of Agriculture, and USDA-TASC. We currently have federal funds that provide partial support to the program over through 2018, after which the merits of continued monitoring will be reassessed.
Updates on the monitoring program are posted at least weekly throughout the growing season via various channels (e.g., PNW Pest Alert, Potato Progress newsletter, Kimberly R&E Center website) in order to help growers, crop consultants and other stakeholders use the information to inform psyllid management decisions.