This article appears in the December 2017 issue of Potato Grower.
In the mid-1990s and early 2000s, potato crops in northern Mexico and Texas were inflicted with a new disease referred to as zebra chip disease due to striped patterns in symptomatic tubers. The pathogen responsible for zebra chip was later identified as a bacterium called Liberibacter. Additional research showed that the bacterium is spread among potato plants by a small sucking insect known as the potato psyllid. The zebra chip pathogen slowly expanded its range northward and westward from its initial appearance in Mexico and Texas, making its way to the Pacific Northwest in 2011.
There are no direct methods for controlling the zebra chip pathogen, so preventing zebra chip relies on management of the psyllid vector. These tactics include calendar-day based applications of insecticides. Complicating these control efforts is the uncertainty in the source and timing of potato psyllids’ colonization of potato fields. The potato psyllid is able to reproduce and develop on many plants in the family Solanaceae, including weeds such as nightshades. With knowledge of which weeds are hosts for potato psyllid during the interval preceding emergence of the potato crop, growers would be better prepared to anticipate when and in what fields psyllids are likely to arrive.
Search for Psyllids’ Weed Source
Surveys in Washington and Oregon led to the discovery that two perennial weed species—bittersweet nightshade and matrimony vine—are year-round hosts for potato psyllid. Bittersweet nightshade is native to Europe but has become naturalized in the northwestern United States. This plant likes moist soils and often grows near streams, canals or boggy areas.
Matrimony vine is native to Asia, where it is cultivated for its edible fruit called Goji berries. This plant is widespread in the Pacific Northwest, having been brought here by European homesteaders and by Chinese immigrants in the late 1800s and early 1900s. Matrimony vine is well-adapted to arid environments and flourishes in the potato-growing regions of the inland Pacific Northwest. One adaptation that allows matrimony vine to survive arid environments is its ability to shut down during hot and dry months of summer, and re-sprout following even modest rainfall in autumn. Thus, these plants have two seasonal intervals in which new foliar growth occurs (spring and autumn) separated by summer leaf drop and two to three months of leafless summer dormancy. The timing of summer defoliation is dependent on spring showers and can be delayed by late-season precipitation or irrigation.
Potato psyllids can be found year-round on both bittersweet nightshade and matrimony vine. This includes during the winter when psyllid populations are low on both species, and during the growing season when populations can build to high levels. But psyllid population dynamics during the warm months differ on these two plant species.
Psyllid populations on bittersweet nightshade seem to increase throughout the warm months and then gradually decrease in autumn. On matrimony vine, psyllids can build to high densities during spring and autumn, but are mostly absent during the summer when matrimony vine enters summer dormancy. The disappearance of psyllids from matrimony vine seems to occur at the same time psyllids begin arriving in potato fields, leading researchers to suspect that summer defoliation of matrimony vine prompts potato psyllids to migrate to potato and other suitable hosts.
Ongoing Research
Research plots were setup during the summer of 2017 to specifically assess whether summer defoliation of matrimony vine prompts migration of potato psyllids. These studies are part of a larger, multi-institutional effort involving USDA-ARS, Washington State University, University of Idaho and Oregon State University scientists to improve understanding of landscape-wide interactions between potato psyllids and their crop and non-crop host plants Funding is provided through the USDA-NIFA Specialty Crop Research Initiative, the Washington State Department of Agriculture Specialty Crop Block Grant, the Northwest Potato Research Consortium, and the Washington State Commission for Pesticide Registration.
While researchers did indeed see summer decline and leaf drop in water-stressed stands of matrimony vine, psyllid numbers were unfortunately too low in those stands to realistically measure dispersal rates (a region-wide phenomenon in 2017 for unknown reasons). Researchers will again be monitoring matrimony vine in 2018, with aims to confirm the hypothesis that summer dormancy by matrimony vine prompts psyllid dispersal into potato. If this psyllid response is indeed demonstrated, the timing of leaf yellowing and defoliation of matrimony vine could be used by pest control advisors, extension personnel or scientists to predict when psyllids will begin to arrive in potato fields.
Finally, the numbers of psyllids occurring on matrimony vine in the spring may be found to accurately predict the numbers of psyllids that will eventually migrate into potato. Indeed, psyllid numbers on matrimony vine in May and June appear to predict numbers that will show up on potato during July and August based on data collected from matrimony vine and potatoes for the growing seasons of 2014 to 2017. Ongoing research in 2018 and beyond will reveal whether matrimony vine can be used by growers as an early warning system to predict the risk of potato psyllids colonizing potato fields.
Jenita Thinakaran is a postdoctoral research associate with the USDA-ARS and University of Idaho. David Horton and Rodney Cooper are ARS entomologists. Alexander Karasev is an entomologist and plant pathologist with the University of Idaho.