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44 POTATO GROWER | DECEMBER 2017 Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In INSECT PESTS | By Jenita Thinakaran, David R. Horton, W. Rodney Cooper & Alexander V. Karasev Pinpointing the weed sources of potato psyllids arriving in Pacifi c Northwest potatoes Origin Story 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 (above) Potato psyllids and eggs (left) Researchers sample matrimony vine for psyllids in the summer as the leaves begin yellowing just prior to leaf drop