2018 ASHS Annual Conference

Zebra chip is a disease of potatoes caused by a bacterium (Candidatus Liberibacter solanacearum) that is vectored by a tiny insect, the potato psyllid (Bactericera cockerelli). Plants infected with zebra chip die early and produce tubers with an internal necrotic defect. The disease, which was known to occur in other parts of the United States for more than a decade, was not found in the Columbia Basin of Washington until 2011. This initial outbreak resulted in the complete destruction of at least two potato fields and symptomatic tubers were reported from several more fields in the region—the losses were estimated at over a million dollars. In 2012, potato psyllids were added to our regional insect monitoring program (established in 2009 to monitor aphids, beet leafhoppers, and potato tubwerworm). After some trial and error, we determined that a combination of yellow sticky cards and leaf samples was the best way to monitor potato psyllids. The most important output from the monitoring program is a weekly e-newsletter, called WSU Potato Pest Alerts, that includes reports about the current prevalence and distribution of potato psyllids, scouting instructions, and management recommendations. This information assists growers in making informed and timely management decisions and may explain why zebra chip has been seen in the Columbia Basin since 2011, but has not resulted in significant losses. The monitoring program also helps us learn more about psyllid biology and behavior. It has revealed details about psyllid migration and distribution in the region, and has led to the discovery of some important overwintering hosts. We have also documented some very large swings in the size of psyllid populations each year; almost 30,000 psyllids were collected in 2016 compared to only 152 psyllids in 2017 using the same number of traps. The infection rate of psyllids has been less varied, with Liberibacter detected in only 0.00% to 0.17% (average 0.14%) of the psyllids each year. These percentages are smaller and less varied compared to the infection rate of psyllids in Idaho during the same period. The low infection rate of psyllids in Washington means that our monitoring and testing program has not been able to predict exactly when and where isolated incidences of zebra chip will occur. But, in the future we will know to anticipate zebra chip if the monitoring program collects unusually large numbers of psyllids or detects a larger proportion of psyllids carrying Liberibacter.