Zebra chip disease, first discovered in the Pacific Northwest in 2011, causes discoloration of tubers that becomes more pronounced when tubers are cooked, especially as chips or fries. The disease is caused by the bacterium Candidatus Liberibacter solanacearum (Lso) and is transmitted by an insect called the potato psyllid. In 2012, we began monitoring commercial potato fields across Idaho for potato psyllids, Lso, and zebra chip. Over the following two years, our monitoring program expanded, each year covering roughly 90 to 100 fields across the potato-growing region of the state.
During 2013 and 2014, each field was monitored either using a “light” approach (weekly yellow sticky card trapping with four cards per field) or an “intense” approach (10 yellow sticky traps, one five-minute vacuum sample, and 100 leaf samples per field). Only the intense sampling approach was used in 2012. Sampling began in May and continued on each field until vine kill.
Each year, psyllid abundance has been very low at first and has gradually increased over the season. Consistently fewer psyllids have been collected in vacuum samples compared to sticky trap samples, and scarcely any psyllid nymphs or eggs have been found in leaf samples over the course of the monitoring program.
Psyllid distribution and abundance patterns over the season from sticky card captures appear to reflect the temperature/elevation gradient across the state. For example, especially during 2013 and 2014, the first incidence of psyllids at each site typically has occurred earlier for western, lower elevation sites; similarly, these sites generally exhibit higher psyllid abundance. Psyllid incidence in eastern Idaho has been rare.
Overall potato psyllid numbers have declined each year relative to the year before, and the incidence of Lso also has been relatively low over the last two years. This is in contrast to 2012, in which psyllid numbers, Lso incidence, and zebra chip incidence all were higher.
Zebra chip incidence in potato fields has been nil during the last two years, which—coupled with the low incidence of psyllids and Lso found—suggests that our monitoring program has been effective at identifying the level of risk of zebra chip to Idaho potato growers.
Updates on the monitoring program are posted at least weekly over the growing season through various means (e.g., PNW Pest Alert, Potato Progress newsletter, Kimberly R&E Center website) in order to help growers and crop consultants use the information to make psyllid management decisions.
The potato psyllid monitoring program has been an enormous undertaking and could not have been successful without partnerships among other University of Idaho faculty and staff as well as several key stakeholders including Lamb Weston, McCain, Simplot, JFR Consulting, Stukenholtz Laboratory and Miller Research. These partnerships facilitated the expansion of the monitoring program through 2013 and 2014, allowing us to process roughly 400 to 500 sticky cards each week over the growing season for a total of over 9,000 sticky cards each year.
Funding for the project has been generously provided by the Idaho Potato Commission, the Idaho State Department of Agriculture and USDA-TASC. We currently have funding to support the monitoring program for one more growing season.