By now it's no secret that a fair number of tuber samples that are infected with zebra chip have been collected from Idaho fields and storages. The causal agent is a bacterium (Liberibacter) transmitted by an insect called the potato psyllid. (See "Don't Be Psyllid" on page 18.)
This disease has been around for a number of years but has been mostly confined to southern U.S. potato production, with Texas especially hard hit. In September it was found in the Columbia Basin of Oregon and Washington. As of October 7, it has now been confirmed in Idaho. Although intensive research on zebra chip has been performed over the last 10 years or so by a group of researchers from across the country, there is still a lot we do not know about this disease and the psyllid vector that carries it.
Zebra chip disease poses absolutely no health or safety issues for consumers. The main problem is quality losses in infected tubers. The disease causes necrotic flecking in the flesh of the tuber similar to net necrosis, but the symptoms often extend throughout the length of the tuber and can be visible at harvest. Figures 1 and 2 are pictures of tubers with typical zebra chip symptoms.
One of the biggest problems associated with infection occurs when diseased tuber tissues are fried to produce potato chips or french fries. The altered starch metabolism occurring in infected tubers causes severe darkening in both chips and fries. Because symptoms can be seen in uncooked tissues, the disease is also a concern for fresh potatoes. One interesting thing to note is that infected tubers typically will not sprout. In other words, zebra chip is not believed to be a seed-borne problem.
Since we know so little about the capabilities of this disease in the Pacific Northwest, the industry is urgently in need of tuber samples. It is imperative that we know the distribution of the disease during last year's production season so we can better formulate and target management recommendations for next season. Samples may be submitted to the University of Idaho for verification. Samples can be taken to your county extension office or taken directly to:
Phil Nolte - Idaho Falls R & E Center, (208) 529-8376
Mike Thornton - Parma R & E Center, (208) 722-6701
Nora Olsen - Twin Falls R & E Center, (208) 736-3600
Please note the county and variety with the sample. No need to keep samples cool now that the weather has changed, but do keep them from freezing. If you are unable to bring samples in, place cut tubers in a plastic bag with insulation around them and FedEx it to one of us. Avoid sending on Friday since the sample may be left out unprotected. Questions can be directed to Phil Nolte at the above number.
As this article is being prepared, we have received numerous samples, but the actual percentage of infected tubers has been very low-less than one percent. Since it is still very early in the season, we don't know if these low percentages will be the extent of the problem or if we will see some lots with higher percentages. Are destructive levels of this disease a possibility in Idaho in the future? Will zebra chip be a major problem or a minor and occasional nuisance? At the present time, we simply don't know. We will keep you posted on the progress of this disease in Idaho and in the rest of the nation. Needless to say, we are all currently on a steep learning curve.
Additional information can be found at http://agrilife.org/zebrachip.