In the early days of 2000, potato growers in the southern U.S. were forced to begin dealing with a new problem in their crop. Vines were showing upwardly rolled leaflets, purple or yellow discoloration, aerial tubers, leaf scorch, and early senescence symptoms. At harvest, tubers were afflicted with vascular browning and areas of the internal tissues that were yellowish, translucent, or otherwise discolored. When affected tubers were sliced to make chips or fries, the symptomatic areas of the tubers turned dark brown when fried, yielding a finished product that showed alternating light and dark bands.
This characteristic symptom gave the name “Zebra Chip” to the new problem. This disease is more than a nuisance in chipping production, but other types of production, including French fry and even fresh potatoes, have the potential to be negatively affected.
Initial attempts to decipher what was causing the new disease met with failure, but scientists recognized early on that an insect called the potato psyllid was involved. After eliminating viruses and phytoplasmas (organisms like the ones that cause witch’s broom and purple top in potato) a team of scientists finally determined that the cause was an obscure, very difficult to work with bacterium. This bacterium belongs to the genus Liberibacter (the same group of bacteria responsible for citrus greening) but the final classification and nomenclature of the organism is still pending.
Potato Losses Mounting
There are no food or health safety issues regarding humans associated with the disease but, over the last 15 years, Zebra Chip has been responsible for some very serious losses to growers in Central America and in the southern U.S. Ominously, the disease was found late this summer in the Southern Columbia Basin of Washington and Oregon. In September, samples with characteristic symptoms were submitted to my laboratory in Idaho.
These also tested positive for Zebra Chip. Fortunately for the producers out here in the Pacific Northwest, the disease has only been found at low percentages in affected lots of potatoes.
Producers in the southern production areas where Zebra Chip has been at its most destructive have been able to utilize insecticide programs to manage the pysllid vector and are able to keep the disease in check At the current time researchers believe that the disease may not pose a significant threat to Pacific Northwest potato production or other more northerly productions areas due to the warm climate requirements of the vector.
Researchers also believe that seed potatoes are probably not responsible for movement of the disease. All of the major varieties currently produced are susceptible to Zebra Chip. Will we be able to breed resistant cultivars? Much remains unknown and only time and more research will answer the many questions posed by this new disease.
Zebra Chip has become important enough that a large group of researchers from across the country are working on it with funding from a Specialty Crops Research Initiative grant. Much of the work performed so far is reported on the grant website at http://agrilife.org/zebrachip. The site contains a wealth of information, including photos of vine and tuber symptoms as well as the problems encountered with finished product. A large and comprehensive research effort is currently underway. I’ll try to keep you all informed of new developments.