First reported in South Texas potato fields in 2000, by 2006 zebra chip of potato (ZC) spread throughout all Texan potato production areas, plus Arizona, California, Colorado, Kansas, Nebraska, Nevada and New Mexico. This new disease now occurs in the Pacific Northwest and in the Northern region, and also as far east as Minnesota and Wisconsin.
To date the Midwest and the Eastern United States have so far escaped its destruction.
When cut, the afflicted tubers of ZC reveal stripes, flecks or streaks. The disease alters the sugars in the tuber. When fried for chips or cooked at high temperatures, the soluble sugars caramelize, producing dark, unsightly stripes. Although harmless to eat, taste is adversely affected.
The pathogen, Candidatus Liberibacter solanacearum, causes ZC. This bacterium is transmitted by the potato psyllid, Bactericera cockerelli.
Foliar symptoms on the plant include yellowing, leaf curling, purpling of shoot tips, wilting, scorching, swollen nodes, axillary buds and aerial tubers. Underground stems can show vascular browning.
Controlling the psyllid vector with insecticides is currently the only effective management. Treating the seed at planting, drenching the soil and targeting foliar applications during the growing season has resulted in aggressive insecticide programs. Scouting, using sticky traps and sweep nets assist the proper timing and application rate. Rotating insecticides with different modes of action can delay insect resistance from developing.
The potato psyllid overwinters in regions such as the Lower Rio Grande Valley, then migrates north each spring. The zebra chip disease was documented in Mexico in 1994, and has been reported in Guatemala, Honduras and New Zealand.
ZC has caused losses of millions of dollars. The state of Texas, the processor Frito-Lay, the National Potato Council, the US Potato Board and others have funded research. In 2009 the USDA Specialty Crop Research Initiative (SCRI) program awarded $6.9 million to multidisciplinary scientists from various states to study this threat. The nature of the pathogen and the vector, biological controls and cultural practices, breeding for resistance, disease forecasting, and extensive education and training together hopefully can combat ZC.
Resistant varieties usually are the first line of defense against vegetable diseases. Today no commercial potato cultivars presently resist the zebra chip pathogen or the potato psyllid.
Research geneticist Dr. Richard Novy, at USDA’s Agricultural Research Service (ARS) in Aberdeen, Idaho, evaluated the zebra chip susceptibility of the most widely produced potatoes in the United States. He found that diseased foliar symptoms and tubers produced yield losses of 50 percent to almost 90 percent.
Novy selected two wild potatoes with several disease/insect resistances to cross with cultivated potatoes. A Chilean potato has shown resistance to green peach aphid, potato aphid and leaf hopper plus resistance to potato leaf roll virus, potato virus Y and potato virus X. A potato from Bolivia has resistance to the Colorado potato beetle, flea beetle, leaf hopper and mites. Novy said wild potatoes present breeding challenges. They do not tuberize easily, requiring multi-crosses. After the fourth cross, using both russets and chipping potatoes, two of his crosses showed resistance to zebra chip. Novy concluded, “This warrants further study.”
When Dr. Joseph E. Munyaneza, research entomologist, ARS, Wapato, Washington, screened 280 potato lines exposed to the potato psyllid, 200 had severe symptoms of ZC in the tubers. Some, though, had light tubers when raw. But after frying, dark streaks appeared in almost all of them. “Something masks symptoms,” Dr. Munyaneza observed about fresh, uncooked potatoes. However, about a dozen showed very light or almost no stripes after frying. After more intense screening, he further evaluated five—one developed by Bejo Seeds, two from Novy’s Aberdeen breeding program, and two from Frito-Lay.
Those five with potential were further evaluated for several years and compared with “Atlantic,” the standard cultivar for potato chips. “Atlantic” yields well and possesses resistance to numerous potato diseases—but is susceptible to zebra chip. Dr. Munyaneza notes that the five selections exposed to the potato psyllid did show above-ground symptoms of ZC, but fewer than “Atlantic. Consequently, he characterizes them as “tolerant.” In the chemical analysis comparisons of fructose and sucrose sugars, the “Atlantic” tubers had considerably higher amounts.
Dr. Munyaneza concluded that those results support the finding that tolerant cultivars respond less strongly to the pathogen’s infection. He said, “This information could quickly help identify the mechanism of zebra chip resistance and the development of resistant cultivars.”
Co-director of the SCRI project, Dr. John Trumble, distinguished professor of entomology, University of California, Riverside, reported that the insecticide abamectin is effective in California, Texas and the Pacific Northwest. Spirotetramat, important to the potato psyllid control strategy, may be developing resistance in Texas, but resistance is not evident in California or the Pacific Northwestern states. Trumble cautioned against a common practice of overusing materials such as the too-frequent application of synthetic pyrethroids. Also proper rotation and the appropriate usage rate, interval and coverage should be followed.
Trumble noted that the psyllids have become resistant to the neonicotinoid imidacloprid in Texas. In California, their program limits imidacloprid to drip irrigation just prior to seedling emergence. He explained, “The effective dose in the plant declines between three and four weeks to ineffective levels.” That is before flowering. Trumble added that although bees will visit potato flowers, the flowers are largely wind pollinated and have low attractiveness to bees.
Other states and other countries use foliar applications of neonicotinoids to control potato psyllids. Trumble does not recommend them in California because of the short effectiveness, plus repeated foliar applications likely contribute to resistance development. He said, “This is probably why imidacloprid is still effective in California.”
Trumble listed tolfenpyrad (Torac), cyazypyr (Verimark/Exivel), and sulfoxaflor (Closer), as potential alternatives. These all have different IRAC chemical/mode of action classification code numbers established by the Insecticide Resistance Action Committee.
The SCRI project has investigated numerous aspects of the pathogen and the vector to better understand and thus ultimately control zebra chip. Project director Dr. Charles Rush, Texas A & M plant pathologist, recently studied the timing of insecticide controls. Disease management programs had focused on early vector repeated usage to minimize the damaging early to mid-season infections. However, his intensive studies of vector transmission and host-pathogen interactions reveal that late-season infections, too, pose a significant threat.
The pathogen can move into the tubers from an infected leaf in two days. Unfortunately, the tubers infected less than a week before harvest remain asymptomatic, with the pathogen undetectable in the tubers, which appear disease-free. But the pathogen can continue to multiply in the tubers during storage. Also, if the plants become infected a few days before vine-kill, the tubers can continue to develop symptoms before harvest.
Rush noted at the conclusion of the scientists’ 2014 meeting that the researchers agree that the SCRI program should continue. According to Rush, these findings to date underscore the importance of research to manage the causes of zebra chip.