By Carolyn King
Zebra chip is an emerging disease that kills potato plants, severely reduces yields and makes the tubers unsellable. This devastating disease hasn’t reached Canadian potato fields, but growers here need to keep an eye out for zebra chip and for the tiny insect that transmits it, the potato psyllid.
By Carolyn King
The name of the disease comes from the distinctive discoloration of the tubers. “The disease causes a higher than normal sugar concentration in the tubers. When fried, the sugar caramelizes, resulting in off flavours and dark brown ‘zebra’ stripes. This defect is harmless to consumers, but it renders the tubers unmarketable, affecting both the processing and fresh market crops,” explains Dr. Eugenia Banks, potato specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).
Expanding disease range
The pathogen causing zebra chip is a bacterium called Candidatus Liberibacter solanacearum. It is causing millions of dollars of losses to the potato industry in affected areas.
“The disease was first found in grower fields near Saltillo, Mexico, in 1994. Potato growers there noticed unusual defects in the raw tubers. Five or six years later, the same defect started to show up in Texas,” says Dr. Don Henne, an entomologist at the Texas A&M AgriLife Research and Extension Center in Welasco, Texas. He is the co-principal investigator of the Speciality Crops Research Initiative Zebra Chip Project and is involved in many aspects of zebra chip and potato psyllid research.
Starting in about 2004, the disease began to appear in other south-central and southwestern states; it has been reported in California, Arizona, Colorado, Nebraska, Kansas, New Mexico, Nevada and Wyoming. Then in 2011, it occurred in the Pacific Northwest (Washington, Oregon and Idaho), and caused significant losses in this major potato-growing region.
Henne notes, “The disease has also spread south into Guatemala, Honduras, El Salvador and Nicaragua. There is concern that it will soon be in Costa Rica and Panama, and possibly spread into South America.”
Zebra chip has spread to New Zealand as well. “It’s thought that the disease arrived there in about 2002 or 2003. It has now spread through the North and South Islands, and is causing very serious problems. Potato psyllids with the disease were probably introduced to New Zealand on infested nursery stock, like an ornamental pepper plant,” says Henne.
The potato psyllid Bactericera cockerelli is key to the spread of zebra chip. Also known as the tomato psyllid or potato-tomato psyllid, this small flying insect prefers to feed and lay its eggs on plants in the nightshade family, including potatoes, tomatoes, peppers, eggplants, and wild species such as hairy nightshade. When potato psyllids carrying the zebra chip bacterium feed on these plants, they infect the plants, impacting crop yield and quality in many of the crops in this family.
Only a small percentage of the potato psyllids in the United States carries the bacterium, but that small percentage can cause a lot of zebra chip.
The disease is new, not the insect
In the United States, potato psyllids have been found for decades in many south-central and southwestern states. The insects prefer areas with less than about 22 inches (560 millimetres) of precipitation annually, so they aren’t typically found east of the Mississippi River. However, winds occasionally carry them farther east. The optimal temperature range for the psyllid is between 20 and 28 C; temperatures above 32 C harm the eggs.
Henne co-ordinates a multi-state program to track the potato psyllid population east of the Rockies, monitoring their distribution and infection levels. His lab processes samples from growers in Texas, Colorado, Nebraska, Kansas, North Dakota, Minnesota and Wisconsin. This psyllid population appears to migrate from Mexico into southern Texas in the fall. The insects stay in south Texas, building up their numbers, from about November to May, which is also the region’s potato-growing season. Then in April or May, when temperatures start to rise above about 32 C, the insects migrate to cooler areas, either higher up the mountains or farther north.
The potato psyllid’s overwintering range seems to be expanding. For instance, the insects used to occur only sporadically in California, but in recent years they appear to be remaining there all year, resulting in earlier infection of plants and greater crop impacts.
Although potato psyllids are able to survive temperatures well below freezing, they don’t overwinter in Canada. However, they do occasionally come into Canada during their summer migration. For instance, in 2012, two were found near Carberry, Manitoba.
“As part of a North American survey in 2012, two potato psyllids were trapped from a commercial field in Manitoba. The potato psyllids were identified and tested at the Texas A&M lab and found to be free from zebra chip bacteria,” explains Dr. John Gavloski, extension entomologist with Manitoba Agriculture, Food and Rural Initiatives (MAFRI).
“Identification of the species of psyllid is important, as over 100 species of psyllids are known from Canada. From the surveys in 2012 conducted by Dr. Vikram Bisht, MAFRI’s potato pest management specialist, other species of psyllids, which are not capable of spreading zebra chip, were found in other commercial potato fields in Manitoba. Similar surveys of potato fields will be organized by Bisht in 2013.”
Gavloski adds, “Potato psyllids are not an insect we would normally see in Manitoba, so it is somewhat unusual but, given the proper weather conditions, not impossible.” He reports that the entomology museum at the University of Manitoba has only one previous record of potato psyllids collected in Manitoba in its collection; that specimen was collected from potatoes at Brandon in August 1946.
There are also records of potato psyllids occasionally occurring in British Columbia, Alberta, Saskatchewan and Ontario, sometimes in greenhouses. “Potato psyllids were detected in one Ontario greenhouse in 2001. It is thought that they arrived on imported crates. Effective sanitation eliminated the psyllids very quickly. Since then, there have been no further reports of psyllids in greenhouses in Ontario,” says Banks. “There are also occasional reports of potato psyllids from B.C. greenhouses.”
The potato psyllid can damage potato plants even if the insect does not carry the zebra chip bacterium. Banks explains, “They inject toxins with their saliva that can cause leaf curling, yellowing or purpling, and small, misshapen tubers. This condition, called ‘psyllid yellows’, is generally less damaging than zebra chip.”
“Potato psyllids acquire the zebra chip bacterium when feeding on infected plants. Once the psyllids acquire the pathogen, they will always carry it. Even eggs laid by infected adults are carriers,” notes Banks.
In the population that Henne monitors, there is a very pronounced northward decline in the percentage of psyllid adults carrying the pathogen. “This past winter, for example, 10 percent of the potato psyllids in the Lower Rio Grande Valley tested positive,” he says. “In the San Antonio area, which is about 200 miles north, it declined to five percent. In the Lubbock area of the Texas Panhandle, it was less than one percent. North of that, it was virtually undetectable. So the disease was confined almost entirely to Texas this year, although the insects were collected as far north as Manitoba and as far east as Wisconsin.”
Henne adds, “Just because we haven’t seen zebra chip in North Dakota, Wisconsin or Minnesota, doesn’t mean it won’t happen next year. The lesson learned from the Pacific Northwest is that typically you start to see the insect in your area for several years and then the disease shows up.”
Once infected potato psyllids arrive in a potato field, they can spread the disease very quickly. “Less than an hour or two of feeding on a potato plant is sufficient to infect the plant and then there is no hope – the plant will die in two or three weeks,” notes Henne.
“Potato psyllids are very active, and one insect can infect multiple plants, if the psyllids are not controlled quickly. As well, they can have a minimum of two or three generations in a potato field if they are not controlled, so you can get a rapid buildup of populations and damage.”
Some countries have banned potato imports from regions with zebra chip. However, Henne and other researchers have determined the risk of disease transmission on the tubers is very low. He says, “Some of the tubers with this disease will sprout and produce plants, but the majority of these plants are very weak and stunted, and most do not even carry the pathogen. Furthermore, the disease cannot spread without the psyllid. If infected potatoes end up in an area where the potato psyllid does not occur, then it’s unlikely to be an issue.”
Tips for Canadian growers
“Zebra chip is on the doorstep, but whether it will become an issue in Canada in years to come remains to be seen. However, we’re seeing a trend in recent years for this disease to move into areas where we’ve never seen it before. The impacts of climate change seem to be happening quite rapidly, with insects and diseases spreading to areas where we’ve never seen them before,” says Henne.
He advises Canadian potato growers to work with entomologists and potato specialists to monitor for potato psyllids and zebra chip. He also recommends keeping an eye on potato psyllid patterns in the United States to get an early warning of the risk level. For interested growers, he issues a weekly report on potato psyllid numbers and infection levels from his monitoring program.
Various strategies can be used to monitor for potato psyllids such as using yellow sticky traps or sweep nets for adults, and leaf sampling for eggs, nymphs and adults.
“Correct identification of all life stages of psyllids is extremely important for effective and successful monitoring,” notes Banks. “Growers, scouts, crop consultants and extension personnel should be trained so that identification is done correctly. A magnifying lens is essential because psyllids are very small: the adults are about two millimetres long, which is the size of a comma of a computer keyboard.
“The eggs are yellow to orange and football-shaped. The nymphs are flat, with a fringe of short spines, and they change in colour from pale brown to green as they get older. The adults resemble small cicadas, with clear wings resting roof-like over the body. Their colour changes from light yellow to pale green at emergence, to brown or green two or three days later, then grey or black with white markings when they are five days old. They jump and fly readily when disturbed.”
She emphasizes, “Since not all potato psyllids carry zebra chip and some symptoms of the disease may be confused with other potato diseases, testing psyllids for the presence of Liberibacter is the only way to determine if zebra chip is infecting the crop.”
Infected potato plants show a number of above-ground symptoms such as stunting, zigzag stems, and misshapen and discoloured leaves. “They look similar to plants with purple top, another disease of potatoes. Affected plants are usually scattered throughout the field. They sometimes appear as clusters of collapsing plants that die early,” says Banks.
Tuber symptoms include enlarged lenticels, collapsed stolons, and brown flecking and streaking that may be confused with tuber net necrosis caused by the leaf roll virus. The key symptom that distinguishes zebra chip from other potato diseases is that the streaking becomes much darker with frying. The symptoms can affect the entire tuber.
“At present, detection of potato psyllids at any level, in any life stage, is the threshold for action. There are insecticides registered in Canada that control psyllids, but unfortunately psyllids do not appear on the label except for Movento,” says Banks. “Jim Chaput, OMAFRA’s Minor Use co-ordinator, is working on submissions for adding psyllids to the label of some insecticides.”
In the United States, various insecticides are registered for potato psyllid control. “Control of the disease right now requires controlling the psyllid. There are a lot of good [insecticide] products that work very well,” says Henne. “However, we need better knowledge of which ones work more quickly, without having adverse effects on natural enemies – without quick control of the psyllids, the disease can spread rapidly.”
Henne and other researchers are working on a variety of integrated pest management strategies to improve control of the insect and the disease. “All of our potato cultivars are susceptible to this disease, as are tomatoes and peppers, but there are some good advances recently in the breeding area and the management area. We’re constantly evaluating new insecticide chemistries. We’re also trying to better understand the biology and behaviour of the potato psyllid; for instance, some of the work in my lab is trying to identify plant species that the potato psyllid would prefer rather than potato, so we can use those as trap crops or to divert the psyllids from potato crops.”
In 2013, Banks will be monitoring for psyllids in Ontario. If she or an Ontario potato grower finds any potato psyllids, she’ll send the samples to Henne’s lab to determine if the insects carry the bacterium. She adds, “Let’s hope our cold Canadian winters keep the psyllids away from our potato fields!”