Zebra Chip and the Columbia Basin experience in 2011
No longer breaking news to anyone in the potato world, Zebra Chip was found in the Columbia Basin in 2011.
This bacterial disease vectored by the potato psyllid is known for causing substantial damage to potatoes in Texas and farther south. Plant death comes relatively quickly once infected. And unfortunately, once the psyllid feeds on an infected plant, the insect will always retain the bacterium and can infect additional plants. The bacterium can also be passed on to their eggs, so even their offspring contain the bacterium and can infect plants.
This past season, the Columbia Basin had infection and damage ranging from none to very light to extensive. Most fields had little or no damage, particularly those located in the central and northern Basin. However, several fields were extensively damaged in the southern Basin.
What we saw
Foliar symptoms caused by ZC are very similar to a common phytoplasma-caused disease called BLTVA (Beet Leafhopper Transmitted Virescence Agent) transmitted by the beet leafhopper. Symptoms are also similar to those caused by the feeding by several psyllids, resulting in the problem called psyllid yellows.
Symptoms caused by ZC, BLTVA and psyllid yellows include purple coloring and curling in leaves, plant stunting and yellowing, and aerial tubers and swollen nodes on stems. Serological tests are not 100% reliable for BLTVA (or ZC for that matter) and there is no test to confirm psyllid yellows.
A few plants in June 2011 with foliar symptoms described above tested positive for BLTVA. Not until much later was that diagnosis questioned, due to a large number of plants that tested negative for that disease.
However, when tubers possessing typical ZC symptoms were found and correlated with the foliage symptoms that were being seen, the diagnoses of ZC was unequivocal. After bringing together growers in the southern Columbia Basin, information was pieced together that suggested that the psyllids carrying the bacterium likely entered the region approximately June 15-25. This is approximately three weeks ahead of when foliar symptoms were first seen in some potato fields.
Interestingly, in 2011, psyllids were not confirmed by Dvac (vacuuming) until the middle of July trapping in Washington (Joe Munyaneza, USDA ARS data) and in foliar and yellow sticky traps sampling until late August in Oregon. In previous years, psyllids in Oregon and Washington were generally found in mid July. Regardless of trapping data, since ZC symptoms developed during that time period, it's clear that potato psyllids were present much earlier than actually found by trapping.
Tubers with Zebra Chip symptoms were first confirmed the last week of August. In the course of a few days, tubers were brought to the Plant Pathology Laboratory at the Hermiston Agricultural Research and Extension Center, originating from multiple locations, and from five different cultivars: Pike, Russet Ranger, Umatilla Russet, Russet Norkotah and Alturas. The disease was also ultimately found in Russet Burbank.
These initial fields were from fields across the most southern part of the Columbia Basin production area. Subsequent samples came from as far north as potatoes are produced, but incidence in these fields was very low.
The hardest-hit fields were for the most part in close proximity to one another and generally close to the Columbia River. This might suggest that psyllids entered the Columbia Basin in greater numbers in that general location by chance, but not likely overwintered nearby due to the apparent random disease pattern found on plants in these fields.
Research information is lacking related to what happens to ZC infected tubers in storage, though preliminary data suggests symptoms worsen. For that reason, actual damage in the region could be slightly higher than the amount going into storage and will not be completely known until all potatoes are removed from storage sometime in late spring.
Home gardens were not exempted from damage. Tomato, pepper and tomatillo were also
found infected with the bacterium. This was not surprising, since plants within the family Solanaceae are hosts of the potato psyllid as well as the bacterium.
Could plants sold at nurseries for home use be a source of either the psyllid or bacterium? Possibly, but it is unlikely garden plants played a major role in the damage seen in potato fields in 2011, given the sudden and widespread appearance of the disease.
What we learned
The occurrence of ZC in the region provided an opportunity to learn a considerable amount of information about this new and damaging potato disease.
Why ZC damage occurred. Of course ZC occurred because both the bacterium and potato psyllid came to the Columbia Basin. However, review of insecticide applications from fields with significant disease incidence showed no insecticides were applied, for the most part, from late May to early August. Remember that potato psyllids were thought to have entered the Columbia Basin with the bacterium around June 15-25. I
Insecticides are used to control a number of important insects in potatoes in the region (e.g. aphids, beet leafhoppers, potato tuberworm), but these insects were not present or not in sufficient levels to trigger insecticide application. Therefore, the psyllids were present but no insecticides were being applied because it was thought that insecticides were not needed.
Two other aspects to insecticide use may have contributed to little, if any, psyllid control: use of ineffective materials multiple times (pyrethroids) or how the insecticides were applied (chemigated verses air or ground applied). Little ground application of insecticides occurs in the region, whereas chemigation and aerial applications are commonly done. Given the discussion during the growers meeting related to ZC, and the apparent results from insecticide applications, a more thoughtful approach to which application method is used may be justified.
This is particularly true since some products are systemic and others are contact materials and therefore may be better applied using one application method over another.
Remember that applying 0.1 inch of water during chemigation means that nearly 2,800 gallons of water per acre is being used. While applying insecticides by this method may work for certain insects, clearly a significant amount of contact material is lost from the plant by being washed through the canopy, reducing the available residue left on the plant and the length of time the product may be effective.
Milder summers may also explain why ZC appeared in the Columbia Basin in 2011. Potato psyllids and the bacterium do not like extreme warm temperatures (above 90oF), and therefore the psyllid may have left areas in the southern U.S. sooner than typical due to hotter weather than usual. Once arriving in the Columbia Basin, they found temperatures, according to weather records, mild and favorable compared to our "normal" warm summer temperatures.
Psyllid movement. Aerial infra red (IR) photos are commonly sequentially taken during the potato-growing season to provide growers information "from a bird's eye view" on disease, water and insect issues. These photos have provided insight on the introduction and spread of the psyllid as measured by plants with ZC symptoms.
Foliar symptoms develop approximately three weeks following infection, and plant death generally follows soon thereafter. Developing small groups of dead plants in IR photos seem to indicate a random pattern of introduction (flight of the psyllid) into the field, suggesting that the source of psyllids carrying the bacterium is not nearby, but coming from miles or hundreds of miles away. This information further supports the theory, after studying air currents, that psyllids coming to the Basin likely came from California (Charlie Rush, Texas AgLife Sciences, personal communication).
In addition, the IR photos also revealed these initial "spots" grew larger with time, presumably as eggs that were laid by the adult psyllid hatched and the young moved to adjacent plants or moved and colonize other areas. These field "spots" had time to expand, since there was a period of time in some fields when insects as a whole were not being controlled.
At the processing facility. In Texas, most potatoes are grown for chips and few are stored, whereas most potatoes grown in the Columbia Basin are grown for French fries and could be stored for many months. What was seen in the processing plants in the Basin was unlike the experience in Texas.
In the Pacific North West (PNW), potatoes enter the processing plant, where they are washed, steam peeled before being cut into French fries and then partially cooked before bagging and freezing. Early harvested fields of Russet Ranger with ZC symptoms showed a marked darkening of the tuber once steam peeled. Tests with Umatilla Russet and Russet Burbank resulted in similar tissue darkening.
Color differences between healthy and disease tubers were remarkable enough to allow workers on the line to remove infected tubers on the grading table prior to cutting into French fries. As what was expected, cooking of fries that were infected with the bacterium resulted in a darkened and unacceptable product. There was some indication of an intermediate infection class, given the fact that mild ZC symptoms in the tubers following peeling and before cooking resulted in discolored, but still acceptable French fries. This may suggest that the bacterium had not fully advanced throughout the tuber due possibly to a later infection, and that quick processing of infected plants may reduce losses from ZC infection.
What we need to learn
Over the last several years, much has been learned about ZC in Texas and much of that has implication to the Columbia Basin. However there is still much to learn, particularly some aspects of this disease related to the PNW experience. Certainly a better way to determine the occurrence of potato psyllids is needed.
This past season, psyllids were not found until weeks after their suspected arrival and certainly, weeks after the bacterium had been vectored to potato plants. But that brings up another question. Do potato psyllids overwinter in the Basin and if they do, can the bacterium also survive or does the insect only move into the area each year? What about infected seed and/or volunteer potatoes, can they be a source of the bacterium to our region? In terms of controlling the insect, is there better ways to reduce populations by how specific insecticides are applied? Are insecticides that are efficacious in Texas also effective in the PNW? And what about what happens to infected potatoes placed into storage. Do symptoms in tubers increase in storage and if so, does the amount of increase damage relate to when the tubers were infected?
Whether or not ZC is found in the Columbia Basin next year is unknown. The fact that this disease has been found for 10 years or more in southern latitudes before being confirmed in the Columbia Basin may suggest that the yearly reintroduction of the bacterium must occur, assuming overwintering of the insect and/or bacterium does not occur. Regardless, more timely and appropriate insecticide programs will ensure much less damage due to this disease in 2012. Careful consideration should be made to preserve current Integrated Pest Management programs.
--By Philip Hamm and Silvia Rondon. Phil Hamm is Extension plant pathologist and Silvia Rondon is Extension entomologist at Hermiston Agricultural Research and Extension Center, Oregon State University.