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Idaho recommendations for potato psyllid and zebra chip management for 2013

During 2011, zebra chip (ZC), a potato disease that was new to Idaho and the Pacific Northwest, was found in tuber samples from the Magic and Treasure Valleys. This disease is caused by a bacterium (Candidatus Liberibacter solanacearum; Lso) that is transmitted by potato psyllids. Scouting during the 2012 growing season showed patchy but sometimes high incidence of the psyllids, and relatively high incidence of the bacterium within psyllids. Low, but widespread incidence of the disease occurred in fields in the Magic and Treasure Valleys, with high (>15%) incidence in a few fields. Few psyllids were found in the Upper Snake River Valley, but at least three fields in the area have shown ZC symptoms during the storage season.

The bacterium that causes ZC produces necrotic flecking in the flesh of the tuber similar to net necrosis, but the symptom often extends throughout the length of the tuber. When affected tubers are fried, the disease causes severe darkening of the chip or fry. The tuber defect is severe enough that the disease is a concern for both fresh and process potatoes. The primary means to control zebra chip is to control infected potato psyllids. Non-infected psyllids will not cause zebra chip. The University of Idaho and Miller Research have developed a scouting and information transfer program for the 2013 growing and storage season. Below is a summary of the program and recommendations for the 2013 season.

  1. University of Idaho and Miller Research will continue a potato psyllid monitoring plan funded by the Idaho Potato Commission and ISDA Specialty Crop Block Grant. At least 13 fields throughout Idaho will be extensively monitored for potato psyllid populations. Sub-samples of potato psyllids will be further analyzed for haplotype (biotype of potato psyllid such as Central, Western, or Northwestern), percent Lso infection, and Lso haplotype (i.e., A or B). Monitoring efforts will focus on fields located in areas where zebra chip was detected in 2011 and 2012. This information will aid in understanding the within-field distribution of psyllids of different life stages, the relative utility of different sampling methods, and the source of psyllid infestations.
    1. Each field will be monitored weekly starting in mid- to late May depending upon growing region.
    2. 10 yellow sticky cards will be placed around the perimeter of each field in addition to vacuum-sampling and in-field leaf scouting. The yellow sticky cards may detect when potato psyllids are moving into the field and the other two methods may detect colonization within the field.
  2.  Approximately 100 additional fields will be monitored throughout Idaho with lower sampling effort (4 yellow sticky cards per field) in order to clarify first timing of arrival of psyllids, and distribution and abundance of psyllids across the state over the summer. A subset of these psyllids will be tested for Lso and haplotype, which will aid in clarifying the source(s) of psyllids and Lso.
  3. Idaho stakeholders will be notified immediately regarding any initial and subsequent detection of potato psyllids observed through our monitoring program and/or submitted samples. Notification will occur via multiple formats and will include the following: email alert system using PNWPestalert (www.pnwpestalert.net), postings in “Potato Progress” and various websites (www.MillerResearch.com, extension.uidaho.edu/kimberly/tag/potato-psyllid-and-zebra-chip/, www.nwpotatoresearch.com), and the Idaho pest hotline (1-800-791-7195).
  4. Various scouting techniques will be used to survey Idaho fields. If you are considering your own potato psyllid scouting program, here are some recommendations. Begin scouting fields around May 15th and no later than June 1st. These dates may need to be adjusted based upon timing of crop emergence. No one scouting method is superior to the others, but some methods will be better for certain life stages. In all cases, scouting for potato psyllids also will provide information on populations of other potato insects such as green peach aphids and potato aphids. You can either evaluate your samples for the presence of potato psyllids yourself or, initially, you may bring them into your local University of Idaho R & E Center to confirm identifications. After submission, subsequent training will occur to allow you to diagnose potato psyllids. Potato psyllid samples submitted to the University of Idaho will be analyzed for the presence of the zebra chip pathogen. Photographs and key diagnostic characters of the different life stages of potato psyllids can be found at various websites listed above or ordered at www.nwpotatoresearch.com/IPM-Home.cfm.
    1. Use of yellow sticky cards – for adults. This method will detect only adult potato psyllids. These cards must be placed within the field but towards the field edge to concentrate on the detection of potato psyllids and reduce the number of other harmless psyllid species on cards. Sticky cards need to be checked and replaced at least weekly since other insects and debris will accumulate on cards which make finding psyllids on the cards difficult. The cards can be easily attached to a wooden lath stake with a binder clip. As cards are replaced, remember to move the card upwards on the lath as the canopy grows, ensuring that the card is located just above the top of the canopy. Before transporting, individual cards should be handled in one of the following ways: placed in a thin, clear plastic bag; separated with a wooden rack; or covered with the paper covering that comes with the card. If using a plastic bag, the sticky face of the card should be carefully adhered to the inside face of the bag to allow observation of insects through the plastic. Ensure the card or plastic bag is labeled. A photograph is shown below illustrating how to manage sticky cards. There are several sources or vendors for yellow sticky traps. Choose a vendor by price and service.
    2. Use of leaf sampling – for eggs, nymphs, and adults. We recommended that you collect 10 leaves from 10 locations within 30 feet of the edge of the field. The best approach would be to sample the expanded leaves towards the middle of the plant. This technique will allow sampling for all life stages including adults, nymphs, and eggs. However, adult potato psyllids are active and often jump abruptly when disturbed and may not be observed. A hand lens or magnifying glass is necessary since the insects are quite small.
    3. Use of sweep nets – for adults. The recommendation is to take 100 sweeps in the field from various areas and focusing on field edges. Collect all insects from the sweep net into a plastic bag, jar, or vial, and freeze overnight. Freezing the sample immediately decreases the potential for sample degradation and loss from predators eating the potato psyllids.
  5. Information/mapping of potato psyllid and zebra chip throughout Pacific Northwest. We will update and post information on potato psyllid detection throughout Washington, Oregon, and Idaho to allow for tracking of the presence and movement of potato psyllids. Information will be reported by county only and all information will be kept confidential. Both the presence of potato psyllids and whether they are infected with the bacterium that causes zebra chip will be reported.
  6. See the following link for a complete list of insecticides registered in potatoes: http://pnwhandbooks.org/insect/sites/default/files/pdfs/pesticidetablesforpotatopests_2.pdf. Recommendations for areas in Idaho that had zebra chip present in the 2012 crop:
    1. We recommend an at-planting application of a neonicotinoid insecticide (seed treatment, in-furrow, or at-hilling).
    2. Follow up with a foliar insecticide application to target adult and nymph potato psyllids once the at-planting material nears the end of its efficacy period. Refer to the “Biology and Management of Potato Psyllid in Pacific Northwest Potatoes” report published in the Potato Progress (and posted at extension.uidaho.edu/kimberly/files/2013/04/02-21-12PotatoProgresszebrachip2.pdf ) for foliar insecticide recommendations. Note that some insecticides target adults, nymphs, eggs, or all life stages. Select an insecticide that is most effective for the psyllid life stage requiring control measures as well as other insects that may be present at the time (e.g., Colorado potato beetles, aphids, mites, etc.).
    3. Avoid foliar neonicotinoid insecticide applications if this class of insecticide was used at planting or at hilling. This is important to avoid insecticide resistance. At-plant neonicotinoids have a long period of control, exposing susceptible insects for several weeks. This period and intensity of exposure may be adequate to foster insecticide resistance in any one (or more) of the pests in potatoes. Adding more foliar applications with the same class of insecticide will only increase this selection for resistance. It is important to remember that Colorado potato beetles, aphids, and other insect pests are also being affected by sprays targeting psyllids. Relying too heavily on any one mode of action will select for resistance not only in potato psyllids, but in all of these pests. Avoiding insecticide resistance in these pests is a critical part of psyllid management decisions.
    4. Avoid pyrethroid insecticides for potato psyllid control. Pyrethroids may flare potato psyllid populations by enhancing egg laying by females and/or killing beneficial insects that attack potato psyllids. Pyrethroids also may flare aphid and mite populations by releasing them from control by natural enemies.
  7. Recommendations for areas in Idaho that might be at lower risk for ZC based on 2012 data:
    1. Follow traditional insecticide program previously used, but consider an at-planting or at-hilling neonicotinoid insecticide if it is not a part of your current insecticide program.
    2. Keep current on survey results in areas with positive detection of infected and non-infected potato psyllids.
    3. If at risk, follow a foliar program as described above and as outlined in the “Biology and Management of Potato Psyllid in Pacific Northwest Potatoes” report published in the Potato Progress.
  8. University of Idaho will hold periodic training sessions throughout the state on psyllid identification and to answer questions regarding the current situation. Three training sessions are scheduled for May 28 (Parma), May 29 (Rupert), and May 30 (Idaho Falls). See the websites listed above for more information. Additional information will be disseminated as necessary via various outlets as described above.
  9. A brief update on the 2012 season regarding potato psyllids and zebra chip. The first potato psyllid was identified on June 19th on a yellow sticky card in Twin Falls County and was positive for Lso. The first foliar and tubers positive for zebra chip was identified July 17th. Soon after the initial findings, numerous fields in both the Magic and Treasure Valleys had Lso infected potato psyllids and had been reported to have zebra chip infected tubers. Positive samples were detected out of storage in potatoes grown in Power County. The incidence of zebra chip in most of the infected fields has been < 1% although some have reported a much higher incidence causing significant loss to quality. Although numbers of potato psyllids caught in Idaho were relative low compared to other regions of the United States, the percent infected with Lso was high (30-50%). Thus far, disease progression in storage has been limited. However, there are many unknowns regarding this disease, and the possible effects of storage conditions over time on symptom development are still not clear.

Recommendations were a collaborative effort of Erik Wenninger, Nora Olsen, Phil Nolte and Mike Thornton of the University of Idaho; Jeff Miller of Miller Research; Andy Jensen of the Idaho, Washington, and Oregon Potato Commissions.

 May 7, 2013