Location: Temperate Tree Fruit and Vegetable Research2019 Annual Report
The long-term objective of this project is to provide basic and applied information for the development and transfer of sustainable and environmentally acceptable methods and technologies for management and control of potato psyllid and zebra chip disease in the major potato growing regions of North America. The objectives of our project are listed below. Objective 1: Determine differences among diverse haplotypes of potato psyllid found in the Pacific Northwest in vectoring capabilities, fitness traits, and overwintering capabilities. Subobjective 1A: Determine if psyllid haplotypes differ in fecundity, development rates, and coldhardiness capabilities. Subobjective 1B: Determine if psyllid haplotypes are equally rapid at transmitting Liberibacter to potato, and examine whether all haplotypes can transmit the pathogen from mother-to-daughter (transovarial transmission) and from male-to-female during mating. Objective 2: Determine suitability of non-crop plant species for development, reproduction, and overwintering of potato psyllid and as potential reservoirs of the zebra chip pathogen in the Pacific Northwest. Subobjective 2A: Compare suitability of non-crop host plants for development, fecundity, and overwintering success among haplotypes of potato psyllid. Subobjective 2B: Compare host preferences of psyllid haplotypes. Subobjective 2C: Develop molecular methods to determine which host species are sources of psyllids colonizing potatoes. Subobjective 2D: Determine whether non-crop Solanaceae are suitable hosts for the pathogen causing zebra chip.
Objective 1: Our hypothesis is that haplotypes differ in biological traits that determine their respective risks to growers as vectors of the zebra chip pathogen. Methods to determine how haplotypes differ in biology will involve: 1). Laboratory-based rearing trials to compare haplotypes in fecundity, egg fertility, and developmental rates; 2). Use of a cold-temperature programmable bath to estimate lower lethal temperatures of each haplotype; 3). Use of electrical penetration graph technology combined with potato grow-outs to estimate how rapidly the zebra chip pathogen can be transmitted by each haplotype; 4). Mating assays between infected and uninfected psyllids to determine if the pathogen is transferred between psyllids during mating; 5). Molecular assays of offspring from infected vs uninfected mothers to determine if all haplotypes transfer the pathogen from mother to offspring. Objective 2: Our hypothesis is that different species of non-crop hosts of potato psyllid will vary in how suitable they are to potato psyllid and to the zebra chip pathogen. Moreover, different haplotypes of the psyllid will vary in what species they prefer for egglaying, and in what species are most suitable for psyllid development and survival. Methods to examine plant suitability to potato psyllid and to the zebra chip pathogen, and to compare suitability of different plant species among the psyllid haplotypes will involve: 1). Standard rearing assays with each haplotype on targeted plant species to determine fecundity and development rates on the different plant species; 2). Choice tests with each haplotype to determine whether haplotypes all prefer the same plant species for egglaying, or whether haplotypes differ in preferences; 3). Development of molecular methods to detect the DNA of specific plant species within the guts of field-collected psyllids, and a comparison of gut contents among field-collected psyllids of the different haplotypes; 4). Inoculation trials to determine whether our targeted plant species are suitable hosts for the zebra chip pathogen, and to determine whether all haplotypes of the psyllid transmit the pathogen to all targeted plant species.
Under Objective 1, research continued in evaluating haplotypes of potato psyllid for differences in fitness, host use and preferences, and importance as vectors of the zebra chip pathogen. Potato psyllid is the insect vector of zebra chip, an economically important disease of potato in the U.S., including the Pacific Northwest, where over 50 percent of U.S. potatoes are grown. The psyllid occurs as three distinct genetic populations (“haplotypes”) in the Pacific Northwest growing region. Unknown differences among haplotypes in vectoring capabilities complicate efforts to manage the psyllid and the zebra chip pathogen transmitted by the psyllid. To examine whether the different haplotypes of potato psyllid have different levels of infection under field conditions requires an effective means for collecting psyllids to assay for presence of the pathogen. Yellow sticky cards are regularly used by the potato industry and university extension personnel to collect specimens of potato psyllid for determining percentage of the psyllid population carrying the zebra chip pathogen. ARS researchers at Wapato, Washington, showed that pathogen deoxyribonucleic acid (DNA) rapidly degrades in psyllids captured by sticky cards, leading to estimates of percent infectivity that are likely to be significantly lower than true population infectivity. Researchers then examined a prototype trap used by the citrus industry that accumulates citrus psyllids directly into preservative and showed that this trap reduces or eliminates degradation of the pathogen signal in potato psyllid. This new trap will allow extension personnel and scientists to more accurately estimate percent infectivity of potato psyllid populations than provided by the more traditionally used yellow sticky card. Under Objective 2, research continued in evaluating non-crop plant species for suitability as developmental hosts of potato psyllid and as potential reservoirs of the zebra chip pathogen. Potato psyllid is able to develop on a number of non-crop weedy plant species whose relative importance as sources of infective psyllids moving into potato fields is not known. ARS researchers at Wapato, Washington, in collaboration with Washington State University (WSU) and the Washington State Department of Agriculture (WSDA), examined 10 weedy annual and perennial plant species that occur in potato growing regions of the Pacific Northwest to determine suitability of each species for psyllid development and as hosts for the zebra chip pathogen, and to determine whether different haplotypes of the psyllid differed in ability to develop on each plant species. The assays demonstrated significant differences among plant species in whether they support psyllid egg-laying and development, and in whether the plants could host the pathogen. The Northwest genotype of potato psyllid exhibited a narrower host range than the California/Oregon/Washington genotype. Scientists used these data to develop a “risk index” ranking of each weed species and both psyllid genetic types, basing each ranking according to the plant’s threat as a potential field source of infective psyllids. These rankings were forwarded to the grower community as an article in the potato industry newsletter, Potato Progress. The “risk index” will help growers determine whether weedy habitats neighboring their potato fields are potential sources of infective psyllids and will additionally allow growers to judge whether there is a need for preemptive psyllid controls. A molecular tool previously developed to identify (to species) plant DNA in potato psyllid has successfully been used to make inferences about field movement by psyllids among plant species preceding their arrival in potato fields. This tool was evaluated to determine whether it could be used to examine dietary history of another phloem-feeding potato pest, the beet leafhopper (vector of the potato purple top pathogen). Like potato psyllid, this insect pest moves extensively through the potato growing region of Washington State and during these movements may feed on many different plant species. It is unclear which of these plants are sources of the purple top pathogen later transmitted to potato by the leafhopper. Researchers at Wapato, Washington, showed that the molecular technology developed for potato psyllid can be adapted to examine dietary histories of beet leafhopper, and using the technology showed that field-collected leafhoppers often contain the DNA of Amaranthaceae, likely from feeding on Russian thistle. These results will assist scientists and extension personnel in understanding landscape level movements by this pest as it moves through the potato growing region and will help pinpoint weedy species that are sources of the pathogen vectored by beet leafhopper into potato. Efforts to better understand the role of non-crop plants as hosts of potato psyllid led to the unexpected discovery that the psyllid is able to develop on some species of morning glories (Convolvulaceae). Other species, in contrast, were found to be quite deadly both to the psyllid and to aphids. ARS scientists at Wapato, Washington, in collaboration with a University of Idaho post-doctoral scientist, and a natural products chemist at Oregon State University, determined that the toxic members of Convolvulaceae exhibit a mutualistic association with a plant fungus that produces a class of alkaloids shown to be the source of the psyllid and aphid mortality. Plants on which psyllids and aphids developed were discovered to be free of the fungus and the insect-protective compounds. Future goals will include efforts to develop methods to incorporate these novel and organic compounds into pest management systems.
1. Development of a weed “risk index” for potato integrated pest management. Managing potato psyllid and zebra chip disease in potatoes is complicated by the psyllid’s ability to develop on weedy non-crop species which may then act as reservoirs of the psyllid as the insect begins colonizing potato fields in late spring. With cooperators at Washington State University and Washington State Department of Agriculture, ARS researchers at Wapato, Washington, screened 10 common weedy plants of the potato growing region of Washington State to evaluate each species as a developmental host for potato psyllid and as a potential reservoir of the zebra chip pathogen. The results of these assays were then used to develop a visual, color-coded “risk index” that ranks each plant species as to its potential importance as a source of infective potato psyllids, with plant ranks ranging from “high threat” (color coded red) to “low or no threat” (green). Each ranking is based upon the weed’s distribution, seasonal availability, suitability to potato psyllid, and suitability to the zebra chip pathogen. This visual index was then forwarded to the potato industry by publication in the industry newsletter, Potato Progress and forwarded to the scientific community by publication in a peer reviewed article in a scientific journal.
2. Discovery of plant-produced compounds having significant anti-psyllid and anti-aphid properties. Psyllid and aphid pests of potatoes exhibit an inconsistent ability to develop on species of weedy and ornamental plants in the Convolvulaceae (bindweeds and morning glories), successfully developing on some species, but showing very rapid mortality on other species. Being unable to predict which species are suitable to these potato pests complicates efforts to understand psyllid and aphid presence or absence in habitats outside of potato fields. Scientists at Wapato, Washington, in collaboration with researchers at University of Idaho and Oregon State University, discovered that plant species susceptible to psyllids and aphids lack a certain class of compounds known as ergot alkaloids. This discovery led to laboratory assays with synthetic analogues of these compounds, and the assays proved that the compounds do indeed have significant anti-psyllid and anti-aphid effects. The discovery of these plant-produced compounds having insecticidal properties could lead to new chemistries with which to control insect pests, even in organically grown crops.
3. Improved detection of the zebra chip pathogen in field-collected potato psyllids. Estimating what percentage of potato psyllids under field conditions are carrying the zebra chip pathogen is done by collecting psyllids with yellow sticky cards and examining trapped specimens for presence of pathogen deoxyribonucleic acid (DNA). Scientists at Wapato, Washington, in collaboration with students and scientists with Heritage University, Washington State University, University of California, and Florida Department of Agriculture (FDA), showed that this method may often significantly underestimate the actual infection rate of psyllids due to degradation of DNA in specimens trapped using sticky cards. Scientists showed that psyllids collected directly into preservative using a trap developed by Florida Department of Agriculture (FDA) cooperators to monitor citrus psyllid substantially improved detection of pathogen DNA compared to what is observed for specimens collected from cards. The new preservative-based trap is now being used to supplement sticky card results and to collect high-quality specimens for pathogen detection as part of psyllid research and monitoring programs in Texas, Oregon, Idaho, and Washington.
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Zhao, Z., Prager, S., Cruzado, R., Liang, X., Cooper, W.R., Hu, G., Rashed, A. 2018. Characterizing zebra chip symptom severity and identifying spectral signatures associated with Candidatus Liberibacter solanacearum infected potato tubers. American Journal of Potato Research. 95(5):584-596. https://doi.org/10.1007/s12230-018-9666-9.
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