1a. Objectives (from AD-416):
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.
1b. Approach (from AD-416):
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.
3. Progress Report:
Potato psyllid is the insect vector of zebra chip, an economically important disease of potato in the United States, including the Pacific Northwest where over 50 percent of U.S. potatoes are grown. ARS scientists in Wapato, Washington, in collaboration with scientists at Washington State University, University of Idaho, and the Washington State Potato Commission, monitored psyllid densities on matrimony vine, a common shrub found within the potato growing regions of the Pacific Northwest. It was determined that both haplotypes of potato psyllid occur on this plant year-round, including in winter. This shrub is an important source of psyllids colonizing potatoes in this growing region. Information from this study will help potato producers manage zebra chip by being better able to predict what potato fields are likely to be colonized by psyllids, merely by determining presence or absence of matrimony vine in the region. Assays continue of annual weedy nightshades for suitability to both haplotypes of potato psyllid and to the zebra chip pathogen. Development and survival on most of these species is seen, although on at least one species, hairy nightshade, development is limited to a subset of psyllid haplotypes. Assays to determine whether annual and perennial nightshades also support survival of the zebra chip pathogen are ongoing. Efforts continue to optimize a molecular tool to identify (to species) plant DNA in potato psyllid. The technology provides a means to study the landscape-level movements of potato psyllid by allowing us to define the dietary histories of field-collected psyllids. Optimization of the technology will allow us to directly determine what species of plants are sources of potato psyllids arriving in potato fields so that growers can better predict whether their fields are likely to be vulnerable to colonization. Monitoring of potato psyllid on shrubby hosts, such as bittersweet nightshade and matrimony vine, showed that both local haplotypes successfully overwinter on these shrubs, indicating that psyllids are cold-hardy enough to survive winter conditions of central Washington State. Laboratory assays using a sub-zero bath showed that psyllids easily survived 24-hour exposure to temperatures as low as minus 12 Celsius, confirming the resilience of this species.
1. Description of traits for separating a pest and a non-pest psyllid. The psyllid Heterotrioza chenopodii is a common species on sticky traps used to monitor potato psyllid, and its presence on traps complicates efforts to count and monitor the targeted pest species (potato psyllid). ARS researchers in Wapato, Washington, in cooperation with scientists at Washington State University, University of Lethbridge, and the Washington State Potato Commission, examined field-collected specimens of Heterotrioza chenopodii to develop a list of characteristics that can be used to rapidly identify the species. The result of this work is an updated description of Heterotrioza chenopodii and a summary of the geographic range of the psyllid. These results will help scientists, field biologists, and others to correctly identify this psyllid and to separate it on monitoring traps from potato psyllid, leading to fewer mistakes in identification of pest and non-pest psyllids.
2. Seasonal occurrence of potato psyllid on matrimony vine. Zebra chip, an economically important disease of potato in the United States, is vectored by the potato psyllid. ARS researchers in Wapato, Washington, in collaboration with scientists from Washington State University and the potato industry in the Pacific Northwest, have determined that matrimony vine, a perennial plant related to potato, hosts potato psyllid at all times of the year. It was discovered that psyllid numbers drop substantially on matrimony vine in mid-summer, and at the same time the potato psyllid begins to arrive in potato fields which indicates psyllids are moving from matrimony vine. This information will help potato growers minimize damage due to zebra chip by allowing them to better predict when and in which potato fields the psyllid is likely to first arrive during the growing season.
3. Screening potato cultivars for resistance to zebra chip disease. Potato psyllid is the vector for the pathogen associated with zebra chip disease of potato, which renders potato tubers unmarketable. ARS researchers in Wapato, Washington, in collaboration with a student from Central Washington University, screened twelve advanced potato breeding lines and seventeen wild potato germplasm accessions for resistance to zebra chip disease. Although none of the wild germplasm accessions were resistant to zebra chip, six advanced breeding lines were found to have no visible symptoms in the raw tuber, and two of those cultivars were relatively symptom-free even after frying of the tubers. These findings will help breeders develop new cultivars that are resistant to zebra chip disease, which would allow growers to reduce or eliminate insecticide applications directed at potato psyllid.
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