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, a new and economically important disease of potato in the United States, including the Pacific Northwest, where over 50% of U.S. potatoes are grown. In collaboration with scientists at Washington State University, we assessed how well this insect pest develops and reproduces when reared on potato or bittersweet nightshade, an important perennial weed commonly found near potato crops in the Pacific Northwest. It was determined that all haplotypes of the potato psyllid develop faster on potato than bittersweet nightshade and reproduce better on bittersweet nightshade than potato, which may lead to production of large populations of this insect pest moving to potato crops and increasing the risk of zebra chip damage in this major potato growing region. This information will help potato producers manage zebra chip by targeting the potato psyllid for control early to prevent this insect from colonizing potato crops and focusing on the management of bittersweet nightshade to reduce potato psyllid populations. Additional experiments are underway to assess whether potato psyllid populations of mixed haplotypes (single genetic types) with and without the endosymbiont, Wolbachia, have lower growth rates than do populations of single haplotypes. Zebra chip, an economically important disease of potato in the United States is caused by the new bacterium Liberibacter, which is vectored by the potato psyllid. In collaboration with scientists at Washington State University, we assessed mechanisms by which this insect pest transmits the bacterium. It was determined that this pathogen was transmitted to potato very rapidly by all haplotypes of potato psyllid, which underscores the urgent need to develop tools that can control this insect vector fast enough to prevent pathogen transmission. Information from this research will help potato producers reduce damage caused by zebra chip by quickly and effectively controlling the potato psyllid. Our examination of potato psyllids collected in winter from bittersweet nightshade, a perennial non-crop host plant of the psyllid in Washington, Oregon, and Idaho, indicated that populations were almost exclusively of a single genetic type (“haplotype”), and that other genetic types common in Washington State potatoes during summer must either overwinter on some other plant species, or must arrive in Washington State potatoes as migrants from more southern growing regions. Psyllids that overwinter in Washington State do so in a mated state and often with mature ovaries, thus egglaying in late winter on nightshade begins as soon as temperatures begin to warm in March. We developed methods to allow screening of weedy nightshades for possible suitability to potato psyllid. These annuals have to be grown from seed for purposes of our assays, and difficulties in getting seeds to germinate led to delays in getting assays initiated. We discovered through substantial trial-and-error that physical nicking of seeds, followed by 24 hours emersion of seeds in a plant hormone solution, led to high rates of germination. Plant suitability tests are now underway. Furthermore, experiments have been initiated to determine whether several non-crop hosts, including jimson weed, cutleaf nightshade, and silverleaf nightshade, and Lycium, are suitable hosts for the zebra chip pathogen. Our studies on a native psyllid Bactericera maculipennis that shares certain hosts with potato psyllid revealed that this psyllid is a carrier for the zebra chip pathogen. This psyllid did not transmit the pathogen to potato, and is not likely a direct threat to potato. Preliminary studies have determined suitable diets needed for the collection of large quantities of salivary proteins from potato psyllid. Using protein electrophoresis, salivary protein profiles of Liberibacter-infected and uninfected potato psyllids have been visually compared to prepare for more sophisticated studies. Using greenhouse studies, we identified several accessions of the wild potato species, Solanum verrucosum, that are resistant to potato psyllid. Ongoing studies are testing whether these resistance traits can be incorporated into cultivated potato. In addition, substantial progress was made to develop a PCR-based gut content analysis technique to identify host plants previously fed upon by potato psyllids. Field studies have been initiated to determine whether this technique can be used to identify sources of psyllids colonizing fields of potato.
1. Gut content analysis of potato psyllid. Potato psyllid is a vector of the pathogen associated with zebra chip disease of potato. This insect relies upon the use of weed hosts to survive during parts of the year when potato plants are not available, but it is unclear what weeds serve as important sources of infective psyllids colonizing fields of potato. ARS scientists in Wapato, Washington, developed molecular methods to identify host plants previously fed upon by psyllids by detecting in psyllids regions of plant DNA which serve as barcodes for plant species. This study is the first to show that plant DNA can be detected in a psyllid, and provides a means to study the landscape-level movements of potato psyllid. Continuing studies using these methods will enable growers to identify fields at risk of being colonized by psyllids carrying the zebra chip pathogen, and to make more informed pest management decisions.
2. Description of traits for separating a pest and a non-pest psyllid. The psyllid Bactericera maculipennis is similar in appearance to potato psyllid, often co-occurring with potato psyllid on weedy plants near potato fields. ARS researchers in Wapato, Washington, in cooperation with scientists at Washington State University and the Washington State Potato Commission, examined field-collected specimens of Bactericera maculipennis 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 Bactericera maculipennis, including a list of traits that will allow growers to separate this species from potato psyllid, and an updated 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 from the closely related potato psyllid, leading to fewer mistakes in identification of this non-pest psyllid.
3. Bactericera maculipennis is associated with the zebra chip pathogen. Bactericera maculipennis is a native psyllid that commonly occurs on field bindweed in the western United States. ARS scientists in Wapato, Washington, discovered that Pacific Northwest populations of B. maculipennis carry Liberibacter solanacearum, the pathogen associated with zebra chip disease of potato. In North America, this pathogen is primarily associated with the potato psyllid, which shares certain weed hosts with Bactericera maculipennis. Our results suggest that interspecific transmission of Liberibacter has occurred between B. maculipennis and potato psyllid on host plants shared by these two psyllid species. Potato is not a host for B. maculipennis, and potato plants exposed to Liberibacter-infected B. maculipennis did not acquire Liberibacter. We therefore conclude that B. maculipennis is not a direct threat to potato despite its ability to carry the zebra chip pathogen.
4. Wild potato germplasm resistant to potato psyllid. The potato psyllid is the vector for the pathogen associated with zebra chip disease of potato, which renders potato tubers unmarketable. Wild potato germplasm provide genetic sources of desirable traits, including insect resistance, which can be bred into marketable potato cultivars. ARS researchers in Wapato, Washington, and in Sturgeon Bay, Wisconsin, screened populations of a wild potato species, Solanum verrucosum, for resistance to potato psyllid. They discovered two populations that are highly resistant to potato psyllid. These findings will help breeders to develop new cultivars that are resistant to potato psyllid, which would provide a cost-effective control of the potato psyllid and zebra chip pathogen without the use of insecticides.
5. The likelihood of potato psyllid transmitting Liberibacter to carrot. Liberibacter is a phloem-limited bacterium that severely affects important crops, including potato and carrot. This bacterium is transmitted to potato by potato psyllids in the Americas and New Zealand and to carrot by carrot psyllids in Europe and northern Africa. ARS researchers in Wapato, Washington, assessed whether potato psyllids can transmit this plant pathogen to carrot, leading to disease symptom development. It was discovered that, while potato psyllids survived on carrot for several weeks when confined on the plants under controlled laboratory and field conditions, the insects generally avoided feeding on the carrot phloem tissue, thereby failing to infect carrot plants with Liberibacter. Information from this study suggests that the risk of Liberibacter infection and spread between potato and carrot crops is negligible.
6. The psyllid Bactericera trigonica identified as a vector of Liberibacter to carrot in northern Africa. Liberibacter is a new and economically important bacterium that severely damages several crops including potato in the Americas and New Zealand and carrots in Europe. This plant pathogen is transmitted to these crops by psyllids, serious insect pests in the United States. ARS researchers in Wapato, Washington, in collaboration with scientists at the Universite de Liege in Belgium, the National Institute for Agricultural Research in Morocco, and the Natural History Museum in the United Kingdom, discovered that the psyllid Bactericera trigonica was the insect vectoring this bacterium to carrot crops in Morocco, posing a serious threat to the vegetable industry in northern Africa. Information from this research will help affected carrot producers in Africa and elsewhere reduce damage caused by this important plant pathogen by effectively monitoring and controlling its psyllid insect vectors to prevent spread of the bacterium.
7. Identification of a spiroplasmal disease on carrot in Mexico. In 2014, carrot plants in Zacatecas, Mexico, were collected showing symptoms similar to phytopathogenic mollicute infection by insect-transmitted phytoplasmas or spiroplasmas. ARS researchers in Wapato, Washington, in collaboration with scientists at the Mexican National Institute for Forest, Agricultural, and Livestock Research in Mexico, used molecular analyses to identify the pathogen causing the diseased carrot plants. It was discovered that symptomatic carrot plants were infected with the plant pathogen Spiroplasma citri, and the presence of this pathogen in its insect host in Mexico was subsequently confirmed. This plant pathogen is capable of causing economic losses in various host crops, and the occurrence of this pathogen in a previously unreported area may raise concern for growers, suggesting that methods to eliminate the insect vector may be necessary to prevent the rapid spread of the pathogen in this area of Mexico.
5. Significant Activities that Support Special Target Populations:
Horton, D.R., Cooper, W.R., Munyaneza, J.E., Swisher, K.D., Echegaray, E., Murphy, A., Rondon, S., Wohleb, C., Waters, T., Jensen, A. 2015. A new problem and old questions: potato psyllid in the Pacific Northwest. American Entomologist. 61(4):234-244.
Swisher, K.D., Velasquez-Valle, R., Mena-Covarrubias, J., Munyaneza, J.E. 2016. Occurrence and molecular detection of Spiroplasma citri in carrots and Circulifer tenellus in Mexico. Journal of Plant Pathology. 98:355-360.
Torres, G.L., Cooper, W.R., Horton, D.R., Swisher, K.D., Garczynski, S.F., Munyaneza, J.E., Barcenas, N.M. 2015. Horizontal transmission of "Candidatus Liberibacter solanacearum" by Bactericera cockerelli (Hemiptera: Triozidae) on Convolvulus and Ipomoea (Solanales: Convolvulaceae). PLoS One. doi: 10.1371/journal.pone.0142734.
Cooper, W.R., Horton, D.R., Unruh, T.R., Garczynski, S.F. 2016. Gut content analysis of a phloem-feeding insect, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae). Environmental Entomology. 45:938-944.
Cooper, W.R., Bamberg, J.B. 2016. Variation in susceptibility to potato psyllid, Bactericera cockerelli (Hemiptera: Triozidae),among Solanum verrucosum germplasm accessions. American Journal of Potato Research. 93:386-391.