Location: Temperate Tree Fruit and Vegetable Research
2020 Annual Report
Objectives
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.
Approach
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.
Progress Report
This is the final report for project 2092-22000-021-00D, "Systems Approach for Managing Emerging Insect Pests and Insect-Transmitted Pathogens of Potatoes". The new project under National Program 304, titled, New Technologies and Strategies for Managing Emerging Insect Pests and Insect-Transmitted Pathogens of Potatoes is currently under evaluation by the ARS, Office of Scientific Quality Review.
In support of Sub-objective 1A, research focused on the potato psyllid, the insect vector of zebra chip, which is an economically important disease of potato in the United States, including the Pacific Northwest where over 50% of U.S. potatoes are grown. Managing potato psyllid is complicated by the existence of distinct genetic subpopulations of the psyllid (haplotypes) which have been hypothesized to differ in biological traits that may affect their importance as vectors of the zebra chip pathogen. Researchers at Wapato, Washington, in collaboration with scientists at Washington State University, Pullman, Washington, the University of Idaho, Moscow, Idaho, and Texas A&M University, Weslaco, Texas, examined this hypothesis in a series of laboratory and field studies. The studies showed that psyllid haplotypes developed at different rates, had different body sizes as adults which likely translate into differing lifetime rates of egg production, varied in host preferences, wintered on different plant species, occupy different geographic ranges, and exhibit low rates of interhaplotype mating success. These results suggest that full understanding of zebra chip epidemiology within any given potato growing region must consider haplotype composition of the resident psyllid population, as rate of spread of the pathogen within any growing season is likely to depend upon what haplotype or haplotypes commonly occur in the region.
For Sub-Objective 2A, studies focused on the potato psyllid's ability 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. Researchers at Wapato, Washington, in collaboration with Washington State University, Pullman, Washington, examined ten 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. The assays demonstrated significant differences among plant species in whether they support psyllid egglaying and development, and in whether the plants could host the pathogen. Scientists used these data to develop a “risk index” for weed species that ranked each species according to its 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 depending upon what species of weedy hosts occur near their fields.
Biotic and environmental factors allowing residence of the zebra chip pathogen in the Pacific Northwest have yet to be fully described, which slows understanding of disease presence and severity year-to-year or potential for spread of the pathogen into other geographic regions. It is especially unclear whether the pathogen overwinters in perennial host plants of potato psyllid. Successful overwintering in psyllid host plants would result in reservoirs of the pathogen in spring that could lead to infection of overwintered psyllids. Researchers at Wapato, Washington, in collaboration with a scientist at the University of Idaho, Moscow, Idaho, inoculated ground cherry (Physalis), a weedy perennial host of potato psyllid, with the zebra chip pathogen under field conditions. Inoculated plants were then allowed to undergo natural winter die-back. Foliage emerging in spring from the overwintered underground storage organs (rhizomes) of the inoculated plants was found to harbor the zebra chip pathogen. Uninfected psyllids allowed access to foliage of overwintered plants readily acquired the pathogen. These results are the first to demonstrate that the zebra chip pathogen is able to overwinter underground in a perennial host plant of potato psyllid. Results also suggest that ground cherry may have a previously underappreciated role in early spring as a source of infective psyllids arriving in potato fields.
Efforts to better understand the role of non-crop plants as hosts of potato psyllid led to the unexpected discovery that potato psyllid is able to develop on some species of morning glories (Convolvulaceae). Other species, in contrast, were found to be quite deadly to the psyllid. Scientists at Wapato, Washington, in collaboration with a post-doctoral scientist at the University of Idaho, Moscow, Idaho, and a natural products chemist at Oregon State University, Corvallis, Oregon, 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 mortality. Plants on which psyllids 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.
Host range of the zebra chip pathogen is not yet known, thus it has been difficult to determine what species of weedy psyllid hosts occurring in potato growing regions are likely to be important sources of zebra chip pathogen entering the psyllid population. Researchers at Wapato, Washington, developed a novel molecular approach that allows scientists to identify plant DNA in potato psyllid, and then showed under field conditions that the tool can be used to describe the feeding histories of potato psyllids that have newly arrived in potato fields. The molecular tool was combined with a second assay to determine whether psyllid specimens providing dietary information also harbored the zebra chip pathogen. The combined approach allows scientists to use the psyllids themselves to pinpoint weedy sources of zebra chip pathogen. A significant correlation between presence of specific plant DNA and presence of the zebra chip pathogen in psyllid specimens would be evidence that the identified plant species is an important reservoir of the pathogen. Researchers at Wapato, Washington, in collaboration with a scientist at Texas A&M University, Weslaco, Texas, used this approach in field trials conducted in south Texas to show that weedy Lycium (wolfberry, matrimony vine) is disproportionately present in pathogen-infected psyllids. This is compelling evidence that Lycium in the southern Texas growing region is a reservoir for the pathogen and source of pathogen for psyllids eventually arriving in potato fields. Foliage from Lycium is being collected to verify that it hosts the zebra chip pathogen in the study area. Knowledge of what plant species are important reservoirs of psyllids and pathogen in a growing regions will allow growers to target and manage specific weeds as a means to reduce incidence of zebra chip disease in their fields.
Accomplishments
1. Field-collected specimens of potato psyllid can be used to identify natural plant reservoirs of the zebra chip pathogen. Zebra chip disease in potatoes is difficult to manage due to difficulties in identifying weedy psyllid hosts that are also reservoirs of the pathogen. Researchers at Wapato, Washington, in collaboration with a scientist at Texas A&M University, Weslaco, Texas, developed new molecular tools that allow them to identify plant remains in field-collected psyllids while also simultaneously confirming presence or absence of the pathogen in those same specimens. Significant correlation between presence of a specific plant species and the pathogen in psyllid populations would be evidence that psyllids feed on those plants and acquire the zebra chip pathogen from those plants. Those data can then be used to guide field-efforts at targeting for eradication species of weeds in potato growing regions that harbor the zebra chip pathogen.
2. Field studies show that the zebra chip pathogen can overwinter in a weedy host of potato psyllid. Slowing spread of the zebra chip pathogen from non-cultivated habitats into potato fields is complicated by not knowing how the pathogen survives winter. Researchers in Wapato, Washington, in collaboration with a scientist at University of Idaho, Moscow, Idaho, have now shown in field trials that the pathogen can successfully overwinter in the underground storage organs (rhizomes) of a perennial weedy host of potato psyllid, ground cherry (Physalis). Scientists inoculated ground cherry plants with the zebra chip pathogen by exposing plants to infected psyllids. As the plants died back in preparation for winter, the pathogen moved to the underground storage organ and successfully wintered in that structure. Foliage emerging from those storage organs the following spring harbored the zebra chip pathogen, and uninfected psyllids acquired the pathogen by feeding on that foliage. These studies are the first to show that the zebra chip pathogen is capable of overwintering in association with a perennial host of potato psyllid that may then be a source of the pathogen the following spring for feeding psyllids.
3. Non-pest psyllids identified as sources of parasitoids that attack potato psyllid. Research on biological control of potato psyllid has been limited to studies of natural enemies and the psyllid in crop environments with no understanding of this process in non-crop habitats. Researchers in Wapato, Washington, discovered that non-pest psyllids which associate with certain herbaceous and woody plant species common in potato growing regions host parasitoids from a taxonomic group (Tamarixia) known to provide biological control of potato psyllid in potato and tomato fields. Sampling of non-pest psyllids confirmed that psyllids from weedy chenopods, willows, stinging nettle, and bindweed are parasitized by Tamarixia. Studies are now underway to verify that these parasites also attack potato psyllid on the psyllid’s weedy nightshade hosts. A lack of research on population biology of potato psyllid in non-crop habitats has slowed our understanding of what biotic factors govern population buildup of the psyllid in these habitats. Identifying non-pest sources of biological control will allow scientists or growers to manage non-crop habitats so as to benefit from the biological control services provided in those habitats.
Review Publications
Cooper, W.R., Horton, D.R., Thinakaran, J., Karasev, A.V. 2020. Dispersal of Bactericera cockerelli (Hemiptera: Triozidae) in relation to phenology of Lycium barbarum (Solanaceae). Journal of British Columbia Entomological Society. 116:25-39.
Wentz, K., Cooper, W.R., Horton, D.R., Wohleb, C., Waters, T., Halbert, S., Ramadugu, C., Snyder, J., Kao, R. 2020. Prototype 3D-printed traps capture Bactericera cockerelli (Šulc) (Hemiptera: Triozidae) directly into preservative for improved detection of "Candidatus Liberibacter solanacearum". Journal of Entomological Science. 55(2):147-155. https://doi.org/10.18474/0749-8004-55.2.147.
Krey, K.L., Nabity, P.D., Blubaugh, C.K., Fu, Z., Van Leuven, J.T., Reganold, J.P., Berim, A., Gang, D.R., Jensen, A.S., Snyder, W.E. 2020. Organic farming sharpens plant defenses in the field. Molecular Ecology. 97(4). https://doi.org/10.3389/fsufs.2020.00097.
Kaur, N., Cooper, W.R., Duringer, J., Rashed, A., Badillo-Vargas, I., Esparza-Diaz, Horton, D.R. 2020. Mortality of potato psyllid (Hemiptera: Triozidae) on host clippings inoculated with ergot alkaloids. Journal of Economic Entomology. 113(5):2079-2085. https://doi.org/10.1093/jee/toaa144.
Swisher Grimm, K.D., Mustafa, T., Cooper, W.R., Munyaneza, J.E. 2020. Growth and yield performance of Solanum tuberosum grown from seed potatoes infected with ‘Candidatus Liberibacter solanacearum’ haplotypes A and B. Plant Disease. 104:688-693. https://doi.org/10.1094/PDIS-05-19-1125-RE.
