Location: Temperate Tree Fruit and Vegetable Research
2021 Annual Report
Objectives
Objective 1: Develop new tools and approaches for examining landscape-scale movement by Hemipteran vectors and plant pathogens between non-crop plant species and potato fields.
• Sub-objective 1A: Identify weedy plant sources of infective potato psyllids and beet leafhoppers entering potato fields of the Columbia Basin growing region.
• Sub-objective 1B: Characterize genetic variation in beet leafhopper populations across geographic areas and between host species within the Columbia Basin and use these data to evaluate host-linked dispersal of leafhoppers into potato fields.
Objective 2: Describe the biology of Hemipteran vectors of potato pathogens in crop and non-crop habitats, including reproduction and development, feeding ecology, chemical ecology, seasonal phenology, interactions with natural enemies, and transmission/acquisition of pathogens.
• Sub-objective 2A: Characterize beet leafhopper feeding behavior and stylet penetration activities to examine acquisition and inoculation of BLTVA across host species.
• Sub-objective 2B: Identify predator species important in reducing densities of potato psyllid and beet leafhopper in stands of weedy host plants.
Objective 3: Develop new or improved integrated management strategies to control emerging insect pests and insect-transmitted pathogens of potatoes.
• Sub-objective 3A: Produce a “risk matrix” that ranks non-crop weedy plants as to importance as sources of infective potato psyllid or beet leafhopper arriving in potato fields and forward those rankings to the potato industry.
Approach
Sub-objective 1A: Identify weed sources of psyllids and leafhoppers. Approach: Molecular gut content analysis will be used to identify plant DNA in insects and define their feeding histories. Both species will be collected as they enter potato fields. Specimens will be tested with PCR for presence of plant pathogens. Presence of a specific plant in insect guts and correlated presence of pathogen DNA will be evidence the plant is a source of infective insects. Contingencies: Plant DNA that cannot be identified to species based on representation in the NCBI database will be identified to genus. Sub-objective 1B: Characterize genetic variation in beet leafhopper populations across regions and host species. Approach: NextRAD sequencing will be used to identify genetically-defined leafhopper subpopulations collected from potato fields and weed hosts. Genetic differentiation among regions and plants will be assessed by analysis of molecular variance. Contingencies: NextRAD sequencing is time intensive which may make it difficult to evaluate all regions and host-sources. We will supplement NextRAD data as needed by analysis of the CO1 gene. Sub-objective 2A: Characterize beet leafhopper feeding behavior to examine acquisition and inoculation of plant pathogens. Approach: Electropentagraphy (EPG) technology to be used to examine how leafhopper feeding behavior affects pathogen acquisition and inoculation in cultivated and weedy hosts. We will record the time required to begin a feeding event and time spent in an event for three behaviors: xylem ingestion, phloem salivation, and phloem ingestion. Probability of pathogen acquisition and inoculation will be evaluated as a function of these time durations. Contingencies: If we encounter difficulties with the EPG assays, we will consult the literature on EPG work with other leafhoppers. Sub-objective 2B: Identify predator species that attack potato psyllid and beet leafhopper in non-crop habitats. Approach: Molecular gut content analysis will be used to identify predators feeding on potato psyllid and beet leafhoppers in non-crop habitats. Insects for molecular assay will be extracted from plant samples in Berlese funnels. The COI gene will be PCR amplified to detect psyllid or leafhopper DNA. We will identify which predatory taxa most readily attack potato psyllid and beet leafhopper by comparing presence vs absence of prey DNA across predator specimens. Contingencies: No difficulties in completing this work is anticipated. Sub-objective 3A: Produce a risk matrix that ranks weedy hosts by importance as sources of infective psyllids and leafhoppers. Approach: Rankings will color-code each plant species according to risk (red, yellow, or green). Host plants color-coded red will be those found to be sources of vectors and pathogens, and to be common in the study region. Rankings will be made available to growers at research meetings and publication in industry newsletter. We will include suggestions of how risk rankings can be used to assist IPM programs through monitoring of at-risk fields or by eradication of high-risk species. Contingencies: No difficulties in completing this work is anticipated.
Progress Report
Several tasks were completed in support of Sub-objective 1A. Matrimony vine was sampled for presence of potato psyllids while various wild mustard species were sampled for beet leafhoppers. Beet leafhoppers were captured from tumble mustard and flixweed but were not present on the widely distributed blue mustard. Greenhouse assays confirmed that leafhoppers developed on flixweed and tumble mustard but failed to develop on blue mustard. Psyllids and leafhoppers were also captured directly into a DNA preservative from potato fields and natural habitats. Molecular gut content analysis was used to identify what plant species they had fed upon before capture. Results of gut content analysis will be used to correlate host plant use with infection by the zebra chip pathogen in potato psyllids or beet leafhopper-transmitted virescence agent (BLTVA ) in beet leafhoppers.
In support of Sub-objective 1B, field sites were located containing stands of winter and summer annual beet leafhopper host plants. Beet leafhopper collections began in spring 2020 from stands of flixweed/pinnate tansy mustard, kochia, Russian thistle, and tumble mustard, as well as at commercial potato fields. Nucleic acid extracted from beet leafhoppers was submitted for sequencing comprising specimens from almost 40 sites encompassing both weeds and commercial potato fields. A pilot sequencing run on eight beet leafhopper extractions was done to determine optimal sequencing parameters. Those results will be used to characterize genetic variation across populations of beet leafhopper on different host plants and to infer dispersal patterns from non-crop host plants into potato fields.
An uninfected beet leafhopper colony has been established to provide a consistent source of leafhoppers for studies to be completed in Sub-objective 2A. Sugar beet, radish, and dodder plants are being grown in a greenhouse in preparation for starting a beet leafhopper colony infected with beet leafhopper-transmitted virescence agent. These uninfected and infected beet leafhopper colonies will ultimately be used to measure feeding behavior related to pathogen acquisition and inoculation on potatoes and non-crop host plants.
In support of Sub-objective 2B, molecular methods for identifying predator species that feed extensively on potato psyllid in non-crop habitats were tested. Primers that detect DNA of potato psyllid but not DNA of other psyllids were developed, and then were confirmed in laboratory feeding trials to detect psyllid DNA in predator specimens that had been allowed to feed on potato psyllid. Collecting of predator specimens from weedy host plants of potato psyllid in non-crop habitats adjacent to potato fields began in summer 2020. Almost 400 specimens of predators are now in storage at -80 C for eventual assay using these new molecular methods.
In support of Sub-objective 3A, the potato psyllid prediction model developed in 2020 was updated with data collected in early spring of 2021. Above- average populations of potato psyllid were detected on matrimony vine suggesting that potato fields are at a high risk of psyllid infestation during the 2021 growing season. This ‘psyllid forecast’ has been communicated to potato growers by Washington State University Extension through their weekly on-line Potato Pest Alert updates.
Accomplishments
1. A highly diverse parasitoid complex attacks close relatives of potato psyllid. Population build-up of potato psyllid on weedy host plants outside of crop fields can be slowed by activities of natural enemies. ARS researchers in Wapato, Washington, along with scientists at Washington State University in Pasco, Washington, collected specimens of psyllid species closely related to potato psyllid, and then examined the insects for the presence of Tamarixia parasitoids, which are known to parasitize potato psyllid in other geographic regions. Results showed that psyllids associated with weedy chenopods, nettle, willows, field bindweed, and bitterbrush were parasitized by previously unknown species of Tamarixia. These findings include some of the first parasite records for several of these psyllid species and suggest that a large and poorly known complex of psyllid parasitoids, feasibly including parasites of potato psyllid, is common in habitats abutting potato fields of the Pacific Northwest.
2. Model that predicts threat of high psyllid pressure developed. Populations of potato psyllid, the vector of the zebra chip pathogen, fluctuate greatly from year to year in the Pacific Northwest. These population fluctuations are difficult to predict and challenge effective management of potato psyllid and zebra chip disease. ARS researchers in Wapato, Washington, along with scientists at Washington State University in Ephrata, Washington, showed that one weed species, called matrimony vine, is a particularly important host plant for potato psyllids in early spring. Fortunately, this plant is not susceptible to the zebra chip pathogen and therefore is not a likely source of infected psyllids that colonize potato. However, psyllid numbers on matrimony vine in early spring are highly correlated with numbers in potato in late August, and this correlation allows scientists and extension personnel to predict as early as March, whether potato psyllid populations will be extremely high or relatively low in July and August. These psyllid forecasts help growers take appropriate actions in years when psyllid outbreaks are expected.
3. The field bindweed psyllid is not a direct threat to potato. Zebra chip disease of potato is caused by a pathogen called Liberibacter that is acquired by potato psyllids when they feed on the phloem of potato plants. A close relative of potato psyllid known as the field bindweed psyllid also carries Liberibacter, but it was never clear whether the psyllid transmits the pathogen to potato. ARS researchers in Wapato, Washington, in collaboration with scientists at Washington State University in Pullman, Washington, and the University of Idaho, in Moscow, Idaho, used electropenetography technology to compare feeding behaviors of bindweed psyllid on host plant (field bindweed) tissues and the non-host potato. Researchers found that the psyllid readily feeds on the phloem of field bindweed but avoids the phloem of potato. Because bindweed psyllid avoids the phloem of potato, it is believed that this species is unlikely to transmit Liberibacter to potato. These results contribute to our understanding of how zebra chip disease is spread and provides important behavioral and biological insight into host plant use by psyllids.
Review Publications
Reyes Corral, C., Cooper, W.R., Karasev, A.V., Delgado-Luna, C., Sanchez-Pena, S. 2021. "Candidatus Liberibacter solanacearum” infection of commercial tomatillo, Physalis ixocarpa Brot. (Solanales: Solanaceae) in Saltillo, Mexico. Plant Disease. 105(9):2560-2566. https://doi.org/10.1094/PDIS-10-20-2240-RE.
Mustafa, T., Horton, D.R., Cooper, W.R., Zack, R., Thinakaran, J., Karasev, A., Munyaneza, J.E. 2021. Stylet-probing behavior of two Bactericera species on host and non-host plants (Hemiptera: Psylloidea: Triozidae). Environmental Entomology. 50(4):919-928. https://doi.org/10.1093/ee/nvab031.
