Recipient Organization
UNIV OF IDAHO
875 PERIMETER DRIVE
MOSCOW,ID 83844-9803
Performing Department
(N/A)
Non Technical Summary
Alfalfa is an important perennial forage crop in Idaho and in Oregon supporting dairy and cattle industries, typically grown in the same field for up to four years. As alfalfa stands age over successive seasons, multiple insect-transmitted viruses accumulate in the crop which leads to productivity decline and possible hay quality deterioration. Here, we will investigate the role of three common viruses found in southern Idaho--alfalfa mosaic virus, bean leafroll virus, and a newly discovered Snake River alfalfa virus (SRAV)--in alfalfa stand productivity, including hay yield and quality. This will be done through yield and quality comparisons between insecticide-treated and non-treated check plots set up in the field. The abundance and diversity of insect vectors will also be compared, and the distribution and prevalence of alfalfa viruses will be assessed using molecular tools, such as RT-PCR. We will also survey commercial alfalfa crops for the three viruses and for potential virus vectors throughout Idaho and in Oregon, to understand the prevalence of SRAV in alfalfa crops in the Pacific Northwest. Finally, we will determine the possible role of thrips in SRAV transmission and epidemiology through transmission experiments in the greenhouse. Comprehensive Extension programming will disseminate research results to stakeholders, providing producers with decision support tools to make informed decisions for management of insect vectors of viruses to improve forage alfalfa yield and quality.
Animal Health Component
60%
Research Effort Categories
Basic
30%
Applied
60%
Developmental
10%
Goals / Objectives
The major goals of the projectare to further characterize the distribution and prevalence of AMV, BLRV, and SRAV in Idaho and Oregon alfalfa fields, determine if and how virus infection affects forage yield and quality, and determine the possible role of insect vectors in transmission of SRAV between alfalfa plants.Objectives:1. Characterize distribution and prevalence of viruses in commercial alfalfa fields in Idaho and Oregon, including the novel alfalfa virus, SRAV2. Evaluate effects of virus infection on alfalfa yield by comparing yield, quality, and virus prevalence (SRAV, AMV, and BLRV) in insecticide-treated versus check plots3. Determine role of insect vectors in SRAV transmission by conducting transmission experiments under controlled greenhouse conditions4. Disseminate results to stakeholders via extension programming that includes field days, workshops, print, and web resources
Project Methods
Objective 1. Samples from commercial alfalfa fields throughout Idaho and Oregon will be collected from alfalfa plants visually assessed to exhibit mosaic, yellowing, vein clearing, leaf distortions, or growth retardation. A sample will be comprised of one complete alfalfa stem with all leaves intact, but without roots; this entire stem will be placed in a plastic bag and kept in a cooler on ice until reaching the laboratory. Samples will be taken from stands of different ages to explore the relationships among stand age and virus prevalence. Tissue aliquots for each sample will be either used for immediate testing by RT-PCR or stored at -80°C until further use.For a subset of plant samples collected from each field, plant stems will be vigorously shaken into a plastic bag to dislodge and collect any arthropods associated with symptomatic plants. In addition, foliar arthropods will be collected using sweep nets. Arthropod samples will be stored in a freezer and possible vectors (e.g., thrips, aphids, leafhoppers) will be identified to species and stored at -80°C until further use.Potential pitfalls: The possibility that SRAV is not widespread. This would be good news for the industry; however, it seems unlikely given our observations of the virus across several samples and in areas in Idaho ca. 50 miles apart.Objective 2. The study will be conducted at the University of Idaho Kimberly Research & Extension Center with field plots using a new seeding of a fall dormancy 3 or 4 variety with disease susceptibility arranged in a randomized complete block design. Plots will be at least 10 feet wide by 25 feet long with at least six replicates per treatment. Fertilization, irrigation, and weed management will be conducted based on University of Idaho Extension Guidelines. Treatments will include (1) a non-treated check and (2) a season-long insecticide spray program designed to limit aphid and thrips virus vectors. The insecticide programs will include a rotation of products that are registered for use against aphids and thrips in alfalfa (or that are known to be effective against these pests, though not necessarily registered in alfalfa), including thiamethoxam (Cruiser) seed treatment followed by foliar applications, in rotation: methomyl (Lannate), flonicamid (Beleaf), tolfenpyrad (Torac), abamectin + bifenthrin (Athena), and neem oil. These insecticides will be applied on a ca. 14-day interval over the season beginning after emergence. A set of study plots will be planted during both the first and second years of the study, with both sets of plots evaluated over two years, providing replication in space and time.Colonizing aphid species will be sampled periodically over the growing season by collecting stem samples from randomly selected plants within the center of each plot. Similarly, thrips will be sampled by shaking stems within a plastic zipper lock bag. All insects will be counted and identified to species and a subset of collected insects will be tested for presence of viruses using methods described for Objective 1. Other potential vectors (e.g., leafhoppers, Lygus bugs) will be saved from stem and plastic bag samples and sampled using sweep nets; these will be later identified and assessed for virus presence if needed (i.e., if thrips are determined not be a vector of SRAV in Objective 3).Alfalfa will be harvested at the bud stage for all harvests. Total herbage yield will be determined by cutting to a ca. 7.6-cm stubble using a research plot harvester equipped with an integrated hopper and load cell system to determine harvested wet weight. An approximate 1,000-g subsample will be collected directly from the hopper, weighed wet and dried at 60°C to a constant moisture and reweighed for dry matter yield and moisture content. Subsampled herbage will be ground using a Wiley Mill and a 2-mm sieve and then processed in a Udy cyclone mill equipped with a 1-mm screen. Yield and quality will be reported on a dry-weight basis and harvest moisture will be reported. Alfalfa herbage samples will be analyzed by near-infrared reflectance spectroscopy (NIRS) for acid detergent fiber (ADF), neutral detergent fiber (NDF), crude protein, degradable protein in crude protein, adjusted crude protein, protein solubility, ash, fat, starch, sugar, and lignin. A select set of random samples will be analyzed by wet chemistry to validate NIRS results. In addition, stem samples will be collected from each plot during harvests and tested for virus incidence as described above under Objective 1.Generalized linear models or generalized linear mixed models (with repeated measures as appropriate) will be used to compare the following among treatments: incidence of SRAV, AMV, and BLRV, yield and the quality attributes outlined above, and densities of known and suspected vectors discussed above.Potential pitfalls: The possibility of low natural virus and vector pressure. This seems unlikely given the prevalence of virus in our samples collected over 2020-2021 in alfalfa stands grown from 1 to 7 years as well as the ubiquity of potential thrips vectors in the study area. We will position field plots as close as possible to known virus-infected fields on our experimental farm.Objective 3. Symptomatic alfalfa plants in the field at the Kimberly Research & Extension Center will be tagged and sampled for SRAV presence. Plants confirmed to be SRAV-positive will be used as source plants for thrips transmission experiments. For each inoculation test, at least ten thrips will be placed on a known SRAV-infected plant within a sleeve cage and allowed to feed for ca. one week before being moved to a SRAV-negative greenhouse-grown plant (the same variety used in Objective 2, above). Transmission efficiency of thrips may be dependent on acquisition of the virus during the first or second instar, hence the need to allow for population development and a long access period on infected plants. Systemic infection of SRAV will be evaluated on test plants using our own RT-PCR test developed in-house. Generalized linear models will be used to compare SRAV incidence among treatments.Potential pitfalls: The possibility that thrips are not the vector of interest. For objective 2, we will collect other insects of potential importance as vectors and, if needed, screen them for virus and conduct additional greenhouse experiments similar to those described here.Objective 4. Results will be reported in peer-reviewed publications and presented at professional scientific meetings. Results also will be disseminated to local and regional alfalfa industry stakeholders through a variety of media: existing web-based tools, written and oral reports and presentations, publication of extension articles (e.g., PNW Extension bulletins) as print or web-based documents, oral and poster presentations at field day tours and workshops. Results will be extended to producers, crop consultants, pest management advisors, and extension educators at local and regional extension events.Extension programming will occur in multiple venues, which may include the following: the Idaho Hay and Forage Conference, the Idaho Association of Plant Protection Meeting, and the Entomological Society of America Meeting. In addition, research plots will be presented each year at the Snake River Pest Management Tour at the Kimberly Research & Extension Center. Moreover, Best Management Practices for managing viruses in alfalfa in the Pacific Northwest will be published as a PNW extension article. Efficacy of extension efforts will be evaluated by conducting surveys of stakeholders to assess changes in knowledge and behavior in relation to this project.Potential pitfalls: Reluctance of growers to adopt new management approaches. We have been working closely with industry partners, including the Idaho Hay and Forage Association, which will help us to educate producers on implementation of our findings.