Abstract
Alfalfa is a perennial legume globally grown as a forage crop and holds significant economic value for the livestock feed industries. Its biological attributes such as higher nutrients content (protein and fiber), better regrowth capacity, multiple cuttings, dense canopy sheltering diverse insect communities and nitrogen fixing ability in soil, make it a high value crop in agricultural system. However, alfalfa is often recognized as a source for several plant viruses and insect vectors facilitating virus transmission. Thus, the study of the impact of plant viruses on yield and quality of alfalfa is critical when considering the long-term persistence of viruses in the field. The two main objectives of this study were (i) Evaluating the effects of insecticide treatments on insect-transmitted virus infection and alfalfa yield and quality and (ii) Do western flower thrips (Frankliniella occidentalis) vector Snake River Alfalfa Virus (SRAV) in alfalfa (Medicago sativa L.)? Chapter one discusses a two-year field experiment comparing two treatments: insecticide rotation targeting thrips and aphids and non-treated checks. Insecticide applications were effective in controlling the aphids and thrips number in the field; however, no statistical significant effects were observed in the yield of alfalfa. Similarly, the forage quality analysis that included crude protein (CP), acid detergent factor (ADF), neutral detergent factor (NDF), and relative feed value (RFV), showed no clear differences between the treatments. This lack of differences may be due to beneficial insect reduction in the treated plot. Despite the vector management, alfalfa viruses; alfalfa mosaic virus (AMV), bean leaf roll virus (BLRV), pea streak virus (PeSV) and Snake River alfalfa virus (SRAV) were widely distributed between the treatments. Our results suggest that insecticide applications targeting insect vectors do not seem promising in reducing virus infection, highlighting complex insect-plant pathogen interaction in alfalfa systems.
In chapter two, the SRAV, a new virus discovered from alfalfa fields in Idaho, was also detected in western flower thrips (Frankliniella occidentalis), raising the vector-mediated transmission possibility of SRAV. To investigate the potential role of western flower thrips, a greenhouse transmission assay was conducted. Alfalfa plants were tested for SRAV using RT-PCR (reverse transcription polymerase chain reaction), and plants that tested positive were used as an inoculum source. After a weeklong acquisition access period, western flower thrips exposed in SRAV positive plant were tested and confirmed to exhibit no virus acquisition. Additionally, thrips in the study were exposed to a weeklong inoculation access period, further affirming no virus infection in the alfalfa plants. The results suggest western flower thrips do not acquire the SRAV virus and are not responsible for virus transmission in alfalfa. The hypothesis that western flower thrips serve as a vector for SRAV has not been supported; however, our study is consistent with the hypothesis that SRAV is seed-borne with almost 83% of alfalfa seedlings testing positive. The widespread prevalence of SRAV and other AMV, BLRV, and PeSV underscores the challenges associated with management strategies.