Development of Monitoring Tool for Managing Tospovirus Damage To Lettuce

Abstract

Impatiens Necrotic Spot Virus (INSV) is a recently introduced, thrips-transmitted tospovirusaffecting lettuce (Lactuca sativa) production in Yuma, Arizona. Since its discovery in Spring 2021 in Yuma, INSV has occurred every year, with incidences as high as 8% symptomatic lettuce plants at harvest during this study conducted in 2022-2023. Using RT-qPCR of lettuce plants, we detected the Tomato Spotted Wilt Virus (TSWV) in co-occurrence with INSV for the first time in Yuma in Spring 2022. However, RT-qPCR of lettuce plants and western flower thrips (Frankliniella occidentalis) populations did not detect TSWV in 2023. RT-qPCR of field-collected thrips coupled with bioassays evaluating their capacity to generate INSV symptoms in petunia leaf disks indicated that amplification of INSV primers alone can overestimate the thrips capacity to generate INSV symptoms in plants. To better evaluate the vectorial capacity of field-collected thrips, we developed a method based on amplification of primers for INSV and a reference gene in groups of individuals containing a known proportion of INSV-free thrips and thrips with empirically established positive vectorial capacity. This method was effective in detecting percentages of vector competent thrips as low as 0.5% (i.e., 1 vector competent individual out of 199 non-infected individuals). In February and March of 2023, surveys of 15 romaine lettuce fields with pairs of yellow sticky traps and transplanted petunia plants placed at the border of fields revealed that petunia plants captured a similar number of thrips as sticky traps. However, the petunia plants did not develop symptoms of INSV infection during the three weeks, even when vector competent thrips were detected on the petunia plants and in adjacent sticky traps. The proportion of vector competent thrips found in sticky traps and on petunia plants was not different. The vector manipulation hypothesis predicts a preference of non-virulent vectors for infected host plants and a preference of virulent vectors for non-infected host plants. We found that the distribution of virulent and non-virulent thrips on INSV-infected and non-infected lettuce plants in the field was consistent with this hypothesis. A key objective of this study was to determine whether INSV damage in lettuce fields at harvest can be predicted by monitoring INSV-competence in thrips and their population density early in the growing season. For each of 3 weeks of sampling, we calculated the risk of virus transmission in the 15 lettuce fields as the product of the number of thrips collected and the estimated proportion of INSV-competent thrips captured in yellow sticky traps at the border of the fields. For each of the three weeks, there was no significant positive association between the risk of virus transmission and the percentage of INSV-symptomatic lettuce plants at harvest. Sample size and the incidence of INSV infection in lettuce fields was relatively low in this study. Accordingly, more work is needed to evaluate whether the risk of virus transmission as estimated here can be used to forecast INSV damage in lettuce fields at harvest.