MITIGATING HIGH CONSEQUENCE DOMESTIC, EXOTIC, AND EMERGING DISEASES OF FRUITS, VEGETABLES, AND ORNAMENTALS

This is the final report for this project.Candidatus Liberibacter asiaticus (Las) in vitro culture was greatly improved with much more consistent results along with the development of semi-selective medium. Las was maintained and propagated in liquid medium for >2 years, and caused Huanglongbing (HLB) via psyllid feeding and citrus inoculation. Selections of citrus seedlings and bud sports were evaluated for HLB resistance/tolerance via graft-based and psyllid inoculations. Resistant/ tolerant selections from greenhouse evaluations were further propagated, and planted in research and commercial grove for field trials where they have been evaluated for 3-4 years. The best new selections have gone through pathogen clean-up by the Florida Division of Plant Industry to be ready for future release. From Las pathogenesis standpoint, ARS scientists at Fort Pierce, Florida, demonstrated that Las 5315 effector induced extreme starch accumulation and chlorosis, typical symptoms of HLB, and revealed that host DEAD-box RNA helicase (DDX3) was targeted by Las5315, and that downregulation of host DDX3 gene was associated with leaf chlorosis. These findings facilitate our understanding of Las pathogenesis and HLB symptom development. Aspects of tissue colonization and population development in different citrus genotypes showed that Las populations peaked within 30 days of tissue colonization and did not increase significantly after this time. Calculations indicated that bacterial populations increased slowly in planta, with a very slow doubling time approaching 48 hours. Transmission of tospoviruses, ilarviruses and squash vein yellowing virus (SqVYV) is continuing to be elucidated. Production scale monitoring of tospoviruses and ilarviruses was investigated. Seeds of fruits from watermelon plants infected with SqVYV were collected for virus testing and seed transmission studies. A novel tobamovirus (Hoya chlorotic spot virus) was isolated from the ornamental crop, Hoya spp., and its biology, host range and genome were characterized. A second novel tobamovirus (Brugmansia latent virus) was identified in another ornamental crop, Brugmansia spp., and its genome was sequenced and characterized with host range ongoing. A viable-cell detection assay for specific detection of living cells of Xanthomonas fragariae (Xf), the cause of strawberry angular leaf spot (ALS) was developed. The assay was used to determine the survival of Xf on the surfaces of 11 materials stored at multiple temperatures and days after inoculation (DAI) to determine the likelihood of those materials serving as source of inoculum for disease. All materials harbored viable bacteria up to 7 DAI when stored at -4°C. Xf survived 270 DAI on cardboard at -4°C. At 20°C, cardboard, cotton balls, cotton t-shirt, and strawberry leaf tissue supported bacterial populations up to 14 DAI. Information from this study can be used to improve sanitation practices for ALS management in strawberry nurseries. In cooperation with industry partners, we are developing technology to identify and map regional distribution of crops on satellite imagery, and produce landscape-level risk maps of whitefly and whitefly-transmitted viruses. Initial testing in southwest Florida showed this to be a good predictor of crop type once the crop has grown sufficiently to be â¿¿seenâ¿