Location: Temperate Tree Fruit and Vegetable Research
Project Number: 2092-21220-003-036-S
Project Type: Non-Assistance Cooperative Agreement
Start Date: Sep 1, 2025
End Date: Aug 31, 2026
Objective:
Research Objectives:
1. Develop and validate diagnostic methods for Spongospora subterranea and Potato mop-top virus to establish inoculum thresholds.
2. Optimize field soil sampling strategies for S. subterranea and Potato mop-top virus detection.
3. Compare soil S. subterranea/Potato mop-top virus levels with S. subterranea/Potato mop-top virus tuber symptoms.
Approach:
Objective 1: Development and Validation of Diagnostic Methods: Spongospora subterranea Detection: A density-based sporosori isolation method using LUDOX colloidal silica will be optimized for quantifying S. subterranea resting spores in soil. Preliminary tests by the collaborator successfully detected S. subterranea in soil samples where quantitative polymerase chain reaction (qPCR) failed. This method will serve as an alternative to molecular diagnostics and will be validated against qPCR-based detection. Potato mop-top virus (PMTV) Detection: A multiplex quantitative reverse-transcription polymerase chain reaction (qRT-PCR) assay using TaqMan probes will be designed to simultaneously detect all three PMTV ribonucleic acid (RNA) segments (RNA1, 2, and 3). Distinct fluorophores synthesized in TaqMan probes will enable differentiation of each RNA in a single reaction. This assay will be validated using symptomatic and asymptomatic tubers as well as roots from infected potato plants. To ensure reliability, key diagnostic parameters (specificity, sensitivity, repeatability) will be assessed.
Objective 2: Optimization of Sampling Strategy: In 2022 and 2023, heavily infested S. subterranea and PMTV seed tubers were planted in a field at the joint Washington State University/USDA-ARS research station in Prosser, WA, in efforts to generate a S. subterranea/PMTV disease field for research purposes. Tomato plants grown in the greenhouse and inoculated with PMTV-infected S. subterranea sporosori were also transplanted into this field in 2023 and 2024. This inoculated field provides an excellent opportunity to assess S. subterranea and PMTV levels across a field to generate a pathogen distribution map. Soil sampling will be done before planting and after harvest using an AMS Field Sampler implement towed by a tractor, and samples will be subjected to pathogen detection using the newly developed and validated diagnostic methods. Understanding pathogen pressure across a field and over the duration of a field season will help determine the optimal number and placement of subsamples for reliable detection in a field plot.
Objective 3: Comparing soil S. subterranea/PMTV levels with S. subterranea/PMTV tuber symptoms: Galena Russet seed will be planted in the new S. subterranea/PMTV disease field at the USDA-ARS research station in Prosser in the spring and the field will be managed to promote good plant growth and tuber development. Tubers will be harvested in the fall using a systematic approach to document general location within the field. Tuber yield will be assessed, and the percentage of tubers with powdery scab lesions on the surface, caused by S. subterranea, or tuber necrosis, caused by PMTV, will be recorded. Internal tuber samples will be collected from a subset of tubers and subjected to pathogen testing using the newly developed and validated diagnostic methods. Correlations between S. subterranea/PMTV soil levels and tuber symptoms/pathogen presence will be analyzed if soil pathogen threshold are associated with yield loss or levels of tuber infection.