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High Plains wheat mosaic virus (HPWMoV) is an emerging problem for the corn seed industry.  First detected in the 1990s in the High Plains region of the US, the virus has been of relatively minor significance for sweet corn and corn growers, as well as seed production, based on evidence of a very low risk of seed transmission. However, more recent evidence that HPWMoV might have been introduced into several countries on sweet corn seed, combined with empirical evidence that the risk of seed transmission might be greater on some corn genotypes, has led to implementation of phytosanitary restrictions on corn seed imported from the US, including stringent testing of corn seed. This has become a major constraint to the US corn seed industry, particularly in the Pacific Northwest where 90-95% of sweet corn seed production in the US occurs in the Treasure Valley of eastern Oregon and western Idaho plus the Columbia Basin of central Washington and northcentral Oregon. The import restrictions by countries such as Chile have affected the ability to export corn and sweet corn seed, and they threaten the winter nursey breeding programs of many US seed companies that use Chile for a second generation of breeding and seed production each year. An emergency ruling from the Chilean regulatory authority, to be implemented in early 2023, only permits corn seed lots from the US that test negative for HPWMoV to be imported into Chile. The rule is to be implemented in early 2023. HPWMoV isolates collected from sweet corn seed and processing crops in the Pacific Northwest in 2022 were far more virulent than isolates from the Midwest in preliminary inoculation tests. The isolates also caused a wide diversity of symptoms. It is not known if the high virulence of these Pacific Northwest isolates accounts for increasing prevalence of HPWMoV in corn seed lots produced in that region. Furthermore, there have been no studies to date on the effect of host resistance on seed transmission of HPWMoV in corn. In this Project Initiation grant, we propose to: i) assess the diversity of HPWMoV isolates affecting corn crops in the Pacific Northwest US, and ii) evaluate the influence of corn genotype on seed transmission rates of the virus using susceptible and resistant cultivars. Understanding nucleotide sequence diversity of the isolates will aid seed testing efforts by the US National Seed Health System to develop a standardized protocol for HPWMoV detection in corn seed. The protocol could also be used for harmonization of seed testing with regulatory agencies in other countries. The project is expected to generate foundational knowledge for developing IPM strategies to manage this emerging viral disease in corn and reduce the risk of seed transmission. The knowledge should inform regulatory policies with science-based insight into relevant seed testing methods to reduce the risk of introducing this virus to other countries while facilitating corn seed trade (see the Logic Model).

1. Assess the diversity of High Plains wheat mosaic virus in the Pacific Northwest United States.

Field sampling will be conducted across the major areas of seed corn production in the Western United States to obtain isolates of HPWMoV for sequencing and further characterization. While HPWMoV disease progression typically proceeds from streaking to mosaic to necrosis, preliminary experiments with isolates collected from the Pacific Northwest in the 2022 growing season indicate that some isolates do not cause the streaking phase while others display reddening prior to necrosis. Hence, there is much diversity in this virus and the associated disease progression and symptomology that have yet to be captured and characterized. Systematic characterization of isolate diversity will inform isolate selection for seed transmission experiments (Objective 2) to ensure isolates representative of the virus diversity in the primary sweet corn seed production region of the US are chosen (see Timeline).

2. Evaluate seed transmission rates of High Plains wheat mosaic virus in susceptible and resistant corn germplasm.

Two seed transmission experiments will be conducted: one to determine seed transmission rates of a single isolate on several cultivars varying in resistance to HPWMoV and another to determine seed transmission rates of several HPWMoV isolates from the Pacific Northwest on a single susceptible sweet corn cultivar. Mite transmission will be used to inoculate corn using the local Ohio isolate since it is higher throughput and more representative of what occurs in the field than VPI. However, VPI will be used for isolates from the Western United States as the USDA APHIS Plant Protection Quarantine (PPQ) permit of the Co-Directors does not permit vector transmission for out-of-state isolates.