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top virus in the northwest U.S. over the past several years. This has included assessment of the current threat of this pathogen to the local industry, assessment of seedborne viral transmission, and development of a new greenhouse screen to identify sources of resistance to Potato mop-top virus. This work has been funded by two different NIFA-SCRI grants focused on tuber necrotic viruses, and the Wash- ington State Potato Commission and Northwest Potato Research Consortium. Seed Lot Testing For Potato Mop-Top Virus Since 2016, potatoes submitted to the Washington State University Othello Seed Lot Trial have been tested for Po- tato mop-top virus by researchers at the USDA in Prosser. Despite only testing four seed tubers per seed lot, Potato mop-top virus has consistently been found in seed lots at rates between 1.73 percent (in 2016) and 6.67 percent (in 2023) (see Table 1). Infected tubers were nearly all asymp- tomatic, suggesting that visual scoring is not an accurate method to detect this viral pathogen. The infected lots originated from six states across the U.S. and one province in Canada. Potato mop-top virus is therefore widespread in seed production areas. Virus infec- tion was found in 24 different cultivars over the eight-year study, highlighting the lack of resistance in commercial cultivars. The finding of Potato mop-top virus infected seed is concerning because it can readily be vectored by S. sub- terranea, which is present in commercial fields across the Columbia Basin of Washington. While the S. subterranea vector by itself is not viewed as a large threat to russet po- tato cultivars grown in this region, introduction of Potato mop-top virus could lead to serious yield and quality losses over time. However, it is currently unknown how long it might take Potato mop-top virus levels to establish and affect grower returns, if ever. It is also unknown whether this seedborne Potato mop-top virus infection has a con- sequence on daughter tuber symptom development when grown in vector-free soil. Assessment Of Potato Mop-Top Virus Trans- mission In Seed To answer these questions, USDA researchers in Prosser began conducting field trials to assess Potato mop-top virus transmission in seed planted in S. subterranea-free soil and in soil recently contaminated with S. subterranea. In an initial trial conducted in 2022, three seed treatments were selected for Galena Russet and Red LaSoda: asymptomatic Potato mop-top virus-free seed, asymptomatic Potato mop- top virus-infected seed, and symptomatic Potato mop-top virus-infected seed. Two seed treatments were selected for the Modoc culti- var, including asymptomatic Potato mop-top virus-free seed and symptomatic Potato mop-top virus-infected seed. Four replications per treatment were planted at two locations (Fields 1 and 2) in S. subterranea-free soil. Emergence rates were comparable between Potato mop-top virus-free and Potato mop-top virus-infected seed treatments for all cultivars. The effect of Potato mop-top virus presence on daughter tuber yield was cultivar- and location-dependent. Modoc showed a reduction in yield when Potato mop-top virus was present in the seed piece, which was statistically significant in field 1, but only numerically lower in field 2. The percentage of daughter tubers with internal necro- sis, as well as disease symptom severity (measured as the Figure 1. Potato mop-top virus causes internal tuber necrosis that can appear as small brown flecks or larger corky blemishes and arcs (A) and causes external symptoms that appear as necrotic arcs or blem- ishes in certain cultivars (B). The virus is vectored to potato by Spongospora subterranea which causes powdery scab on tuber surfaces (B) and root galling (C). WWW.POTATOGROWER.COM 35