Many factors affect symptom expression of PMTV in tubers. Foliar PMTV expression is influenced by environmental conditions such as low levels of light and moderate temperatures. Varieties can be insensitive to both foliar and tuber symptoms, meaning that virus is present with no observable symptoms. Spraing is the term used to describe necrotic tuber symptoms associated with PMTV and with Tobacco rattle virus (TRV). These viruses have two different vectors, but a lab test is needed for positive confirmation of the cause of spraing. PMTV is an emerging disease and is only addressed in the current seed certification system by shipping point inspections with a set of tolerances based on visual symptoms. Recent trials in infested fields in Idaho and Washington examined the percentage of symptoms and the percentage of virus in tubers. Published studies from North Dakota have looked at tuber symptoms, using RT-PCR only to confirm the presence of PMTV and absence of TRV. Phenotyping field studies in Washington state were based mostly on tuber symptoms using a high number of varieties/breeding clones. A small subsample of tubers was tested by RT-PCR with results showing that approximately 20% of asymptomatic tubers were PMTV positive. A phenotyping field study in Idaho examined a much higher number of tubers using RT-PCR. The trial included Castle Russet, Pomerelle Russet, Clearwater Russet, and Russet Burbank. The results show that positive asymptomatic tuber percentages averaged 26%. Pomerelle Russet has the lowest asymptomatic tubers at 7.8% and Russet Burbank had the highest at 55.6%. The presence of virus positive asymptomatic tubers makes control of PMTV difficult and illustrates the need to develop resistant varieties.