In February 2012, commercially produced potato (Solanum tuberosum) tubers, cv. Modoc, grown in southeast Idaho, were observed with internal necrotic arcs and lines. Samples were obtained from potatoes that had been washed and packaged. No external symptoms were evident. Multiple samples were collected from the packing line and cut to check for internal defects as part of the normal grading procedure. The incidence of symptomatic tubers from these samples was determined by personnel at the packaging facility to be approximately 3%. Initially, one symptomatic and one asymptomatic tuber were tested. Total RNA extracted from these tubers were tested by RT-PCR at Aberdeen, Idaho, with primers specific for Potato mop top virus (PMTV) (1) and Tobacco rattle virus (TRV) (4). RT-PCR results showed that the symptomatic tuber produced a band at 416 bp with the PMTV primers, which was also present in the PMTV-positive control. No amplification was observed with the TRV primers. The asymptomatic tuber was negative for both PMTV and TRV. Subsequently, total RNA from four additional symptomatic tubers from the same lot were tested at USDA-ARS in Prosser, WA, by RT-PCR for TRV (4) and with a different set primers for PMTV (2). The tests included two PMTV-positive controls from cv. Alturas tubers (1), a healthy cv. Russet Burbank control, and a water control. Results showed that amplified products of 460 bp were obtained with the PMTV primers for the four symptomatic tubers and the same tubers were negative for TRV. In addition, symptomatic tissue from the four tubers tested positive for PMTV by ELISA using a commercially available kit (Adgen, Ayr, Scotland). Symptomless Russet Burbank tubers and water controls were negative in RT-PCR and ELISA tests. The 460 bp PMTV amplicon from two symptomatic Modoc tubers were cloned and sequenced. The sequences were identical and the sequence (GenBank Accession No. JX239990) was 100% identical to the corresponding sequences of PMTV isolates from North Dakota (HM776172) and Finland (AM503632). There was one nucleotide difference from the corresponding sequence of a PMTV isolate from Washington (JN132117). To our knowledge, this is the first published report of PMTV in Idaho and confirms that PMTV exists in southeast Idaho. A previous report made by Canada in 2004 (Plant Dis. 88:363) indicates that PMTV was found in multiple states and provinces, but no specific locations were identified. This report follows reports of PMTV in commercial potatoes in Washington (1), North Dakota (2), and Maine (3). In 2011, 129,000 hectares of potatoes were grown in Idaho, representing 29% of the fall grown potatoes in the United States. PMTV can cause quality problems and as evidenced by these samples with no external symptoms, problems may be compounded because of internal symptoms that may go undetected. The confirmation of PMTV alerts growers and processors to the presence of this virus in this important potato-producing state.
References: (1) J. M. Crosslin. Plant Dis. 95:1483, 2011. (2) N. David et al.Plant Dis. 94:1506, 2010. (3) D. H. Lambert et al.Plant Dis. 87:872, 2003. (4) D. J. Robinson. J. Virol. Methods 40:57, 1992.