Powdery scab situation
The appearance of the disease is variable from year to year, being dependent upon a conducive environment. Until recently, the disease has been considered rare and unimportant. However, it has become a concern for potato producers in Maine. But In the last years there is only limited interest when the diseases occur on a high level. As there were not two years like that in a row, interest is distracted.
Research
Potato mop-top virus (PMTV) is a tripartite pomovirus vectored by the powdery scab plasmodiophoromycete Spongospora subterranea
pv. subterranea. PMTV occurs on potato (Solanum tuberosum) in Europe, the Andes, Asia, and Canada. Internal necrotic arc
and fleck tuber symptoms ("spraing") may reduce commercial acceptance of some cultivars. PMTV symptoms were discovered in
'Shepody' tubers at the Aroostook Research Farm, Presque Isle, ME in May 2002 and subsequently in 'Russet Burbank' tubers in
commercial storage from the 2001 Maine crop. Symptomatic tubers exhibited single or multiple concentric necrotic arcs that were
partial or complete, but exhibited no distinct external symptoms.
The presence of PMTV in eight 'Shepody' tubers was indicated by positive enzyme-linked immunosorbent assay
(ELISA; Adgen, Ltd., Auchincruive, Ayr, Scotland) and confirmed by reverse-transcription polymerase chain reaction (RT-PCR).
'Russet Burbank' potatoes were visually diagnosed, and the corresponding halves of 128 symptomatic tubers were forwarded to
the University of Maine and APHIS (Beltsville, MD). Of these, ELISA readings in Maine were strongly positive (>3 x background)
for 88, ambiguous (1.5-3 x back-ground) for 13, and negative for 27. Subsamples from these three categories were positive by
PCR in 17 of 17, 9 of 9, and 12 of 14 cases, respectively. A similar rating, positive or ambiguous, in ELISA testing was
identical for all but one case at Beltsville.
Confirmation of PMTV required PCR testing, resulting in a characteristic PCR product of 401 bp that was generated from the coat
protein coding region on RNA 2 (2) using the primer pair PMTV 1 5'-GCAGCCGTCGAGAATAGATA-3' (RNA nucleotides 316-335) and
PMTV 4 5'-GCGAGTTGATGTGCCACATT-3' (complementary to RNA 2 nucleotides 716-697). An immunocapture RT-PCR using this primer set
and the coating antibody from the Adgen ELISA kit was also successful in detecting PMTV. In separate reactions, a second product
of 646 bp was generated from the triple gene block on RNA 3 using the primer pair PMTV 5 5'-GGTGAAGAGGACAAGGT-3'
(RNA molecules 1417-1436) and PMTV 7 5'-AACAGTCCGGTCTTGTGAAC-3' (complementary to RNA 3 nucleotides 2063-2044). The sequence of
these products was 98 to 100% identical to PMTV published sequences.
The discovery of this virus will result in adjustments to U.S. and Canadian seed potato certification standards and symptom
characterization for common North American cultivars.
Contact address:
Steven B. Johnson
Crop Specialist
University of Maine
Cooperative Extension
59 Houlton Road
Presque Isle
Maine 04769
USA
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Last update: January, 2025