Properties of a resistance‐breaking strain of potato virus X

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Abstract
SUMMARY: During indexing of a potato germplasm collection from Bolivia, a strain of potato virus X (PVX), XHB, which failed to cause local lesions in inoculated leaves of Gomphrena globosa was found in 7% of the clones. XHB was transmitted by inoculation of sap to 56 species from 11 families out of 64 species from 12 families tested. It was best propagated in Nicotiana glutinosa or N. debneyi; Montia perfolia and Petunia hybrida were useful as local lesion hosts. Inoculated leaves of G. globosa plants kept at 10°, 14°, 18°, 22°, or 26 °C after inoculation were always infected symptomlessly. XHB caused a mild mosaic, systemic chlorotic blotching or symptomless infection in 16 wild potato species and eight Andean potato cultivars, systemic necrotic symptoms in clone A6 and cultivar Mi Peru, and bright yellow leaf markings in cultivar Renacimiento. It caused necrotic local lesions in inoculated leaves of British potato cultivars with the PVX hypersensitivity gene Nb but then invaded the plants systemically without causing further necrosis; with gene Nx systemic invasion occurred but no necrotic symptoms developed. These reactions resemble those of PVX strain group four. XHB differed from other known strains of PVX in readily infecting PVX‐immune clones 44/1016/10, G. 4298.69 and USDA 41956, cultivars Saphir and Saco, and Solanum acaule PI 230554.XHB had slightly flexuous filamentous particles with a normal length of 516 nm. It was transmitted readily by plant contact and it partially protected G. globosa leaves from infection with XCP, a group two strain of PVX. Sap from infected N. glutinosa was infective after dilution to 10‐‐6 but not 10‐‐7 after 10 min at 75° but not 80 °C and after 1 yr at 20 °C. XHB was readily purified from infected N. debneyi leaves by precipitation with polyethylene glycol followed by differential centrifugation. Microprecipitin tests showed that XHB and XCP are closely related serologically.