Nicotiana megalosiphon, a Highly Susceptible, New, and Useful Host for Potato Virus A.

Potato virus A (PVA; genus Potyvirus, family Potyviridae) occurs wherever potatoes are grown and may reduce tuber yields as much as 40%. Its host range consists of six experimental hosts (Lycopersicon pimpinellifolium (Jusl.) P. Mill., Nicandra physalodes (L.) Gaertn., Nicotiana tabacum L., Solanum demissum Lindl., S. demissum × S. tuberosum [2], and Nicotiana debneyi Domin.) and two natural hosts (S. tuberosum L. and S. betaceae (Cav.) Sendt.) (2). Aphids transmit PVA in a stylet-born manner. Its difficult mechanical transmission, caused by a low virus concentration in potato and other hosts (1), has constrained pathological research on the virus. In routine studies to identify virus isolates from the field, we discovered that N. megalosiphon Van Heurck & Mull. Agr. is a superior host of PVA that markedly facilitated diagnosis, selection for resistance to PVA, and other research applications. The efficiency of mechanical transmission of PVA to potato (5 duplicated assays and 10 plants per assay) ranged from 0 to 10% with PVA-infected potato as the virus source, 0 to 30% with Nicandra physalodes, 10 to 30% with N. tabacum cv. Samsun, and 20 to 80% with N. megalosiphon as the source of virus. The efficiency of mechanical transmission to four systemic hosts of PVA with potato (cv. Russet Burbank) as the source of virus (5 duplicated assays and 10 plants per assay) ranged from 0 to 20% to potato, 0 to 30% to Nicandra physalodes, 10 to 40% to N. tabacum cv. Samsun, and 80 to 100% to N. megalosiphon. The superiority of N. megalosiphon as a host and source of PVA was associated with a high virus concentration in tissues. Infected potato leaves yielded 0.32 to 0.54 mg of virus per kg of infected leaves, Nicandra physalodes yielded 0.37 to 0.66 mg per kg, N. tabacum cv. Samsun yielded 0.78 to 1.22 mg per kg, and N. megalosiphon yielded 5.16 to 9.39 mg per kg of infected leaves in five different purification experiments. These yields are based on the amount of virus isolated in sucrose gradients subjected to rate-zonal centrifugation as the last step in purification (3). The virus antigen concentrations of the original PVA-infected tissues measured using quantitative enzyme-linked immunosorbent assay ranked virus concentrations in the same relative order as purification but were nearly 2 times higher than were the purification yields. Similarly, local lesion assays on S. demissum A leaves (4) ranked infectious virus concentrations in the same order as did purification. Efficiency of aphid transmission from the four hosts was not assayed. Infected N. megalosiphon plants survived and served as sources of PVA for at least 1 year in a greenhouse. N. megalosiphon is an important new host of PVA because it facilitates the routine transmission of the virus and other manipulations essential for efficient research on control of the virus disease. References: (1) R. Bartels. No. 54 in: Descriptions of Plant Viruses. CMI, Kew, Surrey, UK, 1971. (2) A. Brunt. Page 77 in: Virus and Virus-Like Diseases of Potatoes and Production of Seed Potatoes. G Loebenstein et al., eds. Kluwer Academic Publishers, Dordrecht, the Netherlands, 2001. (3) P. E. Thomas and W. K. Kaniewski. Page 285 in: Virus and Virus-Like Diseases of Potatoes and Production of Seed Potatoes, G. Loebenstein et al., eds. Kluwer Academic Publishers, Dordrecht, the Netherlands, 2001. (4) R. E. Webb and R. W. Buck. Am Potato J. 32:248, 1955.