Response of potato, tobacco and Physalis floridana plants to mixed infection with PVX, PVYNTN and PVY° strains

Abstract

Post-harvest screening of potato ‘Kennebec’ revealed a Potato virus Y (PVY) incidence of 15.8%, a rate that is unusually high for a cultivar possessing a high level of field resistance to the virus. Randomly selected tubers were planted in a field plot and the resulting plants were monitored. Approximately 16% of plants developed symptoms ranging from mild mosaic symptoms to severe necrosis/rugosity/stunting. ELISA and RT–PCR analysis revealed that infections with Potato virus S (PVS), Potato virus X (PVX) and PVY, mostly in mixed-infections, occurred commonly in 14 sampled plants. Two strains, namely the common strain (PVY°) and the recombinant tuber necrotic strain (PVYNTN) were identified in the PVY-positive plants. In general, mild mosaic was associated with infections with PVX and PVS; intermediated mosaic was associated with PVS and PVYNTN infections; whereas severe leaf deformation/necrosis/drop symptoms were associated with PVYNTN and PVX co-infections, or with PVY° and either PVS or PVX co-infections. Virus-free plantlets of potato ‘Kennebec’ were mechanically inoculated with PVX, PVY°, and PVYNTN alone or with PVX+PVY° or PVX+PVYNTN combinations in the greenhouse. Single infections with PVY° or mixed-infections with PVX+PVY° or PVX+PVYNTN incited severe mosaic symptoms and systemic necrosis soon after inoculation; whereas single infections with PVX and PVYNTN induced mild to intermediate mosaic symptoms only. The most severe symptoms occurred in the mixed-inoculation with PVX+PVYNTN, demonstrating dramatic synergism between these strains. Similarly, profound PVX and PVYNTN synergism was also found in tobacco and Physalis floridana plants, suggesting that the strain of PVY plays an important role in the level of synergistic reactions between PVX and PVY on host plants.