Effect of temperature on resistance to Potato virus Y in potato cultivars carrying the resistance gene Rychc

The effect of cultivation temperatures on the resistance reaction to three Potato virus Y strains (PVYO, PVYN and PVYNTN) in potato cultivars carrying Ry(chc) was examined. When potato plants carrying Ry(chc) were cultivated at 22 degrees C, a few small necrotic spots developed on inoculated leaves by 5days after mechanical inoculation (dpi), and systemic infection of a few symptomless plants was confirmed at 28 dpi by IC-RT-PCR. At 28 degrees C, distinct necrotic spots developed on inoculated leaves by 5 dpi, and systemic symptoms occasionally appeared at 28 dpi. Thus, high temperature weakens Ry(chc)-conferred resistance. However, the incidence of systemic infection and the titre of virus in resistant cultivars at 28 degrees C were lower than in a susceptible cultivar. In graft inoculation under high summer temperatures, some plants developed necrosis on the leaves and stem, but PVY was barely detected by RT-PCR in leaves on potato carrying Ry(chc). When seedlings from progeny tubers of plants that were inoculated with PVY and grown in a greenhouse at >30 degrees C in the daytime were examined by ELISA and IC-RT-PCR, PVY was not detected in cultivars carrying Ry(chc). These results show that Ry(chc) confers an extreme resistance to PVY strains occurring in Japan.