EPIDEMIOLOGY OF POTATO VIRUS-Y IN NEW-BRUNSWICK, CANADA - A BIENNIAL RHYTHM AND A 9-PLUS YEAR CYCLE

The total area of fields rejected annually for excess mosaic, largely caused by potato virus Y (PVY), in New Brunswick seed potatoes (1920-1991) has fluctuated in a cyclic fashion. Peak epidemic years were 1921, 1931, 1941, 1949, 1959, 1968, 1978, and 1987, with an average interval of 9.4 years. The PVY cycle was in phase, though lagging by about 7 years, with a population cycle in Canadian wildlife. This differs from a 32-year cycle recently reported for the potato leaf roll virus (PLRV). The persistent PLRV is spread largely by the migrant potato colonizing aphid Myzus persicae. The nonpersistent PVY can be transmitted by aphids better adapted to the northern climate. These aphids appear earlier in the season and visit, but do not necessarily colonize, potatoes. Thus, PVY is spread earlier than PLRV. Between double-lows in 1972, 1973 and 1983, 1984, there was significant spread of PVY only in alternate, even years; i.e. a biennial rhythm was superimposed on the 9-plus year cycle. A new cycle has since begun with biennial highs in the odd years 1985, 1987, 1989, and 1991. Thus there has been significant spread of PVY in only 9 of the 20 years from 1972 through 1991. During high-spread years, infections, which eventually reached tubers, began about June 30, increased sharply in numbers, then declined in late July with the onset of mature plant resistance. In contrast, in low-spread years, the first infections did not occur until mid July, shortly before the onset of crop maturity.