POTATO LEAFROLL VIRUS SPREAD IN DIFFERENTIALLY RESISTANT POTATO CULTIVARS UNDER VARYING APHID DENSITIES

An action threshold of 3-10 green peach aphid, Myzus persicae (Sulzer) , apterae per 100 lower leaves is recommended for use in Minnesota to prevent further spread of potato leafroll. virus (PLRV) in potato, Sol anum tuberosum L. This threshold was first developed and validated usi ng the PLRV susceptible cultivar Russet Burbank. Here we report experi ments to determine if higher aphid densities could be tolerated in PLR V resistant cultivars, i.e., Kennebec (moderately resistant) or Cascad e (highly resistant), without an increase in PLRV infection. Insectici dal sprays were applied to plots when predetermined target aphid densi ties, based on number of apterae per 100 leaves, were reached: 3, 10, 30, and 100 (Russet Burbank); 10, 30, 100, and 300 (Kennebec); and 30, 100, 300, and 1000 (Cascade). The response variable was the average p ercentage of PLRV infected plants. Overall mean cumulative aphid-days and percent PLRV infection were 617 and 23.5% for Russet Burbank, 1,29 6 and 10.2% for Kennebec, and 4,816 and 9.5% for Cascade. For each cul tivar, the highest target aphid density tolerated without an increase in PLRV spread was determined by comparing PLRV infection in plots spr ayed on predetermined thresholds to PLRV infection in plots where aphi ds were rigorously controlled. This maximum density was 10 apterae per 100 leaves for Russet Burbank and 300 apterae per 100 leaves for Casc ade. Results using Kennebec were ambiguous, but Kennebec was always mo re resistant to PLRV than Russet Burbank. Excised leaflet tests showed that the cultivars did not differ in resistance to green peach aphid. It appears that action thresholds based on green peach aphid apterae can be different depending upon the inherent PLRV-resistance of the cu ltivar.