POTATO LEAFROLL VIRUS SPREAD IN RELATION TO DENSITIES OF GREEN PEACH APHID (HOMOPTERA, APHIDIDAE) - IMPLICATIONS FOR MANAGEMENT THRESHOLDS FOR MINNESOTA SEED POTATOES

Tolerances for potato leafroll virus in potato, Solanum tuberosum L., seed are low in Minnesota, necessitating intensive insecticide use to control the vector, green peach aphid, Myzus persicae (Sulzer). We sought to determine the relation between low green peach aphid densities and potato leafroll spread, and to learn if insecticide use in seed potatoes could be reduced using green peach aphid apterae action thresholds. Plots containing leafroll infected plants were sprayed with methamidophos on thresholds of 0, 10, 30, and 100 apterae per 100 leaves in 1987 and 0, 3, 10, and 30 per 100 leaves in 1988. Percent potato leafroll infection was related to cumulative green peach aphid apterae-days (number of aphids per 100 leaves x number days present) and time of leafroll source introduction (R2 = 0.79). Leafroll infected sample units were more often adjacent than would be expected if occurrence were random in 17 of 18 apterae action threshold plots. The pattern of leafroll spread and the relation with green peach aphid apterae-days suggested that apterae were primarily responsible for spread of the virus. Percent potato leafroll infection in potato, cv. Russet Burbank, was related to the green peach aphid apterae threshold at which insecticidal sprays were applied, although variation was large within a threshold. Data from these studies suggest that the threshold concept might be employed, but that the threshold for treatment would be very low, perhaps in the range of 3-10 green peach aphid apterae per 100 leaves. Experiments in 1988 showed little spread early in the season, even though aphids and leafroll infected plants were present.