Greenhouse and field experiments: Soils previously described were mixed with 4 mixtures of ALS (0.25, 0.50, 1.0, and 2.0% v/w) and an untreated control. The soils for each treatment were then placed in 15 cm diameter plastic pots (1800 g/pot; 3 replicates/treatment). Six week-old potato explants (cv. Kennebec) were planted into each pot. The pots were then placed in the greenhouse in a completely randomized design and were watered every 2-3 days. To assess the incidence of V. dahliae, a leaf petiole from the lower portion of each plant was harvested every week starting from flowering date. The petioles were surface sterilized for 2 min in 1.5% sodium hypochlorite. Three sections cut from each petiole were plated onto SPT media and the plates were incubated at 25°C in the dark. After 2 weeks, colonies of V. dahliaewere evaluated. Potato scab severity was determined visually by examining tubers for scab lesions. This experiment was carried out twice and the data were pooled together.
Field plots were set up in April, 1998 at a farm near Delhi, Ontario where the potato crop in the previous year had a severe scab problem. Treatments included a control and 2 concentrations of ALS (0.5 and 1.0% v/w). ALS was watered into each plot and rototilled to a depth of 15 cm. Plots were 4 x 7.6 m and there was a 3 m buffer zone between each of the blocks. Plots were set up in a completely randomized block design with 3 replications for each treatment. Seed tubers of potatoes (cv. Yukon Gold) were planted approximately 4 weeks after the treatment application. There were 4 rows of potatoes from which only the 2 middle rows were considered for data collection. All plots received the same recommended fertility. Verticillium wilt and potato scab were assessed as previously described.