Effect of fish emulsion used as a preplanting soil amendment on verticillium wilt, scab, and tuber yield of potato

Fish emulsion is used mainly as a fertilizer for crop production but, in a previous study, we have demonstrated its efficacy in suppressing damping-off [Rhizoctonia and Pythium spp.] in radish and cucumber in a peat-based substrate or soil. In the present study, fish emulsion was tested as a preplanting soil amendment to control soilborne diseases of eggplant and potato such as verticillium wilt [Verticillium dahliae and Verticillium albo-atrum] and scab [Streptomyces spp.]. Tests were conducted on I I soils with different characteristics (pH, 5.2-7.2; organic-matter content, 1.0%-3.7%), originating from commercial potato fields in Ontario, New Brunswick, and Prince Edward Island with a history of verticillium wilt and scab. The amendment of soil with fish emulsion at rates of 0.5% and 1% (m/m) protected eggplant from verticillium wilt, and the 1% fish-emulsion treatment increased fresh and dry plant biomasses in the greenhouse. Microplot experiments with potato showed that the 1% fish-emulsion treatment significantly (P < 0.05) reduced scab severity (by 0.6-2.1 units on a scale of 0-6) in seven soils with low to moderate scab-disease pressure and significantly increased total tuber yield (by 41%-170%) in nine soils, compared with the control treatments. Fish emulsion also reduced potato petiole infection by V dahliae in one soil. In potato field trials at two sites in 2004, treatment with 1% fish emulsion (20 000 L/ha) significantly (P < 0.05) reduced scab severity (by 0.8-1 units) and significantly increased scab-free tubers (by 132%-366%) and marketable-tuber (surface scab < 5%) yield (by two fold), compared with the control treatments. Total tuber yield was reduced at one site and not affected at the other. No effect of fish emulsion on scab and verticillium wilt or tuber yield was observed in a 2005 field trial. The reduction of scab or verticillium wilt by fish emulsion was not soil-specific and varied from year to year. The results indicated that fish emulsion was not effective in soils with high disease pressure.