Reduction of potato tuber disease with organic soil amendments in two Prince Edward Islands fields

Single applications of liquid swine manure (LSM; ca. 49 hL/ha), ammonium lignosulfonate (ALS; a product derived from the pulp and paper processing industry, 50% solids at ca. 10 hL/ha), and Nature Safe (NS) 10:2:8 (N:P:K; an organic fertilizer made primarily of poultry feathers, ca. 8.6 t/ha) were made at two commercial potato (Solanum tuberosum) fields (sites P-I and P-II) in Prince Edward Island in the spring of 1999. Potato tubers were planted 3 weeks later and again in 2000 and 2001 without further addition of amendments. The effects on potato scab, black scurf, silver scurf, fusarium dry rot, verticillium wilt, and tuber yield were determined. Potato scab severity was significantly reduced (77%–84%) by NS compared with the controls at both sites in the first year. Scab severity remained significantly lower in the second year, but there was no difference from the controls in the third year. ALS significantly reduced scab severity (54%) compared with the controls at both sites in the first year, and scab severity remained lower than the control at only one site (P-I) and only for the second year. The mechanism by which ALS reduced scab severity is not known. LSM significantly reduced scab severity (38%) at one site only (P-II) and only in the first year. The control at site P-II was likely due to the favourable pH (5.4) for biotoxic activity of volatile fatty acids (VFAs) in the LSM, whereas the soil pH of 6.4 at site P-I was unfavourable for VFA toxicity to Streptomyces scabiei. The severity of black scurf was significantly reduced compared with the control at both sites by LSM (51% at P-I and 48% at P-II) and NS (40% at P-I and 61% at P-II) but in the first year only. There was no significant effect on the severity of silver scurf or fusarium dry rot or tuber yield by any amendment in any year. Verticillium wilt was significantly reduced compared with the control by ALS (88%) and NS (66%) at only one site (P-I) and only in the second year. Marketable yield was increased at both sites for NS and ALS, and at one site for LSM.