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The Relationship of Irrigation Timing and Soil Treatments to Control Potato Scab. J. R. Davis, Associate Professor of Plant Pathology, University of Idaho College of Agriculture Research and Extension Center, Aberdeen 83210; G. M. McMaster(2), R. H. Callihan(3), J. G. Garner(4), and R. E. McDole(5). (2)(3)(4)(5)Professor of Agricultural Engineering, Assistant Professor of Agronomy, Extension Specialist, and Associate Professor of Soils, respectively, University of Idaho College of Agriculture Research and Extension Center, Aberdeen 83210. Phytopathology 64:1404-1410. Accepted for publication 3 June 1974. DOI: 10.1094/Phyto-64-1404.

Control of common scab (Streptomyces scabies) on the potato cultivar ‘Russet Burbank’ was related to high soil-moisture periods started five days before tuber initiation. No scab reduction resulted with 3 wk of high moisture, but disease was significantly reduced when moisture was held for either 6 or 9 wk above 90% available soil moisture (0.46 bar). Differences of scab severity were not significant between the 6- and 9-wk high-moisture periods, indicating that greatest scab susceptibility occurred between 2 and 5 wk after tuber initiation. Additive effects occurred between irrigation and chemical treatments; e.g., sulfur (897 kg/ha) controlled scab with 3 wk of high moisture, but showed no significant effect when available soil moisture was allowed to deplete to 45% (1.8 bars) throughout the season. Sulfur and irrigation treatments interacted to influence calcium in tuber peelings. The 6-wk high-moisture period appeared to encompass the time of greatest susceptibility, and there was significantly lower calcium in tuber peelings from the 6-wk high-moisture treatment than from the low-moisture treatment. Scab reduction was positively correlated with factors that reduced periderm russeting; e.g., high moisture and soil compaction. However, results indicate that scab reduction with high moisture involved more than influence to periderm russeting. Scab was controlled with either pentachloronitrobenzene [PCNB (28 kg/ha) or a mixture of PCNB (28 kg/ha) and 5-ethoxy-3-trichloromethyl-1,2,4-thiadiazole [Terrazole (7 kg/ha)]; however, the effectiveness of PCNB was reduced by Terrazole. The nitrogen stabilizer, N-Serve [2-chloro-6-(trichlormethyl)-pyridine], increased disease severity and reduced manganese in tuber peelings.

Additional keywords: soil compaction, soil moisture.