INFLUENCE OF IRRIGATION ON SEVERITY OF POTATO EARLY DYING AND TUBER YIELD

The effects of three different irrigation regimes on potato early dying and associated yield loss were assessed in potato cultivar Russet Burbank grown in field microplots at two environmentally distinct locations in Oregon. Fumigated soil was either noninfested or infested with 5, 10, or 25 colony-forming units of Verticillium dahliae per gram of soil (cfu). Microplots were drip-irrigated to provide deficit, moderate, or excessive amounts of irrigation water. Moderate irrigation was approximately equal to estimated consumptive use (ECU); the deficit and excessive regimes were 50 and 150% of ECU in 1987 and 50 and 200% of ECU in 1988, respectively. Severity of potato early dying was greater throughout each season when plants grown in infested soil were irrigated excessively compared to those receiving deficit irrigation. An increase in inoculum density under excessive irrigation significantly suppressed tuber yield. Tuber yield was suppressed (P = 0.05) by 14, 33, and 30% in soil infested with 25 cfu of V. dahliae per gram of soil as compared with noninfested soil at a cool, short-season site in 1988 and a warm, long-season site in 1987 and 1988, respectively. In contrast, under deficit irrigation, an increase in inoculum density had little effect on tuber yield in Umatilla County and there was a slight increase in tuber yield in Crook County. Nitrate-nitrogen concentrations in petioles were significantly lower with excessive compared to deficit irrigation at both locations in 1988. Petiole nitrate-nitrogen concentrations at the warm, long-season site were higher (P = 0.05) for plants grown in soil infested with 25 cfu per gram of soil (2.0%) than for plants grown in noninfested soil (1.7%). Increased disease severity under excessive irrigation was not related to nitrogen deficiency, because all petiole nitrate-nitrogen concentrations exceeded critical levels.