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Plant Water Stress and Development of Fusarium Foot Rot in Wheat Subjected to Different Cultural Practices. R. I. Papendick, Soil Scientist, ARS, USDA, Washington State University, Pullman 99163; R. J. Cook, Plant Pathologist, ARS, USDA, Washington State University, Pullman 99163. Phytopathology 64:358-363. Accepted for publication 18 September 1973. DOI: 10.1094/Phyto-64-358.

Fusarium foot rot of winter wheat caused by F. roseum f. sp. cerealis ‘Culmorum’ was most severe under dryland conditions (<30 cm annual precipitation) with treatments that caused the greatest plant water stress. These treatments were high N [>60 kg N/hectare (ha)] and high plant densities (1 seed/cm length of row; rows 30 cm apart) with early seeding. Plant water potentials were uniform at –20 to –25 bars for all treatments until May. Infection (as revealed by culture plating) was generally completed at that time and was also uniformly high regardless of treatment, but varied with inoculum density. In plots which received low N, plant water potential dropped only to –30 to –33 bars at plant maturity and disease remained mild. With high N. potentials dropped to –35 to –42 bars, extensive basal culm decay developed at about the dough stage, and tillers died prematurely. In plants in rows 60-90 cm apart (low density) water potentials remained near –30 to –33 bars, even with high N; and disease development slowed accordingly, although it was not prevented. The development of low plant water potential in June-July was related to high-use rates of soil water reserves in April and early May, presumably because of high transpirational capacity (high leaf area) of the well-nourished plants. Thereafter, soil water supplies became increasingly more limiting and evaporative demand increased; the result was in diminished use rates and lowered plant water potentials. Fusarium is apparently a slow-decay organism in wheat that becomes no drier than –30 to –33 bars at plant maturity, but it kills its host by aggressive attack if the plant water potential exceeds –35 to –40 bars at maturity.

Additional keywords: thermocouple psychrometry, dryland winter wheat, nitrogen fertilization.