Neil W. Christensen

Rhizoctonia root rot and take-all of wheat in diverse direct-seed spring cropping systems

The percent area of patches of wheat plants stunted by Rhizoctonia solani AG8 in years 3 and 4 of a direct seed (no-till) cropping systems study conducted under dryland conditions of Washington state was the same for continuous spring wheat (Triticum aestivum) (no crop rotation), spring wheat after spring barley (Hordeum vulgare), or first- or second-year spring wheat after consecutive crops of safflower (Carthamus tinctorius) and yellow mustard (Brassica hirta). Similar percent area of patches occurred in plots sown to spring barley after spring wheat and in the safflower and yellow mustard. Greenhouse studies confirmed that safflower and yellow mustard as well as several other broadleaf crops are susceptible to R. solani AG8. Between years 3 and 4, some patches increased in size, some new patches formed, and a few patches present in year 3 were not present in year 4. Seventy to 80% of the wheat plants sampled in year 4 of the study had at least 10% roots with take-all caused by Gaeumannomyces graminis var. tritici, including wheat after back-to-back safflower and yellow mustard (not susceptible to this pathogen). A one-time application of zinc at 1.1 kg/ha at planting provided no visual response of the stunted plants where the application passed through one side of a patch. The effect of crop rotation on grain yield of spring wheat related to water supply, with lower yield after the broadleaf crops because they extract more water (leaving less water for the next crop) than either wheat or barley.

Validation and Recalibration of a Soil Test for Mineralizable Nitrogen

Abstract A soil test for mineralizable soil N had been calibrated for winter wheat in the Willamette Valley of western Oregon. Seventy‐eight percent of the variation in spring N uptake by unfertilized wheat was explained by N mineralized from mid‐winter soil samples incubated anaerobically for 7 days at 40°C. Mineralizable N (Nmin) ranged from 10 to 30 mg N kg−1 and was used to predict N fertilizer needs. Recommended rates of N were correlated (R2=0.87) with maximum economic rates of N fertilizer. Subsequent farmer adoption of no‐till sowing and a high frequency of soil tests>30 mg N kg−1 prompted reevaluation of the soil test. Four N fertilizer rates [0, 56, G, and G+56 kg N ha−1] were compared in 12 m×150 m farmer‐managed plots. Growers N rates (G) ranged from 90 to 180 kg N ha−1 and were based on Nmin and NH4‐N plus NO3‐N soil tests. Averaged across ten no‐till and five conventionally tilled sites, grain yield and crop N uptake were maximized at the recommended rate of N. Results demonstrate that N fertilizer needs for winter wheat can be predicted over a wide range of mineralizable soil N (10 to 75 mg N kg−1) and that the same soil test calibration can be used for conventionally sown and direct‐seeded winter wheat.

A comparison of methods for estimating phosphorus uptake kinetics under steady‐state conditions 1

Abstract Michaelis‐Menten kinetic parameters (Imax and KM) are useful for describing nutrient uptake by plants. This paper compares two methods for estimating the kinetics of P uptake. Both methods employed a steady‐state hydroponic system to measure P uptake by wheat (Triticum aestivum L.) seedlings. In one method, uptake was measured from two P concentrations in nutrient solution, with Imax and KM estimated by direct linear plot (DLP). In an alternate, multiple concentration (MC) method, uptake was measured from five P concentrations, and kinetic parameters were estimated by either nonlinear regression or the Hanes plot. The Imax and KM, estimates obtained by the DLP method were compared to those obtained by the MC method. The MC method offered practical advantages. Unlike the DLP, it allowed estimation of the external P concentration at which net influx = 0 (Cmin), and did not require a priori estimates of KM and Cmin. The MC method provided more precise median parameter estimates as indicated by small...

A simple model for phosphorus uptake kinetics of wheat seedlings

Abstract A simple model to compare and predict phosphorus (P) uptake behavior of plants may be useful to agronomists. A predictive equation based on Michaelis‐Menten kinetics was developed for this purpose. Kinetic parameters for use in the model were determined in an experiment using two cultivars of winter wheat grown for 21 days in 14 soil treatments, including seven P levels, each in fumigated and unfumigated soil. In another experiment, the same wheat cultivars were grown for 7, 14, or 21 days at one soil P level in both fumigated and unfiimigated soil. Using parameter values developed in the first experiment with multiple P levels and one time period, the model closely (R2=0.966, P<0.001) predicted P uptake in the second experiment with one P level over multiple time periods. The model could be a useful agronomic tool because of its simplicity and because no data need be collected from artificial growing conditions.

Genetics of Chloride Deficiency Expression in Barley

Greenhouse‐grown barley exhibited symptoms resembling physiological leaf spot of wheat, a symptom of chloride (Cl−) deficiency. Twenty parents of barley mapping populations were grown hydroponically and exposed to 0 or 2.8 mmol Cl−. Response to Cl− was measured as the percentage of leaf area spotted. Chloride treatment reduced leaf spotting, and barley lines differed in susceptibility. The cultivar ‘Shyri’ was highly susceptible to leaf spotting, whereas ‘Galena’ was resistant. The mapping population derived from a ‘Shyri’/‘Galena’ cross was chosen for further study. The 100 double‐haploid progeny were grown hydroponically with complete nutrient solutions containing 0 or 0.8 mmol Cl−. Measured variables included percentage of leaf area spotted, leaf nutrient concentrations, plant height, and dry‐matter yield. Control plants exhibited 60% more leaf spotting and produced 23% less dry matter than plants receiving Cl−. Leaf Cl− concentrations ranged from 0.8 g kg−1 in control plants to 7.8 g kg−1 in plants exposed to 0.8 mmol Cl−. Chloride phenotypic data were used with map and locus information to conduct a quantitative trait loci (QTL) analysis. Using MapQTL, we mapped genetic determinants of the spotting phenotype to the short arm of chromosome 6H in an autumn study. These results were not confirmed in a winter–spring study, suggesting that unidentified environmental conditions may influence the expression of the leaf‐spotting phenotype.

Effect of Nitrogen Topdressing at Anthesis and the Association of Flag-Leaf Nitrogen with Grain Protein Concentration in Irrigated Spring Wheat

ABSTRACT Grain protein is an important component affecting the market value of hard red spring wheat (Triticum aestivum). In high-yielding irrigated environments, consistently attaining desired protein levels is a chronic problem. Nitrogen (N) management has a strong effect on protein concentration. The objective of this experiment was to evaluate the robustness of using flag-leaf N concentration as a tool for guiding in-season N application in order to obtain high-protein wheat. Three on-farm trials were conducted (each location using a cultivar of the farmers choice) where N rates were varied. Nitrogen rates evaluated were 0, 79, 157, 236, and 314 kg ha−1. To evaluate the benefit of topdressing, all N was applied basally, or 45 kg ha−1 N was reserved from the basal dose for application at heading. Yield and protein response to applied N were variable across the three sites. This study postulated that this response was a function of initial soil-N availability. Where there was a yield response to N, it appeared that reserving a portion of N for topdressing increased protein but tended to decrease yield. At levels of N where yield was not limited, reserving a portion of N for topdressing did not appear to affect yield or protein. Although a linear relationship between grain protein and flag-leaf N was obtained by pooling data across sites, there was enough variation in this relationship to limit its utility as a tool for guiding in-season N application.

Yield of take‐all infested winter wheat as influenced by inhibiting nitrification with dicyandiamide

Abstract The potential for using dicyandiamide (DCD) to enhance yield of take‐all‐infested winter wheat (Triticum aestivum L.) was evaluated in six field experiments on four acid soils (pH 5.7–6.2). Ammonium and NO3 ‐ concentrations and NH4 +: NO3 ‐ ratios in 0–10 and 10–20 cm soil depths were measured for ten weeks after spring topdressing 180 kg N/ha as urea with 0, 13, or 27 kg DCD/ha. Nitrification was strongly inhibited for 6 to 10 weeks by either 13 or 27 kg DCD/ha. Averaged over the ten‐week sampling period, NH4 +: N03 ‐ ratios in the 0–10 cm depth of soil were 36: 1 for DCD‐treated plots as compared to 2: 1 for plots receiving only urea. Ratios in DCD‐treated plots were considerably wider than ratios associated with take‐all suppression (10: 1 to 3: 1) in earlier studies. Extractable NH4 + + NO3 ‐ concentrations in soil were high in DCD‐treated plots after 30 to 40 days, suggested that DCD had reduced crop uptake of N because of the lower mobility of NH4 + as compared to NO3 ‐. In four of the six ...

Pear seedling response to phosphorus in a phosphorus‐fixing soil

The effect of P on growth of ‘Bartlett’ pear (Pyrus communis L.) seedlings was evaluated on the P‐fixing, Parkdale soil (Vitrandepts) from Oregon, USA. The P treatment levels were 0.03, 0.04, 0.06,...