Suboptimal Irrigation Research Articles

Labyrinthula terrestris: a new pathogen of cool‐season turfgrasses

ABSTRACT A new disease of turfgrass known as rapid blight is caused by Labyrinthula terrestris, a newly described member of the Labyrinthulids. This unusual group of microorganisms previously has been found in marine systems, and L. terrestris is the only member of the genus Labyrinthula known to be a pathogen of terrestrial plants. Members of this genus are defined by the formation of ectoplasmic networks in which the unicellular somatic cells move or 'glide'. Infections of cool-season turfgrasses occur most commonly when they are irrigated with suboptimal irrigation water with elevated salinity. Disease has been observed in 11 states in the United States and in the United Kingdom. A phylogenetic study indicates that isolates from turfgrass in the United States fall firmly within a clade containing other Labyrinthula spp. and that they came from a common lineage. Its rapid emergence as a turfgrass pathogen may coincide with increased use of relatively high-salinity water or reclaimed water for irrigation. L. terrestris is a potential problem in susceptible turfgrass varieties wherever soil salinity is allowed to accumulate as a result of poor soil structure or suboptimal quality irrigation water is used for irrigation.

Limitations to carrot (Daucus carota L.) productivity when grown with reduced rates of frequent irrigation on a free-draining, sandy soil

To determine the influence of reduced irrigation input, carrot (Daucus carota L. cv. Ivor) plants were grown on a coarse-textured, sandy soil during summer near Perth, Western Australia. Irrigation was applied daily with sprinklers at 5 rates ranging from 47 to 151% replacement of daily pan evaporation (Ep). A reduction in irrigation from the industry standard of 151% Ep to 124 or 97% Ep resulted in a 17% increase in water use efficiency (WUE) when calculated as total root yield/water applied. However, the percentage of total roots that were marketable declined with reduction in irrigation, being 73, 56, and 63% at 151, 124, and 97% Ep, respectively. This minimised any increase in WUE when calculated on a marketable yield basis. Further reductions in irrigation to 47 and 73% Ep resulted in large reductions in yield, WUE, and the percentage of marketable roots. Leaf biomass was not significantly different between the 151 and 124% Ep treatments and was reduced by 55 and 85% at 73 and 47% Ep, respectively. Soil water tension (ψs) varied from 4 to 80 kPa among the irrigation treatments. Pre-dawn leaf water potential (ψl) was most sensitive to ψs over a ψs range of 0–20 kPa and the influence of ψs on ψl was small (400 kPa) compared with the diurnal variations in ψl of up to 1400 kPa, which occurred independently of significant changes in bulk soil ψs. There was no significant effect of irrigation on the osmotic potential of leaf sap and there was no evidence of osmotic adjustment. For all irrigation treatments, photosynthesis (Pn) peaked in the early morning (0830 hours) at rates of 17–20 μmol CO2/m2.s, after which Pn decreased to a transient minimum value at midday. The diurnal decline in Pn was positively correlated with ψl for all treatments, and minimum values of Pn varied from 12.5 to 7.8 μmol CO2/m2.s for plants grown with 151 and 47% of Ep, respectively. When grown under frequently applied, suboptimal irrigation, carrot productivity is predominately limited by a reduction in leaf growth rates that limits leaf area and matches transpiration requirements with water availability. Furthermore, even under conditions of water deficit, the aerial environment imposes a greater constraint to Pn than soil water availability.

Egg Production and Population Growth of the Citrus Red Mite (Acari: Tetranychidae) on Differentially Irrigated Citrus Trees

We determined the effect of moderate (20%), continuous water stress of citrus trees on the egg production and population growth of the citrus red mite, Panonychus citri (McGregor). We also measured differences in individual and total free amino acids, soluble protein, total nitrogen, and percentage of soluble nitrogen to determine if water stress influenced the concentration of soluble nitrogenous compounds. Although this level of water stress can reduce yield directly, we observed no consistent effect of irrigation on concentrations of nitrogenous compounds. Temporal or ontogenetic variation among trees more strongly affected concentration of plant nitrogenous compounds than irrigation. The three citrus cultivars examined— Citrus limon (L.) Burman f. cv. Eureka, and Citrus sinensis (L.) Osbeck cv. Valencia and cv. Washington Navel—also varied in sensitivity to differential irrigation; lemon was least sensitive and ‘Washington Navel’ orange was most. Egg production rates were independent of irrigation level. Mite population growth on commercial, differentially irrigated ‘Washington Navel’ trees also was independent of irrigation level. When P. citri activity is expected to be greatest, differential irrigation is expected to have minimal influence on P. citri populations. The direct effects of suboptimal irrigation on yield probably are of more economic importance than any indirect effects of altering host tree suitability to mites.

A Simulation Model for Irrigation Scheduling Under Variable Rainfall

ABSTRACT Asimulation model was developed by coupling a simple daily rainfall generator to a water balance model that determines irrigation dates and amounts. The rainfall model uses average monthly data to generate daily precipitation and the water balance uses average ETQ values and estimates separately evaporation and transpiration as well as allowable depletion. The results of 50 years of simulating irrigation schedules for corn and sugarbeets at Cordoba and Badajoz, Spain and Davis, CA were analyzed to evaluate the effects of planting date, soil storage capacity and irrigation system constraints. Probability distribution functions of the date of first irrigation were developed for ideal and practical irrigation situations. A method is proposed based on the simulation to develop a predictive irrigation schedule at selected probability levels for optimal and suboptimal irrigation dates.

Moisture Deficits and Grain Sorghum Performance: Evapotranspiration‐Yield Relationships1

AbstractKnowledge of evapotranspiration‐yield (ET‐Y) relationships is fundamental in evaluating strategies for managing limited irrigation. The work reported here was done to develop ET‐Y functions for grain sorghum [Sorghum bicolor (L.) Moench], to provide base data for a water management improvement study in Nebraska. A field experiment was conducted using a modified line source sprinkler system to establish treatments that resulted in several different ET levels during each growth stage. The system was installed on a Valentine very fine sand (Typic Ustipsament) in west central Nebraska.The specific objectives of the study were: 1) to determine the crop evapotranspiration‐yield relationships for grain sorghum in this location, 2) determine how the timing and intensity of water deficits affect this relationship, 3) compare the evapotranspiration‐yield response of selected genotypes, and 4) to determine whether crop water use efficiency can be increased by judicious limited irrigation.In two seasons the relationship between evapotranspiration (ET) and grain yield (Y) or dry matter was linear. Evapotranspiration reduction under all conditions tested incurred some yield reduction. For a given genotype the slope of the ET‐Y relationship varied greatly depending on the growth period treatment. The lowest slopes were associated with a treatment in which stress built up steadily over the entire growing season. The slope was generally much steeper when irrigation was restricted to varying degrees for only a portion of the growing season. With greater yield reductions per unit of ET deficit, the latter treatments were considered suboptimal irrigation strategies compared to a gradual stress buildup.One hybrid had a lower slope of the ET‐Y relationship than the other genotypes for the treatment that provided water stress of gradually increasing severity. The percentage yield reductions were similar for all hybrids, however. The yield reduction ratios of the genotypes were different for the two growing seasons.Water use efficiency consistently decreased as ET declined below the maximum. It appeared that water stress, regardless of timing, tended to reduce water use efficiency. The reductions were smallest when stress increased gradually throughout the growing season.

Screening soybeans for tolerance to moisture stress: A field procedure

A field experiment was conducted to evaluate the performance of 20 soybean ( Glycine max (L.) Merr.) cultivars under suboptimal moisture. Cultivars were grown on a deep sand soil in replicated 6.2 m 2 plots at a population of 321,000 plants per ha. Contrasting treatments of optimal (0.61 cm/day) and suboptimal (1.83 cm/week) moisture were applied by a solid-set overhead irrigation system through the season. Whole plants were hand-harvested at maturity to assess the effect of drought on vegetative growth, yield components, and yield. Cultivars identified as tolerant of moisture stress were characterized by stable seed yield under the contrasting moisture treatments, but not necessarily stable vegetative growth. Cultivars termed susceptible to moisture stress exhibited decreased seed yield under suboptimal irrigation, but not necessarily decreased vegetative growth. Yield reductions of cultivars susceptible to moisture stress resulted from a significant reduction in both the number of seeds produced per plant and in the number of pods per plant. No other yield component was significantly affected by the suboptimal moisture treatment. Cultivar classification according to moisture stress response in this procedure was only roughly consistent (65%) with our earlier described laboratory and greenhouse procedures for screening for tolerance to moisture stress. We conclude that accurate measurement of moisture stress response among soybean cultivars requires a field testing component.