David A. Goldhamer

Suitability of stem diameter variations and water potential as indicators for irrigation scheduling of almond trees

Summary Successful irrigation often depends upon the development of a precise schedule based on crop monitoring. Measurements of stem water potential (SWP) and of trunk diameter variations (TDV) were performed on almond trees during 1999 in a commercial orchard in the San Joaquin Valley of California. The objective was to determine the feasibility of using SWP and TDV as indicators for irrigation scheduling, for which reference values, representing behaviour of both indicators under non-limiting soil water levels, will be needed. The trees were irrigated in excess of their requirements with microsprinklers and subsurface drip to ensure their responses were affected only by the aerial environment. Regression analyses between three environmental variables (vapour pressure deficit, VPD; air temperature, Ta; and reference evapotranspiration, ETo) and the SWP and maximum daily shrinkage (MDS) gave best results for the correlations of MDS with daily VPD (r2.=.0.64) and Ta (r2.=.0.59). Goodness of fit for the linear regressions of SWP against all environmental variables was lower than for MDS. (r2 values of 0.48 and 0.37 for daily VPD and Ta, respectively). The relation between MDS and SWP was weak and showed significant hysteresis as the season progressed. Both indicatorsresponded sharply to soil water deficits when another set of trees were allowed to dry before harvest, but the signal strength (stressed/fully irrigated) of MDS, which ranged from 0.08 to 0.35.mm, was greater than that of SWP, which declined from –1.0 to –1.8.MPa, during the drying cycle. It was concluded that while there may be uncertainties in establishing reference levels for both SWP and MDS, the use of MDS is a promising approach for the development of automated irrigation scheduling in almonds.

Effects of preharvest irrigation cutoff durations and postharvest water deprivation on almond tree performance

Abstract Plant water relations, tree growth, and yield components of mature almond trees [Prunus dulcis (Mill.) Webb cv. Nonpareil] were evaluated in response to ranges of irrigation cutoff durations immediately prior to harvest and to postharvest water deprivation. The preharvest cutoff regimes consisted of eight treatments that terminated irrigation from 8 to 57 days prior to tree shaking, resulting in 297 mm less preharvest-applied water for the longest cutoff regime. Postharvest irrigation totaled 206 mm. Predawn leaf water potential (LWP) declined rapidly after irrigation cutoff and reached −4.0 MPa prior to harvest in the most severe preharvest treatment. This resulted in reduced tree growth and near complete canopy defoliation. Individual kernel weight and full hull splitting were reduced by up to 17.2% and 71.1%, respectively, by the preharvest cutoff regimes, while bark damage due to mechanical shaking and fruit removal was unaffected. Hull rot at harvest was virtually eliminated with the longer preharvest cutoff durations. Depriving trees of postharvest irrigation had a greater impact on tree productivity than the preharvest irrigation cutoffs, even though the amount of postharvest-deprived water was less than that which occurred with the more severe preharvest cutoffs. In the season following postharvest water deprivation, bloom density and fruit set were reduced by up to 52.2% and 94.3%, respectively. This resulted in fruit load and kernel yield reductions of up to 76.7% and 73.6%, respectively. Even with the latest preharvest cutoff regime, fruit load and kernel yield were significantly lower without postharvest irrigation. We postulate that the sensitivity to postharvest water deficits is due to the late reproductive bud differentiation of almond, which may damage primordial flower parts and/or accelerate the emergence of the stamens during bloom, thus decreasing their pollen receptivity. Since postharvest water deficits are more damaging to sustained productivity than water deficits prior to harvest, growers with limited water supplies should bias irrigation towards the postharvest period.

Yield and foliar injury responses of mature plum trees to salinity

SummaryThe salt tolerance of mature ‘Santa Rosa’ plum trees was assessed on 20-year-old trees grown in the San Joaquin Valley of California. The experimental design consisted of six levels of irrigation water salinity (electrical conductivities of 0.3 to 8 dS/m) replicated five times with each replication consisting of ten trees. Salinity treatments imposed in March 1984 did not influence tree yields harvested in June 1984. In 1985, the second year of treatments, yield from the highest salt treatment (electrical conductivity of irrigation water, ECi, of 8 dS/m) was reduced by half; the number of fruit harvested was reduced 40%, and fruit size was reduced significantly. Foliar damage was so severe by the end of 1985 that nonsaline water was applied to the two highest salt treatments (ECi = 6 and 8 dS/m) in an attempt to restore tree vigor. In 1986 salt effects had become progressively worse on the continuing saline treatments. A linear piece-wise salt tolerance curve is presented for soil salinity values, expressed as the electrical conductivity of saturated extracts (ECe) integrated to a soil depth of 1.2 m over a 2-year period. The salt tolerance threshold for relative yield (Yr) based on 3 years of data was 2.6 dS/m and yield reduction at salinity levels beyond the threshold was 31% per dS/m (Yir=100 − 31 [ECe − 2.6]j). Significant foliar damage occurred when leaf chloride concentrations surpassed 200 mmol/kg of leaf dry weight (0.7%). Sodium concentrations in the leaves remained below 10 mmol/kg (0.02%) until foliar damage became severe. This suggests that chloride was the dominant ion causing foliar damage.

Assessment of peach tree responses to irrigation water ficits by continuous monitoring of trunk diameter changes

Summary Continuous monitoring of trunk diameter and of sap flow were investigated to evaluate and characterize tree responses to irrigation water deficits in peach trees (Prunus persica (L.) Batsch ‘O’ Henry’). A gradual reduction of irrigation during a three-week period followed by a 17 d recovery was applied to field-grown (SF) and lysimeter-grown (SL) trees. Trunk-diameter change (TDC) measurements and maximum daily trunk shrinkage (MDS) were compared with predawn leaf water potential (PLWP) and midday stem water potential (SWP). TDC was also compared with sap flow calibrated in a weighing lysimeter. The experimental calibration of sap flow (r = 0.95) in a lysimeter showed a 45% underestimation of tree transpiration with a coefficient of 0.55, similar to values found in other tree species. The reductions of water supply were reflected clearly by MDS which showed differences among treatments after only 4 d of 75% ET application. MDS was closely related to PLWP and to midday SWP in both deficit irrigation treatments, and some hysteresis between TDC and SWP was detected. There was a strong relationship between the relative sap flow (sap flow of stress treatment relative to control values) and the relative maximum daily shrinkage. Both parameters were sensitive to water stress, as their values declined by 60% with the development of stress. The relationship between sap flow and TDC shows a hysteresis loop whose shape depended on the stress level. As stress developed, the divergence between MinTD (time of day at which minimum TD was reached) and MaxET (time of day when maximum lysimeter evapotranspiration, ET, was attained) increased. It appears that automated trunk diameter measurements provide a precise irrigation management method for the improvement of irrigation scheduling of peach trees.

Regulated deficit irrigation effects on yield, nut quality and water-use efficiency of mature pistachio trees

Summary Regulated deficit irrigation (RDI) was evaluated on deep rooted, mature pistachio trees grown under high evaporative demand in the low rainfall southern San Joaquin Valley of California. The focus of this work was to assess the impact of deficit irrigation during various parts of the season with the goal of determining which period was most stress tolerant in terms of nut production. Pistachio nuts have a unique fruit growth pattern in which rapid kernel growth does not begin until about six weeks after full shell size has been attained. Our hypothesis was that irrigation could be reduced during this period with limited negative effects on production. The season was divided into three preharvest periods: leafout to full shell expansion (Stage 1), full shell expansion to the onset of rapid kernel growth (Stage 2), and rapid kernel growth to harvest (Stage 3). Water deprivation during Stage 1 applied 7.2% less water than the near-fully irrigated Control without any yield loss, based on mean values for the last “on” and “off” alternate bearing years of this study. Shell splitting (endocarp dehiscence) at harvest (a positive impact) was significantly higher (10.4% relative to the Control) but this was offset by a nut weight reduced by 9.4%. Stage 3 water deprivation (53% less applied water than the Control) significantly reduced nut size, shell splitting, mechanical nut removal by tree shaking, and yield, while increasing kernel blanking and abortion. Postharvest water deprivation (5.0% less applied water than the Control) had no significant negative effects on yield components. Six irrigation regimes that applied water at various rates were used to investigate Stage 2 behaviour. While there were no significant differences in yield components among these regimes, the best production occurred with deficit irrigation during Stage 2 at 50% of near-potential ETc during Stage 2 and 25% of near-potential ETc after harvest. This RDI regime saved about 180 mm of water (23.2% of the Control) and water use efficiency was significantly higher (4.69 versus 3.61 kg marketable fruit per mm water, for this RDI regime and the Control, respectively). We believe RDI during Stage 2 and postharvest is a viable irrigation strategy to save water while maintaining top yields of high quality pistachio nuts. Further work is needed to determine if the improved shell splitting harvest that occurred with Stage 1 stress can be exploited to improve grower profits.

Effects of Deficit Irrigation on Hull Rot Disease of Almond Trees Caused by Monilinia fructicola and Rhizopus stolonifer

Almond trees were irrigated from March through November 1994 and 1995 with 70, 85, and 100% of potential evapotranspiration (ETc). Deficit irrigation was accomplished by delivering 70 or 85% of ETc at every irrigation (sustained) or 50% of ETc during 1 June to 31 July (70 regulated) or 1 to 15 July (85 regulated). The natural incidence of dead leaf clusters and dead spurs, twigs, and small branches, measured at harvest, lessened with decreasing amounts of water, and regulated deficits were more effective than sustained deficits in reducing disease. Fruit at early dehiscence on trees in each of the five irrigation treatments were inoculated with 0.1 ml of suspensions of 104 spores per ml of Monilinia fructicola or Rhizopus stolonifer. Monilinia fructicola caused more hull rot than R. stolonifer, and both pathogens responded similarly to the irrigation treatments. The rate of fruit maturation was monitored for approximately 4 weeks before harvest by scoring the percent abscission and dehiscence and measuring the hull moisture content of fruit on trees in each irrigation treatment. Dry kernel weight was determined at harvest. Maturation was slower and kernel weight greater in treatments receiving 85% of ETc than 70% or those under sustained compared with regulated irrigation regimes.

Effects of hull abscission and inoculum concentratrion on severity of leaf death associated with hull rot of almond

Almond fruit of cv. Nonpareil with slightly (1 to 10%), partially (15 to 40%) or almost completely (70 to 95%) abscised hulls were inoculated with Rhizopus stolonifer in 1991 and with R. stolonifer or Monilinia fructicola in 1992. More leaf death occurred both years near inoculated fruits with slightly or partially abscised hulls than near fruits with almost completely abscised hulls. Leaf death was greater near hulls inoculated with M. fructicola than with R. stolonifer in 1992. Disease severity did not differ among fruit with partially abscised hulls inoculated with 10 3 , 10 4 , or 10 5 spores of R. stolonifer per ml in 1991, and with the same concentrations of R. stolonifer, M. fructicola, or M. laxa in 1992. More leaf death occurred near fruit inoculated with M. laxa than with R. stolonifer and M. fructicola in 1992.

Estimation of evapotranspiration on discontinuous crop canopies using high resolution thermal imagery

Efficient water management in agriculture requires accurate estimation of the evapotranspiration (ET) of crops. This work presents the progress made on assessing the water status and ET estimation on discontinuous crop canopies where soil and shadow components affect the remote sensing imagery used. Two orchards used to conduct regulated deficit irrigation (RDI) experiments were monitored between 2004 and 2006 and at three different levels: i) near-field point thermal sensors monitoring single crowns continuously; ii) airborne level using high-resolution thermal and multispectral imagery collected at different times of day; and iii) satellite level using TERRA-ASTER and Quickbird for estimating surface temperature and multispectral imagery, respectively.

Evaluating continuous-move sprinkler machines using time-series statistics

Abstract Traditional methods of describing uniformity of water applied by continuous-move sprinkler machines provide a measure of variability but fail to identify sources of variability. Thus, we investigated the potential of time series statistics as a tool for identifying patterns of variability in catch-can data collected during evaluations of three continuous-move sprinkler machines. Transects of catch cans were installed along the laterals, and the can data were analyzed using autocorrelation and spectral analysis. However, for one machine, transects across individual spans with closely spaced cans were also used. The traditional approach showed similar measures of variability among the machines. The time-series analysis, however, revealed very different patterns of variability along the lateral, attributed to a modification made to one machine and random behavior for another. For a third machine, the uniformity along the lateral was similar to that across individual spans, but along the lateral, random variability occurred, whereas, across the spans, inadequate sprinkler overlap was the major source of variability. We conclude that time-series statistics is an evaluation tool to be used in identifying periodic patterns of variability.

Hedgerows use more water, but increase efficiency, profit in young walnuts

Total water use was 18.8% greater for hedgerow plantings of walnut trees than for conventionally spaced trees during years 2 through 7 of this study. Cumulative yield-and revenue-water use efficiencies were much greater with the hedgerows through year 5, but differences narrowed there-after. The hedgerows had greater total yield, but lower crop value due to smaller nuts. Although individual nut weight was strongly correlated with nut load, the relationship between nut weight and nut density was independent of tree spacing. Total gross and net hedgerow revenues were greater.