Advanced SWP Interpretation in Prune

Allan Fulton, UCCE Water Resources Advisor, Tehama, Shasta, Glenn and Colusa Counties; and Luke Milliron, UCCE Orchards Advisor, Butte, Tehama and Glenn Counties.

Advanced guidelines for prune are provided in Table 1 (below). These guidelines consider the actual orchard measurements and normalized measurements after referencing baseline SWP (i.e. bars +/- Table 2 baseline, end of article). Field research suggests the ideal range in plant water status for prune is between -2 and -8 bars below baseline. This corresponds to orchard measurements from about -6 to -20 bars.

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A measurement of 0 to -2 bars below baseline in prune indicate no or minimal water stress and is more likely to be observed in the spring, when days are shorter, and weather is cooler. Field research suggests the ideal range in plant water status for young, developing prune orchards is -2 to -4 bars below baseline to maximize shoot growth. Readings of -2 to -4 bars below baseline from April through mid-June will also promote shoot growth and fruit sizing in older, producing orchards. From mid-June through July and early August, SWP levels between -4 to -6 bars below baseline will sustain fruit sizing in older, producing orchards.

Beginning early to mid-August when fruit sizing is complete, SWP levels may be elevated to between -6 to -8 bars below baseline to increase sugar and reduce water content in the fruit (reducing “dry away” or drying costs). Once the fruit are harvested, prunes should be irrigated enough to recover the trees back to -2 to -6 bars below baseline, to sustain the tree canopy. The trees will continue to photosynthesize and store carbohydrates in the trunk and root system, and result in stronger bloom and leaf emergence the next season. Prune trees with SWP levels exceeding -8 to -10 bars below baseline over extended periods of the season will exhibit more leaf drop, greater exposure to sunburn, and higher incidence of canker diseases, which can severely reduce the life span of an orchard.

Figure 1. Orchard shows very little shoot growth and extensive limb dieback, often associated with high to severe SWP levels (lower than -30 bars).
Figure 2. The orchard shows more favorable growth and tree health, where water stress before irrigation didn’t exceed moderate to high levels (seldom below -20 bars
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