Adapted from the article “Deficit Irrigation for French Prune” by Richard P. Buchner – UC Cooperative Extension Farm Advisor, Tehama County and Allan E. Fulton –UC Cooperative Extension Farm Advisor, Tehama, Glenn, Colusa and Shasta Counties in the June 2014 Sacramento Valley Prune News.

This article discusses managing irrigation with a severe reduction in irrigation water. A pressure chamber is a very useful tool for measuring Stem Water Potential (SWP) as an indicator of actual tree water stress. The new publication, “Using the Pressure Chamber for Irrigation Management in Walnut, Almond and Prune” (ANR Publication #8503) is posted at: http://anrcatalog.ucdavis.edu/pdf/8503.pdf.

To complement SWP, soil moisture sensors can be used to detect any water loss to percolation below the root zone. A complete discussion of irrigation management is available at cetehama.ucanr.edu. Click on irrigation/water program then select on farm irrigation scheduling tools.

Regulated Deficit Irrigation (RDI) is a strategy of withholding irrigation water to levels less than full evapotranspiration (ET). Water is withheld at specific times and in specific amounts during the season with the goal of conserving water while limiting detrimental effects on the tree, the developing crop and future production. The challenge in manipulating crop water stress is evaluating when and how severe water stress really is.

Fortunately pressure chambers and measurements of SWP are gaining in use and guidelines are available to predict the impact of water stress on tree and crop performance. RDI is a stress management strategy that can help manage relatively small curtailments (10 to 20 percent) in water supply but it doesn’t really address severe reductions in water supply. Research on water stress management for prune is limited so suggestions represent best estimates and glean from experiences in almond. The following are four scenarios for prune drought irrigation management.

#1 — No irrigation water available

In a situation where there is no water, the goal is to keep the trees alive through the drought. The best strategy is to do nothing drastic and conserve as much soil moisture as possible. Yield and fruit size will be low and carry-over yield effects may extend into the next season. Thinning the fruit more than normal to lessen the crop load and weight on the tree limbs may be important to maintain tree structure. This should help shade the fruit bearing wood for next season and protect it from sunburn and secondary diseases.

Prune orchards that endure a whole season with no irrigation may require two years of normal irrigation to once again approach typical production levels. It is unlikely that heavy pruning to reduce transpiration surface will help. In almond, the unpruned trees that survived no water recovered to full production more readily than heavily pruned trees once normal irrigation resumed. Spraying trees with a light reflecting material did not make any differences in tree water stress. Good weed control to conserve moisture makes sense. Mulching to reduce evaporation might be a possibility in some orchards.

#2 — Severe reduction in water available (25-50% ET)

Tree stress and effects on fruit will be inevitable when managing with this little water. Like the no water scenario, the primary goal is to keep the trees alive. More than normal fruit removal could keep limbs upright and protect fruit wood from sunburn particularly. Allocate most of the water evenly throughout fruit sizing from May through July to lessen fruit cracking that can be aggravated by large swings in crop stress. For example, if the water allocation is 12 acre-in consider four 3-inch irrigations spaced evenly throughout fruit sizing, possibly one irrigation in May, June, July, and in early August. An irrigation applied in June, or when water stress is high might aggravate end cracking.

Water applied after final fruit sizing should not promote end cracking since the fruit is no longer increasing in size. Here again, the pressure chamber can be used to monitor for severe water stress. Stress level guidelines are published in the pressure chamber publication # 8503 previously cited. If the goal is to save the trees, fruit defects may be of less concern. A foliar zinc spray in the early fall to defoliate may help conserve soil moisture going into winter. Almost all tree water use is transpiration through the leaves so even though crop removal may help protect tree structure and lessen sunburned wood it does not appear to be a significant way to reduce tree water use.

#3 — Moderate reduction in water available (50-80% ET)

There are two general approaches to working with relatively modest reductions in water availability. Frequent irrigations with less applied water (frequent sips) compared to infrequent irrigation with more applied water spaced farther apart (full drinks). Prune irrigation research is not available to favor one approach over the other. The full drink approach would have the greatest risk of water loss below the root zone via deep percolation but should reduce water loss from surface evaporation compared to more frequent water application.

Either way, the strategy would be to spread the water stress over the season applying 20 to 50 % less water than ET per irrigation. In an RDI strategy, French Prune appears to tolerate mild water stress relatively well after final fruit sizing. In a prune irrigation experiment conducted by Fulton et.al. (2011) fruit continued to size until about mid August so it could make sense to reduce applied water after final fruit size, particularly for a 20 % reduction scenario. Under a moderate reduction in available water scenario consider no irrigation following harvest.

#4 — Full water availability (100% ET)

For prune growers with access to adequate groundwater or a full allocation of surface water, efficient irrigation management is always the objective to optimize costs and revenues. Good irrigation management improves tree health, saves energy, conserves water and reduces the risk of fertilizer leaching. Keeping trees adequately supplied with water through June with mild water stress after fruit sizing in July and early August (-12 to -13 bars, suggested) appears to be a good approach to maintain fruit size and achieve sugar accumulation.