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Managed Deficit Irrigation

Less Water, Reduced Hull Rot, Earlier Harvest

Measuring water stress begins with taking a leaf sample from
a shaded area in the lower canopy. Cover the chosen leaf
with a plastic or foil envelope for a minimum of 10 minutes
before snapping off at the base of the petiole (leaf stem) for
sampling.

A plant-based strategy for managed deficit irrigation (MDI), also known as regulated deficit irrigation, or RDI, targeting stress levels at specific crop stages can effectively manage hull rot and potentially reduce water use without impacting crop productivity.

Prior Almond Board–funded research and a five-year study published in California Agriculture in 2011 found that well-timed deficit irrigation, using midday stem-water-potential readings to measure tree stress, can substantially reduce irrigation requirements with no long-term effect on yields.

Depending on rainfall, location and orchard conditions, MDI can potentially result in water savings of 10–15% for shallow soils and up to 50% in deeper, well-drained soils, annually. Follow-up research has demonstrated that MDI will reduce hull rot by 60–90%, depending on region and other factors. In addition, the practice can provide more uniform maturity and an earlier harvest for improved navel orangeworm control.

The sample leaf’s petiole is then placed in an opening in the
pressure chamber seal. The exact specifications can vary
with different pressure chamber models, but it is always
recommended to make a cut with a sharp razor blade at the
end of the petiole to ensure a good viewing surface.

Hull rot is managed most effectively through deficit irrigation at hullsplit and a balanced nitrogen program. The goal of a managed deficit irrigation program is to maintain tree stress levels between –14 bars and –18 bars from the onset of hullsplit until 90% hullsplit, typically a period of about two weeks.

The onset of hullsplit will vary according to orchard conditions; stress levels vary with soil type and other factors. In shallow soils where trees may dry down quickly, initiate stress when blanks start to split, usually about a week before the onset of hullsplit. On deeper, well-drained soils, it can take up to 20 to 30 days to reach mild to moderate stress levels. Growers in this case may want to use the UC Almond Hull Split Prediction Model available on the UC Davis fruit and nut website, to determine when to initiate water stress.

The most accurate way to practice MDI is with a pressure chamber to accurately measure plant stress. Farm advisors and researchers recommend growers use a pressure chamber to measure actual plant stress through stem water potential throughout the season. Understanding how an individual orchard responds to water use can help guide how long it will take for trees to dry down to –14 to –18 bars. Information about the pressure chamber is available on the UC Davis Fruit and Nut Research and Information website, and a video demonstrating its use is also available.

Using a magnification loupe, this grower is watching for the
point at which water flow is noticeably increased at the cut
end of the petiole. The bars of pressure indicated at this
point show the amount of water stress that the tree is
experiencing. When practicing MDI, maintain stress levels
of –14 to –18 bars for optimal hull rot control.

Growers who do not rely on pressure chambers, particularly those with a history of hull rot and a visual baseline from which plant stress can be observed, may achieve similar results by irrigating at 50% of normal tree demand, using crop evapotranspiration (ETc) calculations during hullsplit. Dr. Ken Shackel of the Plant Sciences Department, UC Davis, emphasizes that as a start, “normal demand” should be based on calculated almond evapotranspiration (ET). Methods for this calculation can be found at the California Irrigation Management Information System (CIMIS) website and on the Almond Board’s website. From that 50% of normal tree demand, adjustments can be made based on observed levels of disease and stress.

Brent Holtz, San Joaquin County farm advisor, noted that in a water short year in which growers may be limiting irrigations already, hull rot pressures may be lower than average, and managed deficit irrigation practices should be adjusted accordingly.

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