Ensuring a robust crop for the next year, hull rot prevention, and producing an earlier and more uniform harvest are all reasons to maximize irrigation management in almonds.

University of California, Davis researcher-engineer Shrini Upadhyaya is testing a continuous leaf monitor system to conduct precision irrigation in almonds based on plant water status at a Nickels Soil Lab trial in Arbuckle, Calif.

The monitors are part of a wireless mesh network with sensors to monitor soil and plant water status, and controllers to implement precision irrigation in almonds.

The continuous leaf monitor is a plant water stress sensing system which measures leaf temperature, ambient temperature, relative humidity, incident solar radiation, and wind speed to better estimate plant water status.

The system measures and monitors leaf temperatures continuously - 24-7. Data collected from noon to 5 p.m. helps detect plant response and assist with irrigation decisions. A minimum of three monitors per zone are suggested to obtain an average reading.

“The trial attempts to maintain a stress level recommended for the tree,” Upadhyaya told participants at the annual field day. “We observe plant response and make a decision on irrigation.”

Plant water status is a term generally used to express the stress status in plants. Stem water potential (SWP) is the most widely used method to measure plant water status level. Other parameters, including stomatal conductance, also respond to plant water status. Upadhyaya says SWP is a well-accepted method to express plant water status.

The most recent trial at the Nickels Soil Lab includes five acres divided into two management zones based on soil variability (texture and electrical conductivity at two different depths), elevations, and plant light interception.

The researcher says zones are considered stable over the years as they are primarily based on static characteristics of the soil. Within each management zone, both conventional irrigation timing and precision irrigation based on plant water status were used.

“We are looking at precision irrigation based on plant water status as an opportunity to save water,” Upadhyaya explained.

In the trial, precision irrigation management based on plant water stress was implemented during the 2016 growing season. Upadhyaya says attempts were made to maintain SWP at about 13 bars during the pre-hull split and post-hull split periods, and about 16 bars during hull split. During the season, crop water stress index values were used to indicate stress.

Using the plant water status-based variable rate for irrigation management resulted in 75-86 percent water use in Zones 1 and 2 respectively, a savings of 25 and 14 percent in the irrigation. Crop yield and quality were not significantly different between plant water status-based precision irrigation and the conventional grower practice. Bud formation was not reduced.

Hull rot prevention

Another part of the Nickels Soil Lab trial is focused on hull rot prevention. Moderate drought stress from completion of kernel fill to 90 percent hull split lowers the potential for hull rot and assists with a more uniform hull split and harvest.

Strategic management can also bring on an earlier harvest and help minimize exposure to late season Navel orangeworm and potential aflatoxin contamination.

The system can also sense moisture levels in the root zone. Using the stem water potential as a measurement, Franz Niederholzer, UC Cooperative Extension farm advisor for Colusa, Yuba and Sutter counties, warns that more than -18 bars the trees can begin to defoliate and next year’s crop may be affected.

Niederholzer says it’s best to aim for -16 bars, though it is best for each grower to determine their tree stress level based on factors particular to each block of trees.

“It will be up to the grower to decide on the level of stress,” Niederholzer said. “There are still some questions that must be answered to improve the system.”