Scientists with the U.S. Department of Agriculture, looking for ways that growers can save water without affecting fruit quality or yields, see potential for using deficit irrigation on early season California peach trees. Their research includes development of a tool that can tell farmers precisely when irrigation is needed.
USDA researchers Drs. James Ayars and Dong Wang collaborated in a project studying regulated deficit irrigation in peach trees and the use of infrared temperature sensors to measure plant stress. Ayars’s deficit irrigation work is completed, but Wang is continuing his work to test the infrared sensors in irrigation scheduling of commercial orchards.
Water is a major issue in California. Growing populations, endangered species, and groundwater quality and supply problems all compete for the state’s limited water supply. In the San Joaquin Valley, home to some 25,000 acres of peaches and thousands of acres of other crops, irrigation is the primary source of water for agriculture during the summer months when temperatures and water demand are at their highest.
Timing of use
Early season peach varieties were identified as a candidate crop to reduce water use because fruit is harvested in late May and early June, and most of the tree’s watering needs are during the hottest months of the year from June through September—after fruit is harvested.
“For early varieties, the grower’s irrigation strategy shifts after harvest from producing a crop to keeping trees alive and setting up the crop for the next year,” said Ayars, who is based in USDA’s San Joaquin Valley Agricultural Sciences Center in Parlier, California.
With early variety peach trees, more than two-thirds of the water is applied after harvest. One of the drawbacks to all of that watering is additional canopy management.
“Trees grow like weeds here with our deep soils,” Ayars said. “Vigorous trees can require summer pruning.”
The thrust of the peach deficit irrigation study was to find out to what water stress limit the trees could be pushed. Deficit irrigation is used to produce wine grapes in Washington State and other regions and it’s been studied for its potential in fruit tree and row crop production.
In the four-acre trial, Crimson Lady peaches were fully irrigated from March through the May harvest. In the postharvest phase, between June and September, trees were irrigated at 25 percent of normal, 50 percent of normal, or 100 percent. Three irrigation methods were compared: microsprinkler irrigation, subsurface drip, and furrow. Soil moisture was measured weekly and standard practices of fertilization, pruning, and fruit thinning were followed.
Ayars also considered the impacts of deficit irrigation on the overall water management, going into winter months. “Part of the strategy was to create room in the soil profile to serve as a reservoir for winter precipitation,” he explained in a phone interview with Good Fruit Grower. The San Joaquin Valley receives an average of 10 to 12 inches of rainfall annually, with most coming between November to April. “If you’re already fully irrigating, then you don’t have the same storage potential in your soil as you would if the soil is partly dry from deficit irrigation,” he said.
Fruit quality impact
Research showed that reducing water to only 25 percent of what the tree normally receives negatively affected the following year’s fruit. At 25 percent, yield was reduced and fruit defects, such as doubles, increased. However, cutting water in half had a minimum effect on fruit yield and quality, while having a maximum effect on water savings.
He reports that growers could see almost a 70 percent savings of postharvest water use by following the 50 percent irrigation deficit, with minimum impact on the following year’s crop.
Less pruning and maintenance of tree growth were other benefits of the deficit irrigation that the scientists observed. The subsurface drip irrigation treatments tended to have the lowest yields within a given year, but the differences were not significant.
Ayars believes that standard cultural practices, such as fruit thinning to remove lower quality doubles and defective fruit, could be used to minimize fruit defects from the deficit irrigation.
While some deficit irrigation strategies shut tree growth down completely by not watering for a couple months, in the USDA study they maintained an even amount of stress until after harvest, which helped to minimize fruit defects.
One of the reasons deficit irrigation has not been adopted more widely by growers is because it has a small margin of error for irrigation timing. That’s where the infrared temperature sensor comes in—as a potential tool to help growers decide when to irrigate.
USDA agricultural engineer Wang and colleague Jim Gartung, also based in Parlier, are evaluating the use of infrared sensors and thermal technology to assess plant water stress and help growers know precisely when to schedule irrigation. Infrared sensors, around since the 1970s, have been used in cotton and other crops to measure plant health.
In the two-year study, 12 infrared temperature sensors were installed in the same peach orchard subjected to deficit irrigation to measure midday canopy and surrounding air temperatures.
When trees are water stressed, the stomata begin to close, and less heat is carried away by water vapor transpiring from the leaves. While stomate closing helps the tree conserve water, it also results in a warmer tree and adds to the water stress. Wang said that dry, water-stressed leaves have higher temperatures than a tree with normal transpiration.
“The infrared sensors measure the temperature of the leaf canopy, which reflects the overall thermal footprint of the tree,” he said.
The scientists calculated a crop water stress index based on the differences between tree canopy temperatures and surrounding air temperatures. Higher index numbers indicated more stressed trees. Wang and Gartung found that midday canopy-to-air temperature differences in the water-stressed trees were in the 10 to 15°F range, consistently higher than the 3 to 4°F range found in the trees not water stressed.
To see if data from the sensors were meaningful, Wang used a pressure chamber to measure stem water potential to compare tree stress to sensor data. He found a correlation of about 70 percent between pressure chamber leaf measurements and the canopy to air temperature differences. “Seventy percent correlation is not perfect, but it’s close and gives us some confidence that the sensor can be used to tell growers what’s going on in the trees,” he said. “It means the temperatures obtained from the sensors could be a reasonable indicator of plant stress.”
In the next phase, the scientists are focusing on identifying specific levels of water stress that peach trees can tolerate and identify relationships between stress and canopy temperatures. The sensors, which are commercially available for around $600 to $700, will be tested in irrigation scheduling to see if the technology is viable for commercial orchards.