It may be difficult to think of over-irrigation as a likely culprit for damage to walnut trees during a drought. But that’s exactly what two University of California (UC) researchers believe occurred in some Tulare County walnut orchards.

Not only did they find foliar chlorosis they blame on over-irrigation in some walnut blocks in the county in late May and early June. They also came across foliar scorch in August believed caused by standing water that reflected extreme heat, much as heat reflected from a lake can give you a sunburned face despite you wearing a broad-brimmed hat.

The research was conducted by Elizabeth Fichtner, UC Cooperative Extension farm advisor in Tulare County, and Bruce Lampinen, UCCE specialist with UC Davis.

Fichtner conducted field observations while visiting growers, snapped pictures of injured trees, and sent the photos to Lampinen. Her suspicions were validated by Lampinen who is conducting research on irrigation management.

Fichtner said the foliar chlorosis she saw was identical to symptomology in research blocks that are maintained wetter than baseline. In research plots, foliar chlorosis occurs within just days of an excessive irrigation event.

The yellowing associated with overwatering affects all leaves on a shoot down to the nuts, Fichtner said. Newly emerging leaves may initially appear yellow; however, they gradually turn to green three or four leaves back from the growing tip.

Over-irrigated walnut trees in research plots also have a tendency to drop one of two nut doubles, resulting in lower overall nut retention in the canopy.

When this situation occurs, Fichtner said, you can grasp a number of pairs of double nuts on the tree and one of each pair growing close together will often come loose with almost no force. These nuts will exhibit blackening while still on the tree and squirt water vigorously when cut.

Anaerobic conditions in the root zone may cause root rot which may lead to overall tree decline and mortality, Fichtner said, explaining that when the porous spaces in soil are filled with water then less air can get to tree roots.

She said over-irrigation of walnuts is common in young blocks where growers cultivate an annual crop in tree row middles to maintain an economic return on the land in advance of tree maturation and productivity. Walnuts generally start bearing at fourth leaf.

Fichtner has seen annual crops grown between rows of trees including corn and eggplant. The challenge is these annuals likely require more water than the trees.

Over-watering to keep the annual crop thriving can affect the future economic productivity of the orchard. Similarly, Fichtner said, replants are often subject to over-irrigation because irrigation events are designed to meet the needs of the more mature trees.

The root rot and tree decline resulting from over-watering is similar to the symptoms of root rot caused by Phytophthora. But Fichtner said isolation of these pathogens from the roots of overwatered trees is rare, suggesting that over-watering is the primary cause of decline.

Even if there is no annual crop in the orchard, she said irrigation events may be timed around factors other than tree physiology. For example, many growers in Tulare County were irrigating walnut orchards approximately a week in advance of the anticipated codling moth flight around May 30.

These irrigations were timed to allow for the orchard floor to dry to bring in a spray rig.

“In some orchards, irrigation events timed around pest management operations may be ill-timed with respect to tree water needs,” Fichtner said. “Additionally, some irrigation districts in Tulare County had a release of Class 1 surface water in mid-May 2016.

“The timing of surface water availability and anticipated codling moth flight activity might have contributed to the over-irrigation and subsequent chlorosis in several Tulare County walnut blocks. Finally, over irrigation symptoms can occur when there is a period of hot weather with high rates of irrigation followed by a cool cloudy period.”

Fichtner recommends using a pressure chamber, also termed a “pressure bomb,” to measure stem water potential, which allows direct measurement of tree water stress and assists with irrigation management.

Most irrigation managers measure stem water potential prior to an irrigation and one to two days after an irrigation event. To reduce variability and achieve consistency in readings, measurements should be made on bagged interior branches in the lower canopy during the afternoon, Fichtner said.

To maximize shoot growth and allow for nut sizing, she said readings should be maintained from -4.0 to -6.0 bars (low-to-mild stress) from leaf-out until mid-June. To control vigor without adverse effect on kernel development or fruitfulness in the successive season, trees may be maintained at -6.0 to -8.0 bars (mild-to-moderate stress).

For a comprehensive understanding of the use of the pressure chamber for measuring SWP in walnut, download the UC Agriculture and Natural Resources article entitled “Using the Pressure Chamber for Irrigation Management in Walnut, Almond, and Prune” at http://anrcatalog.ucanr.edu/pdf/8503.pdf.

The article was composed by UC farm advisors Allan Fulton, Joe Grant, Richard Buchner, and Joe Connell. It provides background on the use of the pressure chamber and interpretation of SWP data.