Physiologist says more walnut trees die from too much water

By TIM HEARDEN

Capital Press

RED BLUFF, Calif. -- When it comes to walnuts, the decision of how much or when to irrigate can be challenging, experts say.

Walnut irrigation is dependent on many factors, including weather and climate, soil interactions, root development and system engineering, said Rick Buchner, a University of California Cooperative Extension farm advisor.

"The problem with irrigation is ... everything is kind of a moving target," Buchner told about 60 enrollees of a walnut irrigation short course on Dec. 17. "The walnut is sensitive to moisture stress. There's not a whole lot of give and take to getting this irrigation thing right or wrong."

Buchner and a dozen other researchers have done an extensive study of irrigation-related stress on walnut orchards, which has been identified as a major factor in such problems as root rot, walnut decline and deep bark canker.

Growers often think that walnut trees must be kept very wet to get good production, the researchers found.

Drought can definitely cause havoc in walnut orchards. But more walnut trees die from too much water than from too little, said Ted DeJong, an environmental plant physiologist at UC-Davis.

It's hard to kill a tree with drought, but "flooding will kill a tree every time," DeJong told the gathering.

A telltale sign of water loss in a tree is that the leaves dry out. They're the final destination for water that's drawn from the soil and up through the roots, trunk, branches and stems.

But stress begins to affect tree growth and other processes before the leaves start to wilt, said Ken Shackel, a UC-Davis plant biologist. That's why it's important to use pressure chambers and other devices to monitor moisture levels, much like with a blood pressure monitor for humans, Shackel said.

In some walnut orchards, researchers tried moderate deficit irrigation, which has helped improve the quality of almonds and prunes and saves water and pruning costs. But the practice resulted in yield losses by the third year, as walnuts are much more vulnerable to low-to-mild stress than are almonds or prunes, Shackel said.

"This would argue that no stress is the sweet spot for walnuts," he said.

Waterlogging at the base of the tree can cause little roots to die of lack of oxygen, Buchner said. So it's important to think about water placement in irrigation, he said.

Measures to reduce stress can include stream splitters on sprinklers to keep the water off the trunk of the tree, placing berms around the trees and managing drainage, Buchner said.

Knowing when to irrigate is also a plus, as when growers irrigate before the fall cold to prevent damage from frost.

"I think we just dodged a bullet with the recent freeze out there," Buchner said.

Online

University of California Walnut Research Reports: http://walnutresearch.ucdavis.edu