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New Mexico State University

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NMSU researcher finds disease-causing bacteria in pistachio trees

In November 2013, Jennifer Randall, New Mexico State University plant pathology professor, was contacted by pistachio growers in California and Arizona wanting her to look at their oddly shaped trees.

Two pistachio plants, one much larger than the other.
Jennifer Randall, New Mexico State University plant pathology professor, found two bacteria called Rhodococcus, which usually distress ornamental plants, but instead has affected more than 20,000 acres of California’s pistachio industry. (Courtesy photo)

Randall traveled to California to inspect the pistachio trees. Instead of growing lean and tall, they were stunted and bushy with twisted roots, which in Arizona, resulted in three-year-old trees being lifted by the wind.

“These trees were abnormal. We used the term ‘Pistachio Bushy Top Syndrome’ to describe these trees,” Randall said.

Funded by the California Pistachio Research Board and the NMSU Agricultural Experiment Station, Randall’s laboratory investigated the cause of these abnormal trees.

The lab identified two bacteria, called Rhodococcus, which usually distresses ornamental plants, not trees, but instead has affected more than 20,000 acres of California’s pistachio industry.

“In the lab, we put the Rhodococcus bacteria onto healthy pistachio trees and the trees developed the same symptoms that were in the affected pistachio orchards,” Randall said. “The trees without the bacteria grew normally. This proved the bacteria caused the problem.

“One of the main issues with these trees was that the budding efficiency was reduced; only 30 percent of the trees were successfully budded. However, trees that were budded typically developed huge bark-cracking areas where normally they have a smooth surface. This meant that the top of the tree was not stable and if grown to harvest it is possible that the trees may not survive the shakers commercial growers use.”

Many of the affected pistachio trees were removed and growers must decide whether to plant their new trees in the same holes.

“My collaborator, Dr. Elizabeth Fichtner in California, is testing the soil to determine if a newly planted tree root might be contaminated by the bacteria from the previous tree,” Randall said. “We have also encouraged growers to disinfect their tools.”

Rhodococcus bacteria can live on leaf surfaces for months without causing any symptoms, but once it appears on the plant’s surface, the bacteria infiltrate the tree, altering the hormones that control growth and development, Randall explained.

“The problem for pistachio growers is that it takes seven years to produce a significant crop,” Randall said. “Now, they will not have the amount of production they projected because they had to take out the contaminated trees and plant new ones.”

Randall explained this issue would also affect consumers in a few years as demand for pistachios grows.

Although continuous testing has been done in the lab, Randall and her team don’t have a treatment for these bacteria.

“These two bacteria work together and affect a wide range of plants,” she said. “This bacteria has caused problems in nursery settings, but this is the first time it has been a problem for trees. One pistachio orchard in New Mexico was found to be infected, and
in New Mexico the concern is that we need to understand this and make sure it doesn’t affect our regionally important crops, such as pecan trees or chile.”

Randall and her team are trying to find an easier and faster way to detect the bacteria, especially for those growers who are replanting.

“Our research will also focus on ways to treat it because it is a different type of pathogen,” she said. “There are pathogens that kill plants and this one is different because it doesn’t kill the plant, but changes how the plant grows.”

The next step of her lab research is to experiment, in a controlled setting, how it might affect pecan trees, given that pecans are one of the largest industries in New Mexico.