Jennifer Randall, New Mexico State University plant pathology professor, has identified two Rhodococcus bacteria, which usually distress ornamental plants, not trees, that have affected more than 20,000 acres of California’s pistachio industry.
“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.”
Two years ago, Randall was contacted by pistachio growers in California and Arizona wanting her to look at their oddly shaped trees. During inspection of California orchards, instead of viewing trees growing lean and tall, Randall found trees 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.
Rhodococcus bacteria can live on leaf surfaces for months without causing any symptoms, but once they appears on the plant’s surface, the bacteria infiltrate the tree, altering the hormones that control growth and development.
“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 of the research that was funded by the California Pistachio Research Board and the NMSU Agricultural Experiment Station.
The disease has an economic impact on the growers because it reduces the tree’s nut production.
“One of the main issues with these trees was that the budding efficiency was reduced; only 30 percent of the trees were successfully budded,” Randall said of her survey of the California orchards. “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.”
Another economic impact is the replacement of the infected tree in the orchard.
“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’s collaborator in California, Elizabeht Fichtner, is testing the soil to determine if a newly planted tree root might be contaminated by the bacteria from the previous tree.
Although continuous testing has been done in the lab, Randall and her team don’t have a treatment for these bacteria. They 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.