Growers Claim Pistachio Trees Diseased – Nursery Maintains Problem Is A Genetic Disorder

Several pistachio growers in California’s San Joaquin Valley have filed lawsuits against one of the nation’s leading nurseries, alleging they were sold trees with a syndrome caused by bacterial infection. However, the president of Duarte Nursery, John Duarte, disputes that claim, saying the trees have a genetic disorder.

John Duarte

John Duarte

“If you’re saying this disease causes the problem, you would have to find the symptoms of the disease in the greenhouse,” Duarte says. “You don’t want to find it, but if you did, you’d certainly find plants with disease symptoms in the greenhouses where the conditions are more conducive to the organism’s survival. What we do find is plants that are phenotypically different but that can be clearly linked to a genetic disorder in our clonal line.”

Advertisement

But Rod Stiefvater, the only grower who has filed suit who was willing to talk on the record to American Fruit Grower® and Western Fruit Grower™ magazines, is convinced many of the trees he purchased from Duarte had the bacterial infection, Rhodococcus. Stiefvater, who farms more than 3,000 acres of pistachios in Fresno, Tulare, and Kern counties, says he had the trees, which exhibited what has been dubbed Pistachio Bushy Top Syndrome (PBTS) tested by university researchers.

Stiefvater, a member of the California Pistachio Research Board, says he switched to Duarte trees in 2012 because he was impressed in the past with their performance in other orchards. But the trees he planted not only grew slowly, by the spring of 2014, they were exhibiting some unusual characteristics.

Top Articles
What Is Climate Smart Agriculture? Here's One Take

“The trees flushed out, then the leaves wilted. We started seeing problems in the tops, but once we started pulling trees, we realized how bad it was,” he says. “Because we could literally pull two-year-old trees out of the ground; their root structures were just bizarre.”

Stiefvater has planted trees from other nurseries both before and after planting the trees in question, and they all had UCB-1 clonal rootstocks, the industry standard, and he says hasn’t had a problem with those. Also, he says he’s not the only one who had a problem; some industry leaders have estimated that at least 20,000–35,000 acres of pistachios planted in California and Arizona between 2011 and 2014 are at risk.

“If you look at the size of the problem, it seems extraordinarily unlikely it’s a genetic mutation because a genetic mutation doesn’t grow exponentially,” he says.

Stiefvater hopes the problem can get resolved out of court. It’s not just the $7 per tree he planted, or the cost of replanting, or even the two years of lost production.

“The bottom line is we have filed a lawsuit and resolution could take years,” he says. “As growers, we want to get trees in the ground.”

First Rhodococcus Link
PBTS was first ascribed to Rhodococcus by Dr. Jennifer Randall, a New Mexico State University research associate professor, who was called in November 2013 by an Arizona grower. Randall is a plant pathologist, not a pomologist, and in fact had never been in a pistachio orchard before.

Jennifer Randall

Jennifer Randall

Randall says she saw symptoms that would look strange in any plant, and in those trees found Rhodococcus. She didn’t find it in healthy orchards, nor did she find it in over 350 plants on UCB-1 clonal rootstocks the university ordered from other nursery sources for testing purposes. The California Pistachio Research board funded further experiments.

Introduction of Rhodococcus to healthy UCB-1 rootstock resulted in symptoms observed in the field. Dr. Elizabeth Fichtner, a University of California Cooperative Extension farm advisor/plant pathologist, has also shown that the Rhodococcus bacteria, when introduced to roots of UCB-1 plants, also produces the same symptoms. Also, Fichtner’s work shows that trees planted immediately after the bushy top trees were removed can be infected by Rhodococcus.

Organism Is Everywhere
Duarte insists it is not a bacterial infection, saying Rhodococcus is a group of microorganisms that are virtually omnipresent.

“If I go to your back yard and look hard enough, I can find it,” he says. “It’s a very general family of bacterium; it’s used to gobble up oil spills, it can cause acne, all kinds of things.”

In addition, he says Rhodococcus simply doesn’t behave that way, noting he and Kendall Ash — the Ph.D. researcher he has on staff — have devoted a lot of time to PBTS. Rhodococcus simply doesn’t seem like it would be a factor on pistachios, says Ash, noting that historically its most often found in moist environments on herbaceous plants.

The California Department of Food and Agriculture has reviewed the data and opted not to change the organism’s “C” pest rating , he says. According to the CDFA that “means the organism is subject to no state enforced action outside of nurseries except to retard spread.”

Duarte says the CDFA has previously documented “widespread distribution in California,” and its “wide host range.” This finding by the CDFA concurs with the fact that the organism can be a natural plant epiphyte, Ash says, which has been isolated from nursery plants, field plantings and water from wells and canals.

“In all the studies we have found,” Ash says, “we have found no documented epidemiology on trees or in desert environments.” Over the intervening months of testing for Rhodococcus, many of the California laboratories involved have noted difficulty in achieving consistent results when analyzing the same suspect cultures from field samples, he adds.

At a recent open forum, hosted by Duarte Nursery, a diverse group of laboratory scientists, plant pathologists and horticulturists came together to discuss the problems associated with the inconsistent PCR results. Representatives from CDFA, University of California Agriculture and Natural Resources/Cooperative Extension, UC-Davis Foundation Plant Services, USDA-ARS, UC-Davis Department of Plant Pathology, independent plant pathologists, private laboratories and members of the California Pistachio Research Board were in attendance.

Duarte says “the group, with no reservation, concluded that the PCR primer set (VicA) (a DNA test) commonly used and recommended for the identification of Rhodococcus by Dr. Randall, produces false positives, due to its lack of specificity.”

Duarte says the clonal rootstock line UCB-1 has been around since 2000, and though it became the industry standard — as high as 70% of the pistachio plantings in some years — it has become degraded. At his laboratory in Hughson, CA, they have developed five new and improved clones for which they are seeking patents.
“Whatever’s happening in the old clone is not happening in the new clones,” he says, “and that brings us to believe it’s a genetic meltdown.”

Duarte notes that the five new rootstock clones will be much more salt tolerant. That’s a tremendous advantage, not only in the current drought when growers are forced to pump groundwater that is much saltier than the surface water they had available in past years, but it might open up areas to planting where the soil was thought to be too salty.

Advice For Growers
Currently, Duarte is advising growers to stay away from trees with the old clone of UCB-1 rootstocks, and to use other clones, seedlings or to plant the new clones when they are available.

The nursery is conducting 10 trials up and down the San Joaquin Valley, says Ash, who adds that the initial results are extremely promising.

“We think with these new clones we will increase yields by 50%,” he says “There is a very high correlation (Correlation Coefficient equal to = 0.858) between trunk circumference and yield in these new clones, indicating that the bigger the trunk, the greater the yield.”

Even before the PBTS problems cropped up with UCB-1, Duarte says they were already working on new clones to improve performance.

“We’re going to move the ball for growers not only on vigor,” he says, “But also on salt tolerance.” ●

 

 

0