Three publications just released by researchers on bushy top syndrome in pistachios confirm there is considerable risk that comes with replanting trees into soil from which infested trees have been removed.
Researchers also found there is a risk of transmitting the bacteria that cause the disease on infested pruning equipment.
Hundreds of thousands of pistachio trees have been removed in the San Joaquin Valley alone because of a disease caused by a bacterial plant pathogen (Rhodococcus). The disease is called PBTS: pistachio bushy top syndrome.
“Infested soil can colonize the roots of healthy replants,” said Elizabeth Fichtner, University of California farm advisor in Tulare County. She is among researchers who looked at risks associated with pruning and with replanting into holes previously occupied by trees showing advanced PBTS.
Researchers recommend disinfecting tools used within and between orchards. They say growers unaffected by pistachio bushy top syndrome should request pruners to disinfect all tools prior to entering unaffected blocks of pistachios.
“We don’t’ know if it will be a problem on mature trees, but it can wreak havoc on young plants,” Fichtner said.
Pruners and other tools could be disinfected with solutions of isopropyl alcohol or bleach. For research conducted by Fichtner and Jennifer Randall, an entomologist with New Mexico State University, uninfested pruners were sprayed with 95-percent ethanol and flamed prior to making cuts on test plants.
Fichtner and Randall found that Rhodococcus was detected on 20 percent (5/20) and 15 percent (3/20) of plants cut with naturally-infested and artificially-infested pruners, respectively. The pathogen has not been recovered from the negative control plants.
Randall said some owners of orchards that are free of infection provide their own pruning tools to work crews to ensure the disease does not spread into those orchards.
Speaking at a Pistachio Day gathering in Visalia in January, Randall said nearly 2 million trees planted on UCB-1 rootstock in California and Arizona from 2011 showed symptoms of the disease.
Those symptoms include shortened internodes, stunted growth, swollen lateral buds, bushy/bunchy growth, and twisted roots with minimal lateral branching. During the second and third years of growth, many trees in affected orchards exhibited large stem galls and unusual cracking around the bud union.
Randall closed her January talk with three questions:
“How likely are the bacteria to be transmitted via farm tools, i.e. pruning shears, grafting knives, etc.?
“Is it safe to replant in the same holes where PBTS trees were planted?
“Is there a cure for Rhodococcus infection?”
The research papers just released address the first two questions. The third remains unanswered.
Randall said researchers are working on trying to find “a cure,” or at least to mitigate the effects of infection.
“The lab is about to start looking at ways to treat the soil and will likely test fumigants and solarization,” she said. “There is a plant in Madagascar on the endangered species list that could be a biocontrol for the bacteria. If we could find a cousin to that plant that would make some of the same compounds as that plant, that could work.”
Rio Stamler, a post-doctoral scientist at New Mexico State University, joined Fichtner and Randall in looking at the risk of replanting in PBTS affected orchards. Craig Kallsen, a UC farm advisor in Kern County, joined Fichtner and Randall to research management of the disease.
Researchers into the latter issue stated, “As part of the objectives of a grant provided by California growers through the California Pistachio Research Board, we were able to make an initial investigation of the fate of new roostocks or budded trees planted into holes previously occupied by trees showing advanced PBTS. We want to make it clear this was not a scientifically designed study.”
The researchers said they “do not know the eventual fate of large, healthy replant trees that test positive for Rhodococcus as a result of being planted in locations previously occupied by PBTS trees.”
They said it is “advisable for growers to mitigate the risk of potential infection of replants from residual inoculum in affected orchards. Therefore, observational evidence suggests that replants should not be placed in the same holes that were formerly occupied by symptomatic plants.
“Secondly, it may be appropriate in orchards where significant tree removal occurred as a result of PBTS and in which original trees were retained and look symptomless, to closely observe these trees for early nutlet abortion as they come into bearing.”
Fichtner, Stamler and Randall found that infested root fragments remaining in the field after removal of affected trees “may serve as a source of inoculum.”
Rhodococcus has been detected from a PBTS-affected orchard using polymerase chain reaction (PCR) tests.
“It is important to note,” the researchers wrote, “that this method relies on detection of pathogen DNA in soil and does not address whether the pathogen is viable (alive). A confounding issue is the fact that the natural distribution of the pathogen in soil is unknown; consequently, it is premature to conclude whether the pathogen DNA detected in the soil originated with PBTS-affected plants. Future survey-style studies are planned to methodically address pathogen presence/absence in orchards with and without a history of PBTS.”