Lab Opens to Test for Pistachio Problem

A Tulare, CA, lab testing for Rhodococcus bacteria in pistachio, AgExperts Inc., is now accepting samples.

AgExperts Inc.’s lab facilities will be housed in an 8,000-square-foot facility soon to be under construction. (Photo: AgExperts Inc.)

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.

As reported two years ago, growers have reported trees exhibiting what has been dubbed Pistachio Bushy Top Syndrome (PBTS), confirmed by university researchers. Tens of thousands of acres of pistachios have been reportedly affected.

In the past the lab focused on animal health diagnostic capabilities such as isolation and identification of fastidious bacteria causing diseases on animals, said AgExperts Inc. President Alfonso Lago, who thought they could overcome some of the problems encountered by traditional plant disease diagnostic laboratories to identify Rhodococcus spp and other pathogens causing plant diseases.

Interested growers should sample a minimum of eight trees in a block of 1,000 or nine in a block of 10,000 or more, says Lago, in order to get 90% confidence to identify at least one infected tree if at least 25% of the trees in a block are infected.

Alfonso Lago

Symptomatic trees should be sampled, if possible. Each sample should contain 10 rootstock leaves. If symptomatic stem tissue is present, such as swollen nodes, swollen/cracked graft union, or galls, sample approximately a a three-inch symptomatic stem segment.

Lago says growers collecting samples should change gloves between trees. Pruning shears should be surface-sterilized between trees with either a bleach solution, or an ammonia chloride product like Lysol.
Individually package tissue from each sample tree in a one-quart zip-lock bag and ship overnight with ice packs (place ice pack in plastic bag to protect shipping container) or deliver to: 1101 B Security Court, Tulare, CA, 93274. Phone: 559-331-9164. The cost is $125 per tree, and no fewer than five sample trees per block will be accepted.

Keen Interest In Lab
There has been a lot of interest from growers and associated industry people to bring a lab on line that includes culturing of the bacteria in the laboratory protocols, according to University of California Cooperative Extension Horticulture Advisor Craig Kallsen.

“Weekly, I have received calls asking if a lab is available that will test for Rhodococcus using protocols more similar to those used by U.C.,” Kallsen wrote in a recent newsletter. “Apparently, we now have a lab that will do this.“

Because of problems caused by the bacteria — slow growing, tough to bud, trunk-galling, declining and dying pistachio rootstocks, both clonal and seedling — Kallsen says he is becoming hesitant to suggest that growers plant pistachio at all.

“Granted, we have a majority of nurseries that have not had problems with their rootstocks, either clonal or seedling, and many orchards continue to go in the ground and grow normally,” he reports. “However, until we can better explain the mechanism of infection and other possible causes of these symptoms and poorly performing trees, how does one lessen the risk in choosing a rootstock? Based on my discussions with several researchers active in this area of research, the basic problem with some clonal, and possibly seedling, rootstocks, appears to be related to Rhodococcus (Rf) bacterial infection.
It is not a problem originating in growers’ orchards.”

Bottom Line Headaches
Many growers have suffered huge losses without even knowing the risk existed, Kallsen reports. Some growers are already working on the complete removal of their second set of collapsing rootstocks.

“With what we currently know, and even if Rf is not thought to be the causal agent by some researchers, I believe that it would be prudent for those involved in producing pistachio rootstocks, both seedling and clonal, to make every effort to ensure that their propagative plant material, clonal growth media, facilities and media for stratifying and germinating the seed and irrigation/misting systems are not contaminated with Rf bacteria,” he advises. “Growers would not be amiss in asking their nursery rootstock suppliers how they are ensuring that their rootstocks have not been contaminated with Rf bacteria. For those still interested in planting new blocks of pistachios, buying older, pre-budded trees that exhibit observable good size and vigorous growth of the scion, to me, would be prudent until we can get a better handle on this problem.”

For pistachio growers, it is definitely not business as usual, Kallsen reports, and they will have to adjust to the fact that tens of thousands of acres have been affected.

“I realize that the pistachio industry is not, generally, set up to produce nursery-budded trees, but in the absence of knowing how to prevent the problems we have been seeing, sufficient damage has been done to the industry that suggests we should be moving in this direction,” he concludes.

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