California Pistachios: Rhodococcus Bacterial Infection – Dealing With Uncertainty

Cluster showing nut shriveling and failure to develop in bearing pistachio trees. Picture taken April 29, 2014. Photo by Craig Kallsen.

With the continued problems with slow-growing, tough to bud, trunk-galling, declining and dying pistachio rootstocks, both clonal and seedling, I am 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.

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 scientists 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.

That Rf can cause many of the problems we are seeing in the field has been documented in recent ongoing and as yet unpublished research, as well as in published and anonymously, peer-reviewed scientific publications (the gold standard for scientific work).

In my opinion and, more importantly, in the opinion of several of those working closely with this problem, judging by the almost 100% of trees affected and the degree of decline in some young orchards, whatever is causing the worst of the symptomology with these trees, appears to occur early in the life of clonal and seedling trees at the nursery.

It is not a problem originating in growers’ orchards. For such a high percentage of trees to be affected so early in the growth of the orchard, a hypothesis, shared in some form by several researchers working with Rf, is that the worst of the symptoms must begin when that little piece of tissue is being turned into a complete plant in the test tube or, possibly, with seedlings when the seed is first germinated or very shortly thereafter.

Exposure of older trees to Rf does not appear to be associated with the risk to tree growth and productivity that early infection entails. Generally, I try to avoid discussing hypotheses prematurely, but with the lack of scientific data, the current pace of research with this difficult organism, and the continued collapse of orchards, we need to talk now about ways to lessen the risk of losing thousands of dollars per acre related to this problem.

Many growers have suffered huge losses without even knowing this risk existed. 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.

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.

I realize that the pistachio industry is not generall, 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.

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