Lab tests confirm bacteria of the genus Rhodococcus as culprits behind pistachio bushy top syndrome (PBTS) moving the pistachio industry one big step closer to finding a way of controlling a disease that has forced growers in California and Arizona to remove thousands of acres of young trees in the past three years.
Jennifer Randall, a plant pathologist at New Mexico State University, is leading the research that involves scientists there and at the University of California. Their work, which is being funded by the California Pistachio Research Board and the NMSU Agricultural Experiment Station, is focused on two species of Rhodococcus bacteria.
So far PBTS has been found only on UCB-1 rootstock trees from 2011 or later. “In greenhouse trials, the Rhodococcus that we isolated from diseased UCB-1 rootstock was introduced onto small UCB-1 pistachio trees, which were free of Rhodococcus,” Randall reports. “The Rhodococcus isolates caused bushy top syndrome to develop. The trees from our study are small and not yet big enough to bud or graft.”
Some of the first reports of abnormal growth of pistachio trees, now attributed to PBTS, trace back to 2011 in the southern end of California’s San Joaquin Valley.
“Budders were having problems grafting buds onto rootstocks in several fields of young pistachio trees, which didn’t appear to be growing normally,” recalls Craig Kallsen, University of California Cooperative Extension Service farm advisor for Kern County. “As many as 60 percent of the buds didn’t take in some orchards.”
While new to pistachio growers in Arizona and California, Rhodococcus species have long been known in the scientific world. The bacterium has been found elsewhere in the United States, including Oregon and Pennsylvania, and other areas of the world where it has caused PBTS-like symptoms on herbaceous and ornamental plant species.
“We have not found Rhodococcus on the trees in a healthy pistachio orchard,” Randall says. “Even when the bacteria does get into an orchard, they can live on leaf surfaces for months without causing symptoms of bushy top syndrome. But, once the bacterial population on the plant surface reaches a certain level, the bacteria enter the tree, modifying the hormones that control growth and development.”
These hormonal changes, say researchers, lead to such tell-tale signs of Rhodococcus infection as bushy growth at the top of trees, resembling witches broom.
“It’s as if these hormones are keeping the trees in a juvenile stage and producing lots of adventitious shoots,” Kallsen says.
In the pistachio fields affected by PBTS, Randall reports finding from 30 percent to 60 percent to be abnormal. “However, in one orchard I visited, the farm manager estimated that figure at about 90 percent”, she says.
The symptoms of PBTS appear early on.
“If you look at infected rootstock, you’ll see that it’s stunted with shorter-than-normal spacing between the nodes or branches,” Randall says. “In infected first-leaf trees, many of the lateral nodes become swollen. By second- or third-leaf, galls develop from these swollen nodes. A number of buds begin emerging from each gall, leading to the characteristic bushy appearance of the top of tree. At the same time, growth of the trees is stunted.”
So is the root system, which becomes very twisted with no lateral branching which makes it easy to pull trees out of the ground. Randall notes a case in Arizona where such weakened roots caused infected three-year old trees to blow over from the wind.
In some orchards, grafting is successful on no more than about 30 percent of infected rootstock, she adds. Even then, it may take two or three attempts.
“The majority of trees that are t-budded develop an unusual graft union and the bark around the graft cracks”, Randall says.
Typically, these symptoms develop in stages over several seasons. However, last October, Randall, Kallsen and Elizabeth Fichtner, UCCE farm advisor for Tulare County, viewed a California pistachio orchard, planted earlier that year in January and budded in July, where galls had already formed and bark was cracking around the bud union.
Knowing the cause of PBTS answers just one of a long list of questions growers are asking in dealing with the Rhodococcus threat to their pistachio orchards. For example, which rootstocks are susceptible to the bacterium? How does it spread? How fast? How far from the site of an infected tree can a new tree be planted without acquiring PBTS? What do you do with infected trees? Are older trees at risk?
Researchers have found no evidence that Rhodococcus is transmitted by insects. In greenhouse trials, it has been spread by water splash, Randall reports. Fichtner is now testing if the bacteria can be spread by pruning tools or grafting knives.
Until more is known about PBTS, Randall recommends sanitizing any tree tools used in PBTS-infected orchards before using them in another orchard. This can be done using a 10 percent solution of bleach or quaternary ammonium.
Randall and her colleagues have now turned their attention to developing practical field methods of detecting Rhodococcus and, eventually, controlling it in pistachio orchards.
Currently, they detect the presence of Rhodococcus by growing out lab cultures taken from tissues of plants suspected of infection. This process takes several weeks. The goal is to develop a faster, simpler way for growers to test their trees for PBTS.
Other studies, involving molecular biology, are designed to learn more about the bacterium and how it affects pistachio trees in hopes of developing antibiotics, natural biocontrol agents or other methods of killing the bacterium without harming the tree.
Until then, some growers are responding to PBTS in their fields by removing entire orchards and bearing the extensive costs of replanting with new trees.
Others are removing only those trees showing signs of PBTS and replanting with new trees.
However, Randall cautions, trees showing no symptoms in an orchard where PBTS is present in other trees, may be infected with Rhodococcus.
“We’ve detected Rhodococcus on trees that looked healthy in some infected orchards and, when we’ve gone back later to check, we’ve found these trees had developed PBTS symptoms,” she says.
If you suspect PBTS, she advises contacting your local farm advisor or nursery for more information.