California walnut growers know about crown gall, a disease that saps young trees and triggers root-destructive diseases, and now, with increasing reports of it in walnuts, some suspect it of expanding also in almond orchards, without growers being aware of the threat.
Lynn Epstein, an associate professor in the Department of Plant Pathology at the University of California, Davis, has worked with the disease in walnuts and recently started collecting data on it in almonds.
Few facts are available to indicate whether it is economically worthwhile to treat walnuts for it, so she has set out to develop some guidelines.
Although even less is known about the pathogen, Agrobacterium tumefaciens, in almonds, she talked about it during a recent almond field day near Escalon.
In the case of walnuts, she said severe infections can cause significant losses in yield. “Bill Olsen, farm advisor in Butte County, found that for walnut trees planted in 1996, there was a 12 percent decrease in cumulative yield for each 25 percent of the circumference of the trunk infected with galls.
“It appears,” she added, “to be increasing in walnuts and we are also getting calls about it in almonds. Younger trees are the most susceptible and become stunted, while older trees develop secondary root rots.”
The characteristic irregular, rough-outside, soft-inside galls form on roots and then encircle the crown of three-year-old trees. A telltale sign is soil pushed up around the crown.
The bacterium lives on the surface of many plant hosts found throughout California. It invades roots and inserts genetic codes to cause the host to provide it with shelter for reproduction, in the form of the galls, along with a tailored food source for it.
In time the galls dry up and fall away, leaving sites for wood-rotting fungi to invade. Once roots are destroyed, trees collapse or blow over in a wind.
Natural wounds in the plant, such as points where roots or suckers emerge, are enough to let in the bacterium already on the surface. Epstein said it can form a “biofilm” coating on young trees before they are planted.
The disease is unpredictable, so neither nurserymen nor growers can say where it may occur, but Epstein said it is linked to nurseries, especially where plant densities are high. Peach-almond hybrid rootstocks are known to be more susceptible than Nemaguard rootstocks.
Although conventional wisdom has been that growers likely caused the crown gall entry points, Epstein said she sees no evidence that standard cultural practices contribute to incidence of the disease.
Severe cuts or scrapes from equipment can be an entry site, but “we do know that in walnuts the bacterium does not like to infect through pruned tissue. Pruning in walnuts causes release of phenols known to be toxic to the bacterium.”
A standard recommendation for prevention — but not eradication — of crown gall in walnuts has been treatment prior to heeling in or planting with a biocontrol agent, Agrobacterium radiobacter 84, also known as K-84, Galltrol, or Norbac.
“If you have trees that are already infected,” she said, “use of K-84 will do no good. It is only effective when applied to a clean tree that is to be planted in soil known to be infected. Of course, if the bacterium isn't there, the K-84 offers no advantage.” The bacterium has also developed strains resistant to the biological control agent.
Not long lasting
“And K-84 is a relatively wimpy colonizer that tends to stay where it is applied and doesn't last for long.”
On the other hand, she added, “It is always a real question whether it is economical to treat, but it is good insurance if you have had crown gall in the orchard before.”
Some attempts to remove galls are very time- and labor-intensive. Their economic value is unknown, and Epstein said she hopes to learn more in trials with cooperating growers. One method is to dig away soil to expose the galls and use compressed air to dry out the bacterium, which thrives on moisture. Another is flaming to kill the galls.
The kerosene-derivative Gallex can be used in walnuts but galls, whose outer tissue resists penetration, must be cut open for the material to reach the sensitive internal tissue. She said the use of Gallex, or a grower alternative of applying straight kerosene with a brush, may be of some use on almond but is not entirely effective alone on walnuts.
Using her experience with walnuts as an example, she said trees infected with the bacterium will only produce more galls when planted in fumigated soil. Fumigation destroys the other competing soil-borne organisms, and then the bacterium has more opportunity to produce galls.
She does not recommend another remedy used by some growers, treatment with household bleach, because it sets up a semi-sterile condition that does not eliminate the crown gall disease but discourages natural competitors.
On the nursery front, she said some nurserymen use K-84 after digging, “although it is unclear whether this helps or not. We recommend a good rotation with two years without tree production.”
Epstein distributed questionnaires on crown gall at the Escalon meeting, but too few were returned to provide any impressions, suggesting growers may not know if they have it in their orchards.