A Terrible Sinking Feeling; Bacterial Canker
Two years ago, Bill Van Ryn would cruise by his three-year-old almond trees to check on their progress and to simply admire them. "They were the most beautiful trees you ever saw in your life," he recalls. The second generation almond orchard in good sandy loam near Manteca had gotten off to an excellent start. "There was never any hint that there was something wrong with the soil or with the trees," Bill says.
Then along came the monsoon-like rains of the spring of '96. That was the beginning of the end for the young orchard. "That spring we could see that something was wrong with the trees," he explains. "They just never came out of dormancy. Some tried. They put out a few leaves, but no nuts. Others looked sick and many died." Another wet spring in '97 added to the young orchard's misery. "This year, the whole field was affected," Bill continues. "It was devastating. I looked at the orchard and got a terrible sinking feeling."
Young Tree Disease
The disease that has wiped out over two thousand of the 2,500-plus trees in Van Ryn's three-year-old block is bacterial canker. It is wreaking havoc in many other young almond orchards in California, primarily in Stanislaus, San Joaquin and Merced counties.
"The severity of bacterial canker problems in the three-county area has increased greatly in the past two years," says Paul Verdegaal, San Joaquin County farm adviser. "This year the problem was not quite as bad as last year, because of the warm spring," he adds. "The bacteria prefers cold, rainy weather."
But the outlook is not good. "In San Joaquin County, many of the orchards that are being replanted to almonds will have problems with bacterial canker," Verdegaal predicts. "Some will have severe problems."
Also known as gummosis or sour sap, bacterial canker occurs on all commercially grown stone fruits. In almonds, according to the University of California, the disease is more common in areas with coarse-textured, shallow, or nitrogen-deficient soils, or on soil containing a high population of ring nematodes. Van Ryn believes that the ring nematode "creates the weakness and opens the door to bacterial infection." The same bacterium Pseudomonas syringae causes both bacterial canker and bacterial blast in almonds.
"Bacterial canker generally affects young trees, two to seven years old," explains Roger Duncan, Stanislaus County farm adviser. Bacterial blast, another form of the disease which causes blighting of the blossoms, can occur on older trees.
No Easy Answers
Exactly how and why bacterial canker gets started in almonds is not well understood, says Verdegaal. "It is a complex disease that involves a number of environmental and pest factors."
This much is known about it. According to the university, Pseudomonas syringae survives the summer on leaves and bark of trees. Winter rains move the pathogen about, enabling it to enter dormant trees through buds or natural openings, such as leaf scars. The disease grows until spring, forming cankers.
If the tree is invaded early enough in dormancy to allow the pathogen time to kill a large branch or even the entire above-ground portion of the tree, the affected wood will "sour out." This refers to the vinegar-like odor that rises from the dead wood. In spring, when other trees bloom and leaf out, diseased trees produce a watery, sour smelling sap that runs down the tree.
Stress appears to be a major factor in the onset of the disease, says Duncan. "Vigorous trees have less trouble with bacterial canker. " He advises minimizing stress through adequate fertilization and irrigation to encourage good plant growth. But even those measures may not be enough if a new orchard is planted in ground that has held almond trees before, as Bill Van Ryn and many other central valley growers are finding out.
"Bacterial canker is not usually a problem in first generation orchards," Duncan observes. "It is more of a problem in second, third and multiple-generation orchards. If you are planting in virgin soil, you don't usually have the nematodes and other soil problems that go with replanting an orchard, consequently the disease is not as prevalent."
Fumigation May Help
How can a grower reduce the possibility of bacterial canker when replanting an orchard? Verdegaal says fumigation helps, but not just in strips or where the backhoe has dug a planting hole.
"No fumigation is better than spot fumigation," he counsels.
Even done correctly, fumigating with methyl bromide doesn't solve the problem, Duncan says, "It really only gives you a couple of year's head start. Then the ring nematode comes roaring back. By the third year, its populations can be very high."
What does a grower do then? Duncan sighs and says, "We don't have a cure, because there are no really satisfactory methods for reducing ring nematode counts." "In nonbearing trees, you can apply Nemacure," he explains, "but by the third year, you can no longer apply it."
Verdegaal has seen some beneficial effects from the use of Enzone in post-planting situations. "Enzone breaks down into carbon bisulfide, which helps control ring nematodes," he says.
Enzone, which is registered for bearing trees, works best when applied with drip or microsprinkler systems, says Duncan. "But it can be very expensive to use in flood irrigated orchards, which is the predominate method in this area."
The problem with using Enzone with flood systems, he explains, is that the concentration has to be kept above 500 parts per million. "So in flood irrigation, that gets expensive, probably not even cost-effective," he says. "Enzone is not a cure," Duncan cautions, "it is a management tool that needs to be used annually."
The most effective nematicide application timing for managing bacterial canker appears to be in October as nematode numbers begin to climb.
Resistant Root, Copper Sprays?
Does nematode-resistant root help ward off the disease? According to Verdegaal, Nemaguard rootstock can actually present a problem. "Nemaguard works well against root knot nematodes, but is not effective for ring nematode," he explains. "So the problem becomes less competition for the ring nematode. When that happens, its numbers increase."
One theory advanced for preventing bacterial canker involved copper sprays to protect the leaf scars through which the bacteria enters the tree. Duncan tested that idea on peaches with copper applications once a month, October through March, but without favorable results. "I don't think the use of copper sprays for this disease does much," he declares.
Cleanse the Soil
Van Ryn thinks growers have to go back to some old methods to prevent future severe outbreaks of bacterial canker. "What we have in replanted orchards is an overwhelming dominance of a bad bacteria and pests, and the only way to break the cycle is to rotate our crops," he says. "We have to put some other crop in the ground before putting trees back in where orchards have previously stood. That is one of the steps we have to take to get back to a healthy soil."
Hope for Answers
Given all the uncertainty about the disease and the increasing severity of its attacks on young almond orchards, what kinds of new materials or methods can growers expect for controlling this problem?
"Unfortunately, there are people who have worked their entire careers on this disease and we still don't have a good answer," says Duncan, "but maybe we will in the not-too-distant future. There is renewed interest at the university and more resources have been directed at studying this disease, so hopefully we will be getting some new information."
In the meantime, more growers will be getting that "sinking feeling" as they watch their young orchards wither and die.