There is still no silver bullet for controlling crown gall in walnuts, one of the gnarliest maladies California producers face.
However, a combination of approaches can help minimize the problem, Dan Kluepfel, USDA bacteriologist and research leader, UC Davis, told attendees at a recent UC sponsored walnut meeting in Yuba City, Calif.
Crown gall disease is a persistent nemesis for walnut growers. The disease is caused by the bacterium Agrobacterium tumefaciens. It mostly infects trees one to eight years old and is more devastating to certain rootstocks such as Paradox.
University of California researchers have been looking at different management strategies to minimize the impact of the crown disease in walnuts.
“It likes to grow on the roots of just about everything,” Kluepfel says. “The other thing that is annoying about this is that it can persist in the soil in absence of a host for a very long time.
“The reason it is probably the best known of any plant pathogen is because it has the ability to transform plants with a piece of DNA,” he says. “It's a natural genetic engineer. Probably 90 percent plus of the transgenic plants are created using a highly modified form of the Agrobacterium tumefaciens.”
It has the widest host range of any known plant pathogen and appears to have no particular species specificity, according to Kluepfel. The only thing it needs to infect a tree is a wound. “You can dunk a plant into a suspension of this material, pull it out, wait two months and you will not see a problem if there is not a wound,” Kluepfel says.
The first line of defense is a resistant rootstock, he says. “When you work on any plant pathogen, there are certain steps you want to take to control that disease. The very best in any situation is resistant host gene types.”
Biological control such as modifying cultural practices is another option, and the “big gun” is fumigation.
How growers handle trees once they receive them for planting from the nursery can also impact the potential for problems such as crown gall disease. Research looking into the effect of leaving trees from one to three days under a tarp before planting showed the mortality rate of trees after one year went from basically zero to over 70 percent when inoculated with Agrobacterium tumefaciens. The trees were subjected to a very high rate of inoculum which could explain the very high mortality, but researchers think that time under tarps in the field plays a fairly significant role in the incidence of crown gall disease.
Temperatures under a tarp consistently reach about 130 degrees, regardless of ambient temperatures, he says. “That in itself is not necessarily a good thing.”
Results of the trial showed that if a tree was planted immediately, there was virtually no crown gall disease.
Using detergents on seedlings is another option and may help manage crown gall. Questions have been raised about how those types of products affect the health of the plant, but researchers have not seen much negative impact so far.
Yet another option is K84 which works by producing antibiotics that kill the virulent forms of Agrobacterium tumefaciens. It's a numbers game. You've got to get enough out there to do some good, according to Kluepfel.
“Some growers swear by it,” he says. “Others see little benefit.”
Part of the issue could be the diversity of strains that cause crown gall tumors. Lab assays have shown that isolates collected from different areas of the state are genetically different and therefore respond differently to K84.
“Isolates that we collect up and down the state vary as a function of geography,” he says. “We can show that they are genetically different from other parts of the state.”
If K84 isn't working, a grower could very well have an isolate of the Agrobacterium that is not susceptible to it.
Fumigation is another option however, it is not entirely effective.
“From speaking with a wide variety of growers, I've heard all ends of the spectrum from ‘fumigation doesn't work worth a darn’ to ‘I swear by it’,” Kluepfel says.
In vitro research has show varying results on the pathogen, while field research is still being conducted to find the best option.
“Methyl bromide decreased the population down to zero or at least you can't detect it anymore, whereas Telone does virtually nothing to populations of Agrobacterium,” Kluepfel says. “Methyl bromide knocks everything down.”
However, one problem with using methyl bromide is that it knocks out the beneficial organisms in the soil as well as the pathogens, according to Kluepfel. “All of the fumigants we've tested so far leave a fairly considerable component of the microbial component there,” he says. “It's altered, no doubt. Methyl bromide, however, is far more devastating to the total microbial population in the soil.”
A combination of approaches appears to be the best management strategy at present. Pre-plant fumigation offers some degree of protection depending on the material. Bio-control is also somewhat beneficial depending on the situation. Growers need to be careful handling planting stock as well as carrying out other operations associated with planting.
“Elevated temperatures when planting trees have a big impact — not only on crown root gall, but also on plant mortality,” Kleupfel says. “Avoid mounding soil on newly planted trees. Do not plant too deep. Keep the base/crown of the tree as dry as possible. I've seen so many growers who have irrigation that wets the base of the tree for days or months at a time. That is very conducive to Agrobacterium infection.”
In a new orchard planting, it's important to avoid water filling up sleeves that keeps new tissue wet for extended periods of time. Finally, if galls are present, surgical removal can help improve the situation if the trees are three to eight years old.