Researchers target fungus that attacks tree fruit, nuts

Pear growers may be one step closer to having an answer for a widespread fungus that grows and survives for decades in wooded soils while waiting to attack tree fruit and nut crops with devastating impact.

Armillaria, commonly known as oak root fungus, is a native pathogen that survives invisibly on dead wood and root material deep beneath the ground until new trees are planted, and then it can spread, causing wilt and decline.

A recent University of California, Davis, study has developed protocols that will let researchers and plant breeders more quickly detect whether a pear rootstock is resistant to the most lethal strain of armillaria, at least in the lab.

"There is no pear rootstock known to be resistant to armillaria," said Carolina Tweedy, the UC Davis graduate student who adapted an in vitro technique for screening armillaria resistance in pear rootstocks. "This limits options for replanting trees in infected orchards. Due to the broad host range of armillaria, other tree crops face a similar challenge. In order to identify sources of resistance to be utilized in rootstock breeding programs, a rapid and reliable screening technique for pears is necessary."

Many pear varieties were considered resistant or at least tolerant of armillaria, but changes in irrigation practices may be creating conditions that encourage the disease.

The switch from flood or furrow irrigation to sprinklers that keep the top of the soil and upper roots moist more often may be the reason armillaria has become more of an issue in pears in recent years.

"Armillaria root and crown rot most often occurs when the soil is cool and moist," according to UC pear pest management guidelines. "Recent infections of armillaria root rot in pear orchards may have been caused by a switch in irrigation practices from flood or furrow irrigation to permanent under-tree sprinklers where water is applied more frequently and the soil layer close to the upper roots and tree crown is kept fairly moist through the entire growing season.

"Most infected orchards have been found in the North Coast with fewer in the Sacramento Delta," the guidelines added. "Most affected orchards are on sandy loam to loam soils along streams or rivers originally lined with native oaks."

The disease can show itself as a gradual decline of trees in a circle extending out from the center or may result in the sudden collapse and death of trees that appeared for years to be healthy.

The native oak root fungus pathogen is already in much of California's soil, even before susceptible tree fruit or nut crops are planted.

"If an orchard is planted on a site where oak, black walnut or willow trees once grew, chances are very high that the inoculum already exists in the soil on old decaying roots," according to the UC guidelines.

Once an orchard is infested with armillaria, farmers can limit the spread by machinery or slow the damage through irrigation management, but there is no reliable way to rid ground of the pathogens.

UC Cooperative Extension Lake County farm advisor Rachel Elkins has conducted pear rootstock field trials of armillaria resistance but has yet to find the silver bullet. It can take years to evaluate rootstock resistance to armillaria in greenhouse trials with plants grown in soil. And the results may not always be consistent and reliable.

But Tweedy developed an in vitro screen to identify pear armillaria resistance in six weeks, adapting a technique used first for grape rootstocks and later for walnuts and almonds by U.S. Department of Agriculture plant pathologist Kendra Baumgartner, who runs a research program on sustainable viticulture.

The plants are micro-propagated, rooted in vitro and inoculated with the disease pathogen on a tissue culture rather than in soil.

Using this technique, Tweedy was able to distinguish two pear rootstocks that are already in production as more resistant to armillaria, at least under laboratory conditions.

"From my findings, there were two rootstocks that showed greater resistance that were already available to growers," Tweedy said. "OHxF 87 and OHxF 97 are the two commercial rootstocks that had great resistance to armillaria and also fire blight."

There is a difference, however, between rating the development of the disease over six weeks in a laboratory and seeing how those rootstocks would hold up in infested soil over years or even decades.

"My findings were only in the in vitro form, so I do not know how they will hold up in the field setting," Tweedy said.

The process of breeding new rootstocks is painstaking, and it takes years to produce reliably resistant rootstocks.

The in vitro lab test should give researchers clues as to the genetic material that could be used to add armillaria resistance to rootstocks that already have other valued characteristics.

But it takes years of field trials under different conditions to confirm that OHxF 87 and OHxF 97, or other rootstocks, have genetic resistance to the disease that will hold up over the long life of a pear orchard.

(Bob Johnson is a reporter in Sacramento. He may be contacted at bjohn11135@gmail.com.)