Ganoderma fungus: A big threat for Almond trees?


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The term “conked out” has a whole new meaning in almond production.

Conks, the shelf-like mushrooms found growing on the trunks or branches of almond trees and stone fruit, are a sign the tree is infected with Ganoderma or another wood-decaying fungi. The slow-moving fungus degrades the lignin in the tree, leaving it susceptible to being blown over by wind.

“Once you see conks on the tree it is essentially dying from the inside out,” said Bob Johnson, a University of California plant pathology PhD student who has studied wood decay fungi in almonds and prunes for two-and-a-half years.  

He notes that a species of Ganoderma, not previously documented in California, has been implicated in the removal of 120 acres of nine- and 10-year-old almond trees in Kings County. The species was also isolated at two sites in Fresno County.

There are about 80 known species of Ganoderma. Johnson is working to identify the exact species causing the cases of wood decay and tree loss. Unknown is whether the recent findings in almond trees are tied to a new strain or if the fungi is gaining momentum as almond acres increase in California.

Some species of Ganoderma, including lucidum, have medicinal properties.

A blown-over almond tree does not mean it’s due to infection, notes Johnson. Not every tree that blows over is due to decay.

Often there are no outward tree signs until the tree blows down, and decayed roots are revealed as the cause. When a tree develops a conk which can take years after the initial infection, decay inside the tree can already be substantial. In some cases, loss of vigor is observed in trees infected with Ganoderma. Infected trees may still be productive but other stressors can hasten tree death.

Johnson is examining the route of infection and points to root damage as a possible entryway for the fungus. Trees without damage can become infected, he notes, but there may be an unseen injury that opens the way for Ganoderma.

“It is possible that injury to lateral roots during harvest can lead to an infection. There is no evidence that the infection moves from root to root. Our evidence suggests each infection is spore-related,” Johnson said.

Understanding the distribution and incidence of Ganoderma can help develop management strategies to limit the impact of the disease.

Johnson says Ganoderma is more of a rootstock problem than a scion problem. By looking at the decay pattern, there is not much observed above the graft union.

Based on preliminary data, the PhD student says trees on peach rootstocks appear to be affected more by Ganoderma. About 25 percent of peach trees surveyed by other researchers in 1989 had one of four species of Ganoderma.

In almond, Ganoderma was only reported on 3 percent of the surveyed trees which suggests that peach may be more susceptible to infection by Ganoderma than almond. No information is available about the presence of the previously undocumented Ganoderma in peach.

Johnson suggests that growers remove active conks from trees. Live conks are a brilliant white color on the underside and release rust-colored spores which often accumulate on the upper surface of the conk making it appear red.

Mae Culumber, a UCCE tree nut farm advisor in Fresno County, says there is speculation that restrictions on burning orchard prunings may have lead to a build up of fungi inside almond orchards, and possibly spurred the number of infections.

Johnson has found different, less aggressive species of Ganoderma in Solano and San Joaquin counties, yet the distribution and incidence is largely unknown which is part of his research.

In the Kings County where Ganoderma infected an orchard, Johnson says the nine- and 10- year-old trees should have been in prime production  but the trees were falling over due to infection. In Fresno County, there were reports of large conks on nearly every fifth tree in a block. With this many spore-producing conks in an orchard, the inoculum level was high and the fungus was most likely spreading.

Growers may see conks on trees, but may not realize they could be spreading Ganoderma, Culumber said. Owners of two other orchards within a square mile have reported active conks on trees.

For more information on Ganoderma, contact Johnson at

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