- Author: Raymond Mireles
- Author: Mohammad Yaghmour
- Author: Santosh Bhandari
- Author: Elizabeth J Fichtner
Within the past decade a wood decay fungus with no prior record in North America has emerged as a pathogen of almond, prune, and peach (rootstock) in the San Joaquin Valley. Since 2016, Ganoderma adspersum, has been associated with decay symptoms on almonds in Kings, Tulare, Kern, and Madera counties. Unlike other endemic species of Ganoderma that were previously recognized in California almond orchards, G. adspersum appears to be aggressive on young trees, particularly those on ‘Nemaguard' rootstock. Additionally, G. adsperum infection appears to be prevalent in orchards with high incidence of crown gall caused by Agrobacterium tumefaciens, leading plant pathologists to suspect that the bacterial disease may predispose infected almond trees to the decay fungus. After a recent farm call with almond growers and crop consultants, UCCE Advisors Mohammad Yaghmour, Elizabeth Fichtner, and Raymond Mireles have initiated a series of studies to further investigate the potential relationship between crown gall and G. adspersum in the field, as well as evaluate techniques to limit the spread of G. adspersum in orchards.
Ganoderma adspersum is a wood decay fungus that infects the roots and butts of trees causing white rot and leading to tree blow-over and mortality especially after windy and rainy storms, hence, it is also called butt rot. Infected trees may appear healthy but are more likely to collapse during storms or harvest activities than uninfected trees.
In February 2024, Yaghmour, Fichtner, and Mireles established new research studies in Tulare County orchards to address the hypothesis that G. adspersum infections are more prevalent on trees infected with A. tumefaciens. With the assistance of Santosh Bhandari, Associate Specialist with UC ANR, over 6,000 trees were surveyed across two orchards and rated for incidence of each disease. Both orchards were on ‘Nemaguard' rootstock and included ‘Nonpareil,' ‘Monterey,' and ‘Fritz' varieties. Results of statistical analyses indicate a significant association between G. adspersum and crown gall infection on all varieties in both orchards. For each incident of crown gall, the probability of infection with G. adspersum increased by over 80-fold across all varieties in both orchards.
Studies conducted by Daisy Hernandez, a PhD student in the Department of Plant Pathology at UC Davis, suggest that stem infections with crown gall do not alter the tree's susceptibility to future infection with G. adspersum at another location on the stem. This work suggests that the association of the two diseases may not be related to a physiological change in the plant resulting from prior infection with A. tumefaciens; however, it does not rule out other mechanisms of predisposition. Mireles, Yaghmour, and Fichtner are interested in studying whether the crown gall itself presents a unique infection court (ie. opening) that facilitates infection with windblown spores of G. adspersum that may encounter the gall years after crown gall development.
In 2024, new studies will be initiated to evaluate the potential benefit of phosphites (ie. K-Phite®) in limiting the progression of new infections of G. adspersum in affected almond orchards. Phosphites offer a cost-effective approach at managing several plant diseases, including Phytophthora on almonds. Phosphites are known to induce plant defense responses to disease and are often better at preventing than curing disease. Additionally, some growers have expressed interest in using conk-removal as a strategy for reducing G. adspersum inoculum in orchards. UCCE Advisors plan to evaluate the influence of phosphite treatment on the rate of fruiting body regrowth with the anticipation of identifying multiple techniques that growers may employ to mitigate the economic effects of these diseases in California orchards.
- Author: Jeannette E. Warnert
An airborne fungus from Europe, ganoderma adspersum, has been killing almond trees in the San Jaoquin Valley since it was discovered in the area five years ago, reported John Cox in the Bakersfield Californian.
The fungus rots wood from the inside out, usually weakening the trunk a ground level.
Three kinds of ganoderma fungus infections were identified recently in California almond orchards; University of California researchers say 94 percent of the cases were of the adspersum variety.
"We are seeing those trees collapsing at 11, 12, 15 years old,” said UC Cooperative Extension orchard systems advisor Mohammad Yaghmour. The infections have results in the removal of orchards at less than half their typical 20- to 25-year life span.
Spraying for the fungal disease is ineffective. Yaghmour believes that in time researchers will identify a root stock that is resistant to the fungus.
- Author: Jeannette E. Warnert
“Ganoderma's been around for a long time,” said Bob Johnson, a UC Davis graduate student who is leading the study for his doctoral thesis under the direction of UC Davis plant pathologist Dave Rizzo. “The tree failure we're seeing may be that we've now reached such a density of almonds here, that the problem just seems more widespread. Or it may be a new Ganoderma species in our state.”
The UC Cooperative Extension nut crops advisor in Fresno County, Mae Culumber, speculated that air quality regulations prohibiting the burning of orchard prunings may have allowed fungi to grow in slash piles in agricultural areas. However, the cause of the problem is currently unknown.
Ganoderma is a genus of fungi with about 80 known species. It is typically considered a forest pest in the U.S. and, in terms of agriculture, poses problems for the palm tree industry in the tropics. The Ganoderma now being found in California agriculture grows in the living heartwood of almond, peach and other stone fruit trees. The presence of the fungus doesn't appear to impact tree production. The only outward sign is development of rather large shelf-like mushrooms on the trunks called conks.
“Once you see conks on the tree, it is essentially dying from the inside out,” Johnson said. “The conks release trillions of spores which wind and water move through the orchard and to neighboring orchards. It's next to impossible to stop the spread.”
A farmer in Hanford recently pulled out and destroyed every tree in his 120-acre orchard because of Ganoderma infection.
“This was a 9- or 10-year-old orchard, just when the grower starts making money after investing in its establishment,” Johnson said. “Instead, his trees were just falling down right and left and he pulled out the orchard.”
Culumber called Johnson out to a Fresno County orchard where the farmer is starting to lose trees.
“In his orchard, every fifth tree has a massively sporulating conk,” Johnson said. “Growers see the conks, but they don't realize they are the infectious part of Ganoderma fungi.”
Culumber said other orchards in the area also have active conks.
“I know of at least two locations within a square mile,” she said. “One is very progressed, and I talked with another grower with early symptoms.”
Johnson is calling on farmers to contact him if their trees have fallen over due to decay in the trunk or if they have seen conks on tree trunks.
“We need to understand the distribution and incidence of Ganoderma infection in order to develop management strategies that will limit the impact of this disease,” Johnson said.
To report trees potentially infected with Ganoderma, contact Johnson at (530) 302-6301 or bobjohnson@ucdavis.edu.