(Jan. 31, 2020) – How threatening is Ganoderma adspersum to California almond orchards?
This almond tree is one among many in the South San Joaquin Valley that has been impacted by Ganoderma. Photo courtesy of Bob Johnson.
Imagine being able to walk up to a tree and push it over on its side, discovering that a fungus had rotted out the base of the tree. Imagine losing 120 acres of 9- and 10-year-old trees, or losing trees as young as 4 years old.
That is the real threat of Ganoderma adspersum.
At The Almond Conference 2019 in Sacramento, UC Davis graduate student Bob Johnson shared his first-hand experience with the fungus, stating that he has confirmed the presence of Ganoderma adspersum in at least 68 almond orchards in the Southern San Joaquin Valley. Many growers have been forced to remove trees at less than half of their productive lifespans. Based on his research, Johnson said the average age of affected trees is about 14.
Currently, there is no cure.
Johnson, writing in the March 2017 issue of Growing Produce, said most species of wood decay fungi are non-aggressive and opportunistically colonize and decay dead wood. These fungal species pose little threat to the overall productivity and longevity of almond orchards.
Ganoderma, unfortunately, is anything but benign. It has many forms, of which G. adspersum is just one that is relatively new to the United States. It is most likely native to Europe and Asia.
UC Davis professor David Rizzo, who also presented at The Almond Conference, said there are several Ganoderma species in California and that they can be difficult to differentiate. While different species affect hardwoods, conifers and almonds, however, they do have one thing in common.
“They decay the inside of a tree,” Rizzo explained, “so that eventually, it falls over.”
Ganoderma is spread from tree to tree by spores in the air. Typically, it attacks trees that have been wounded by shaking during harvest or as part of winter sanitation.
Infected trees often have large conks at their base, often growing near a V-shaped cleft. The conks can produce 12-to-40 million spores a day, Johnson said, dramatically increasing the odds of widespread infection across multiple acres. Other symptoms of infected trees include a less dense canopy, a later-than-normal bloom, shallow root systems and flat strips on their trunk.
Johnson described a typical Ganoderma outbreak to occur as follows:
- During harvest, shaking may create wounds below the soil line and on the trunk’s base.
- Sweeping spreads spores from conks through the air within and between orchards.
- Post-harvest irrigation and rain then allow spores to filter into the soil.
- During dormancy, winter sanitation shaking reinjures trees, potentially making them even more susceptible to infection.
The experts’ advice to growers about Ganoderma is to avoid machine-shaking their trees any sooner than necessary after a new tree has been planted.
“The longer that you can put off shaking a new, young tree, the better,” Johnson said. “If you infect a little tree, the disease has a head start. A bigger tree has a better chance. A young tree might fall down at 9 or 10 years old. An older tree might last 20 years.”
Rizzo, Johnson and their associates at UC Davis continue to research ways to detect Ganoderma infections before the removal of complete orchards is required, but the short-term outlook is not good for affected growers.
“It is a destructive pathogen,” Johnson said.
Stay tuned for future issues of “In the Orchard” to receive updates on how research is discovering methods to combat this dangerous wood decay fungi.
