Last year, Cindy Watter, a UC Master Gardener from Napa County, discovered a sinkhole in her yard while cleaning out an old shed. The cause was traced back to the remnants of a curly willow tree that had been previously infected with Ganoderma lucidum fungus.
Watter recalled noticing unusual shelf-like growths on the trunk of her willow tree years earlier. “I told my husband it was a Ganoderma fungus (Ganoderma lucidum) and it meant the tree was dead,” she said. Despite initial skepticism from her husband due to the presence of leaves on the tree, an arborist confirmed Watter’s assessment and recommended removal.
The tree, estimated to be at least 40 years old, was cut down and removed; however, its trunk and roots remained in the ground. Over time, these decayed further under fungal attack, causing soil structure loss and resulting in the formation of a hole. Watter reflected on best practices: “I should have had the trunk and roots ripped out and hauled to the dump. The detritus from a tree with a fungal infection should go to a landfill, not a compost pile, and the soil should be disinfected, fumigated, solarized or removed.”
Ganoderma is known as both a saprophyte—feeding on dead wood—and as a parasite that can kill living hosts by breaking down cellulose within trees. While this fungus plays an ecological role in forests by decomposing wood and providing habitat for animals, it poses challenges when it infects food crops or landscape trees.
In North America, Ganoderma commonly affects hardwood species such as oaks. In Asia, it has significant economic impact by attacking palm oil trees—a major crop industry.
Watter also identified honey mushrooms (Armillaria mellea) growing from another willow stump using iNaturalist’s plant identification tool. Like Ganoderma, honey mushrooms are generally harmless in wild settings but can damage cultivated plants such as roses and hydrangeas by spreading underground via rhizomes.
When discussing edibility concerns about wild mushrooms found in gardens, Watter advised caution: “Yes [they are edible], if they are boiled before cooking, and no, I am not eating them. Always check and double-check your mushroom identification, consulting several sources.” She also warned that honey mushrooms near certain trees like buckeyes or hemlocks may cause stomach upset if consumed.
To prevent fungal problems in home gardens, experts recommend maintaining plant health through proper fertilization and watering routines; removing dead wood; avoiding root or bark injury; preventing soil compaction; refraining from pruning during wet weather; ensuring good drainage; and practicing overall preventive care rather than relying solely on restoration efforts.
The UC Master Gardeners of Napa County will host several upcoming workshops for local residents interested in rose care (January 10), starting vegetable seedlings (January 10), and winter fruit tree care (January 24). Details about times and locations can be found through their reservation links.
Gardeners seeking advice may contact the Master Gardener Help Desk online or visit their office at University of California Cooperative Extension at 1710 Soscol Avenue in Napa for assistance with garden-related issues.
