Which commonly available specimen plants make the most spectacular instant impact when transplanted into landscapes around Napa Valley? The top two are olive trees and Canary Island date palms.
And which commonly available specimen plants become the most expensive loss when the transplant dies? The same two. Though transplanted olive trees rarely fail, the cost can be thousands of dollars for a single tree. Transplanted palms, too, rarely fail, but the cost of mature palms is greater, and there is a wild card at play, making the risk greater: a fungal vascular wilt disease called Fusarium. It can infect them through contaminated chainsaws and other pruning tools and can even be present in a palm when it arrives from the grower.
Fusarium wilt is insidious. It can lurk indefinitely in the soil, waiting to infect susceptible palm roots. The disease may be present in a palm for some time with no visual symptoms, and symptomatic older fronds can be pruned off at the growing grounds before shipping. I am just saying circumstantial evidence suggests this has happened.
After writing a column on a different species of Fusarium earlier this year ("Signs of a New Disease of Mexican Fan Palms in Napa" March 2025), I had occasion to examine some old, established Canary Island date palms in Napa Valley, showing signs of disease. The visual symptoms of some of the cut fronds were typical of Fusarium wilt, and laboratory analysis confirmed it. How did the disease get there? I do not know.
Fusarium is a death sentence for the infected palm. What is worse is the likelihood that other palms in the landscape could be infected by pruning tools.
Palm expert Mark Robinson reviewed the history of the disease, diagnosis of symptoms, sanitation practices and how the disease spreads. ("Fusarium Wilt of Canary Island Date Palms" 5-15-12). He stated: "I think the main culprit is the contaminated chainsaw."
The common emphasis on pruning tool sanitation is often overstated, but not in this case. Fusarium wilt infections in Canary Island date palms via contaminated tools is for real.
With respect to this palm disease, UC Pest Notes Publication 74148 advises avoiding pruning palms at all, if possible. Other sources advise pruning only to remove dead fronds and dead flower parts. The dead tissues have no active vascular connections that would serve as infection courts. Only-the-dead-parts-pruning is not as neat and pretty as traditional, formal palm pruning, but it is safer with respect to Fusarium wilt. Additionally, retaining even partially green fronds is good for palm health. They cannot store energy the way woody plants do, so they rely on living fronds for energy.
The UC article recommends thoroughly cleaning all tools and disinfecting them "by soaking for 10 minutes in a 1:3 pine oil to water solution." Alternatively, handsaw blades can be effectively sanitized by heating them with a butane torch for at least 10 seconds per side. They admonish that chainsaws are "difficult if not impossible to clean and disinfect adequately."
When a palm dies of Fusarium the disease is likely to remain in the soil and infected roots that will infect the replacement palm. It is unwise to replant with the same palm species.
Palm expert Dr. Don Hodel states this disease is specifically "Fusarium oxysporum f. sp. canariensis" ("f. sp." means "forma specialis) and it only infects Canary Island date palms. But in a field trial it infected California fan palms and Senegal date palms. California fan palms are scarce in Napa Valley, and Senegal date is too frost tender to grow here.
Some parting thoughts on Canary Island date palms:
Do not use chainsaws unless you are absolutely sure the saw has not been exposed to Fusarium.Sanitize other pruning tools with the disinfectant solution or by flaming blades with a butane torch before moving on to the next palm.If you plan to import a mature one, inspect it at the growing grounds before the grower has a chance to clean it up for shipping.When removing a dead or dying Fusarium-infected palm, remove the entire thing, roots and all, and send it to the landfill, not for green waste recycling.Do not replace a Fusarium-infected palm with another of the same species.