Palm Tree Disease

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Any gardener who vacations in California is usually enthralled by our palm trees, more so when they see them thriving so well in the chilly, clammy climate of the Bay Area’s decidedly less-than-tropical summer. The matched sets of Phoenix canariensis recently transplanted on upper Market Street and on the Embarcadero medians (running all the way, now, to the Caltrain station) have added unimagined elegance to San Francisco thoroughfares. Small wonder the palm is once again becoming an option as a garden tree.

Small palms are easy to find in nurseries that carry a good selection of containerized trees. Larger palms are usually brokered or sold wholesale through landscapers, dug up from old homesteads around the Southwest, then shipped and replanted with their tidy small rootball. That palms are easily transplanted is obvious from the City’s trees: of the hundreds planted in the past four years, only a handful have had to be replaced.

But all this moving of palms appears to have a price. The fly in the ointment, the worm in this apple of apparent paradise, is actually the white, wormy grub of a flying insect pest known as the giant palm borer.

Biologists at the University of Arizona know the most about this pest, although it is native to desert California. According to Dr. David Langston, an entomologist at University of Arizona’s Maricopa campus, the borer is a ticking time bomb for palms. “It’s the mother of all borers,” says Langston. “And you never know you’ve got it, until a strong wind comes up and snaps the tree right in half.”

As the larval stage of a two-inch beetle, the giant palm borer is about the size of your thumb, and will spend anywhere from (ital)three to nine years (endital) hidden within the trunk of a palm tree, chewing miles of tunnel from the woody inside. “Since it doesn’t eat the green growing parts, you have absolutely no clue that it’s inside,” Langston says. “But over time it ruins the structural integrity of the trunk.” Slimmer palms such as washingtonias suffer the most and are most likely to snap when stressed in a winter wind storm. But the borer has also been found in Phoenix palms and Mexican fan palms.

Dr. Langston says Arizona landscapers are still piqued that this California insect crossed state lines in the late 1980′s to become a serious pest there. Once thought to be nearly extinct in its native habitat — the area around Palm Springs — the giant palm borer was hemmed in by colder mountain regions and escaped only as a passenger on palms that had been dug up and transplanted to other parts of the Southwest.

There is no way to tell if your new palm has the grub, unless you find a bit of sawdust-like frass at the point where new fronds start as buds. The only other clue is an 3/4-inch exit hole, made by a grub after it pupates and turns into an adult beetle.

“We tell people who work on cleaning palms that if they see such a hole, they should stop working and get down from the tree immediately,” Langston says. “Because it just isn’t safe.” Insecticides are useless once the tree is infected, but Langston thinks there may be some hope in controlling the adult beetle, which flies at night.

“What we’re advising now is to apply an insecticide during May or June,” he says, “when the beetles are emerging and the females are laying eggs in the green portion of the tree.”

Other than that, keeping palms healthy can mitigate borer damage if there are not too many grubs involved. Warm weather boosts palm growth, so the balmier weather of early fall is a good time to fertilize and irrigate landscape trees in Northern California. Until winter rains arrive, hosing down the fronds of palms helps remove dust and small insects such as spider mites and mealybugs that can weaken new foliage growth.

Note: Entomologists at the University of Arizona are very interested in tracking the spread of the Giant Palm Borer, which mainly infests native California stands of the palm Washingtonia filifera. If you encounter this grub in your palm tree, please contact: Dr. Carl Olson, Dept. Of Entomology, University of Arizona, Tucson, Arizona, 85721.

Common Problems With Palms

Hardy as palms are, there are a few pests that trouble them. Scale insects and mealybugs make an ugly mess when they appear in hordes, as a gray or brown, powdery encrustation on the fronds. By sucking plant juices, they make foliage weak and yellowish; whole fronds may die back. Their insect wastes also become a breeding ground for a sticky, black fungus known as sooty mold. Usually, these can be kept down by natural predators, such as a tiny parasitic wasp that unfortunately is easily killed if you attempt to spray the tree with insecticide. Once in the hard-shell stage, scale insects are impervious to chemical sprays, but scraping their crusty bodies off the leaves may bring some relief. The sooty mold can be washed off with a mild soap and water rinse.

If you notice a gradual yellowing and dieback of palm fronds, look for these insects first. If the fronds are clean but the foliage still looks sickly, the problem is most likely poor irrigation or bad soil. The use of herbicides in other parts of the garden to kill weeds can also affect palms, if the herbicide was applied upwind or uphill from the tree.

Though long lived, palms do die of old age, and a venerable specimen may simply stop producing new fronds at its crown. If a landscape palm is in decline, you might wish to install its replacement — as a younger, less expensive tree some feet away from the trunk of the first. With mature Phoenix palms priced anywhere from $2,000 to $3,500, smaller plants from nursery containers can be a bargain, and you’ll have the added pleasure of watching them grow into stately and valuable property trees.

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