July 6, 2019
As UC Marin Master Gardeners, we have an obligation to provide science-based information. Sometimes, though, the science we need is elusive.
Take the strawberry tree, the Arbutus ‘Marina’ or unedo. This colorful evergreen alternative to the difficult-to-grow madrone (Arbutus menziesii) is cited as a durable street or patio tree.
The strawberry tree is summer dry once established, deer-tolerant and has low root damage potential. A backdrop of dark green leaves showcase pendulous clusters of bell-shaped pink and white flowers and edible (although not delicious) little balls of fuzzy red fruit. Smooth mahogany bark peels to reveal a cinnamon-colored trunk or multiple trunks.
Unfortunately, two mysterious problems are targeting this tree: An unnamed insect and an unnamed disease. The bug harms the plants. The disease kills them. Neither is clearly understood. So, there are no solutions. Yet.
Steven Swain, environmental horticulture adviser for Marin and Sonoma counties, is studying both and may be close to a breakthrough.
First, the insect.
In its adult form, it’s likely a tiny moth with a quarter-inch wingspan that may be targeting madrones and large-leaved manzanitas as well as strawberry trees.
Swain posits that the insect’s life cycle works like this: A moth lays eggs on leaves and the resulting 1- to 2-millimeter-long larval caterpillars, too tiny to bite into smooth bark, chew maze-like circles in leaves like a leaf miner until they find a lateral vein. They follow that vein to a mid-vein, on into a petiole (the stalk joining a leaf to a stem), and into the cambium layer of a twig. Everything dies from that cambium dinner to the tip of the branch, and the roots suffer from lack of sugar.
“It doesn’t cause mortality unless the tree is really young and the insect hits all the branches,” Swain says. “But we have seen mature trees disfigured.”Once the tiny caterpillar chews its way out of the cambium, it drops into the soil, forms a cocoon and pupates.
“We usually identify insects by their adult form,” Swain says. “But, I’ve been working on this for five years, and never seen an adult moth. We still don’t know what it’s called. We’ve done DNA work on the larvae, but we don’t have enough data yet to form a conclusion. And, until we can identify the pest, we can’t treat it.”
After a few unsuccessful attempts to catch the elusive adults, Swain has now put an infested plant in his backyard hoping to funnel any caterpillars into the soil, and bag them. If successful, he may collect enough adult moths to identify the insect at last.
The second mysterious problem is the disease that’s killing strawberry trees.
“The symptoms start with lesions forming in the bark near the base of the tree, a foot or two above the soil line,” Swain says. “Sunken, dark cracks that turn charcoal gray. I’ve never seen a tree survive.”
An infected tree cut down and sectioned looks discolored inside. A dark stain extends up from the roots, which leads Swain to think the disease is soil borne. The difficulty in identifying root pathogens, though, is in getting infected wood to a lab before the organism dies.
“We had a little success recently,” Swain says. “We drove a root ball straight to Sacramento, and got some candidate diseases.”
To successfully identify the cause, though, researchers must isolate the organism from the diseased host, grow it in pure culture, inoculate it into a healthy host, and reproduce the disease. That hasn’t happened.
“We’ve been trying for five years,” Swain says.
So, you can help.
“If your strawberry tree has this disease, please call the Cooperative Extension at 415-473-4204,” Swain says. “We can’t save it, but if you let us dig up the root ball and send it in for research, we might pin down what it is.”
When science-based advice is not yet available, it’s good to know researchers are looking for the answers.