Do you have toenail fungus?
If so, you have something in common with California sycamores, avocado trees, date palms, and tomato plants. You see, toenail fungus, as well as certain fungi from which these plants are imperiled, are all identifiable as Fusarium fungi, even if different Fusarium species are involved in the pathology of each case. Since the species differ, you do not need to be concerned if, walking barefoot in your backyard, you stub your toe on a surface root of a sycamore tree: your toenail will not be affected by a sick sycamore anymore than a healthy sycamore would be affected by your infected toenail.
However, when it comes to the spread of Fusarium dieback, a disease that infects more than 100 species of trees, the picture changes. There is a complex of three different Fusarium fungi involved in Fusarium dieback. Moreover, these fungi are not spread on their own but rather by a tiny symbiotic beetle know as polyphagous shot hole beetle, a type of ambrosia beetle, which is no larger than a sesame seed. The beetle carries fungal spores in sacs located in its head as it flies to a new tree for the purpose of boring into it. Upon arrival, it burrows through the bark, creating tunnels or galleries. Within these galleries, it secretes fungal spores into its mouth from where they are removed by mandibles (claw-like appendages extruding from the head) and deposited onto the gallery walls. With an impeccable sense of timing, the beetle lays its eggs just as the fungus starts to grow so that by the time the eggs hatch out into larvae, there will be a ready supply of fungus to eat.
The beetles themselves do not endanger the trees into which they burrow. Unfortunately, the fungus they bring along with them and nurture turns out to be a destructive force. This fungus, actually a complex of three fungi, grows into the vascular tissue of the tree, clogging the xylem vessels, that carry water and minerals up from the roots, and phloem vessels, that carry carbohydrate down from the leaves. Branches may suddenly turn brown and death of the entire tree may follow soon after.
The most famous local case of Fusarium dieback occurred in Long Beach in 2010. An entire city block of box elder (Acer negundo) trees was stricken. The trees died and had to be removed. Unfortunately, there is no cure for Fusarium dieback, to which avocado trees and more than 100 other common ornamental and shade trees are susceptible .
Symptoms include a sugary, crusty white ring around the entry and exit holes of the beetles. The wood surrounding these holes may also appear water soaked or oil stained, accompanied by discolored leaves and wood, leaf wilting and branch death.
The recommended preventive measure regarding this disease is maintenance of the trees’ health. Vigorous trees are the least susceptible to attack and even if beetles do start to tunnel, sap will quickly fill the tunnels and stop the beetle-fungus festivities before they start.
Note: Two weeks ago, I recommended trailing lantana as a lawn substitute. Connie Vines emailed to inform me that lantana is toxic to both pets and people. “I have an 8 pound dog and young grandchildren,” Vines wrote, “both of which are guilty of chewing on all sorts of items.” Under such circumstances, you might want to put a fence around your lantana if you decide to plant it after all.
Tip of the Week: If you are a gardener, there is nothing like the discovery of a new plant to put a spring in your step. Just the other day, I bumped up against a hibiscus species (Hibiscus sabdariffa) that I had never seen before. Not only does this hibiscus, native to West Africa and known commonly as roselle, have the most unusual looking fruits, resembling large, scarlet Hershey kisses, but the plant grows with the greatest of ease from both seeds and cuttings. The red calyces of the fruit and its seeds, as well as the leaves, are edible.