by Margie Siegal
I came back from a short trip and did a tour around the back yard. The Gala apple tree was drooping. I knew what it was – oak root fungus. I started soaking the tree with fungicide and clearing out the ground around the roots, but it didn’t work. A few weeks later, most of the leaves had turned brown.
Oak root fungus (more properly known as Armillaria Root Rot), is caused by an organism known as Armillaria mellea. The name, “Oak root fungus” comes from the association of the fungus with native oaks. During wet weather, the fungus grows into honey colored mushrooms. It is deadly to susceptible trees and other plants. Apple trees and avocado trees are often victims, as are grape vines.
I originally thought the fungus was some sort of import, like elm tree disease, but I was wrong. The fungus is, unhappily, a California native. It has existed for centuries along with all the native plants that thrive alongside it. Unfortunately, a lot of fruit trees (which are imports from other places) are susceptible to the fungus. I know of three apple trees in my yard that were killed by the fungus and another one that had been around at least thirty years whose life span was shortened by the fungus. I mourn the apple trees. Friends have lost several trees. Part of the problem is that you don’t know the fungus is there until the tree starts dying. It spreads underground.
Spurred by the desire to grow fruit, I have done research on the fungus, and how to control it. According to the University of California agricultural service, Armillaria can remain alive in pieces of wood, roots and stumps. When trees are planted, if new roots grow into contact with Armillaria-infected roots or infested wood pieces, they can become infected as well. The fungus can only reach a short distance through soil, and spreads through contact with infected roots. It also may grow dark, root-like structures, called rhizomorphs, that will spread the disease.
The Louisiana university extension service states that Armillaria can cause disease on almond, apple, apricot, avocado, citrus, loquat, and walnut trees, as well as blackberries and raspberries. Oak root fungus is also known to infect grape vines. However, in my yard, my forty year old apricot tree seems to be perking along happily, and the blackberry bushes that periodically threaten to overwhelm the area near the back fence are totally unaffected. Since fungal diseases are exacerbated by humidity, it is possible that apricots and blackberries are able to fight off the fungus in our dry climate.
Several experts have stated that over-watering will help the virus spread, as will vegetation around the base of a tree. Keep at least a foot of ground around the bottom of the trunk free of weeds. Don’t water any more than absolutely necessary. Some people have had success in stopping the fungus by digging down to the top of the roots (the root crown) and letting them dry out. This approach is suggested by University Extension for grapevines.
Once the fungus infects a tree, there is very little that can be done. Copper sprays don’t seem to work, and no anti-fungal tree spray has been approved against this fungus. According to an article in SF Gate, sodium tetrathiocarbonate can be effective. This is the first I have heard of an effective chemical solution. Reading up on this chemical, it is a soil fumigant that can produce severe burns- not the sort of thing I want to take chances with. Also, the University of California extension service says that fumigant may be ineffective if diseased roots are left in the soil. A less hazardous (and possibly more effective) approach is isolating the infected area from the fungus. The SF Gate article suggests digging a four foot deep trench around the infected area, lining the trench with plastic and then back-filling the trench with clean dirt. The only problem with this is that you probably don’t know which areas of your yard are infected. I am currently trying a variation of this. I hired someone to dig a three foot deep hole, lined it with hardware cloth (stainless steel mesh) back-filled it with clean dirt, put in another hardware cloth liner, filled the center with more clean dirt, put in a half barrel with the bottom cut out and planted a grapevine in the barrel. Will see if it works. The grape is just waking up from winter dormancy.
Oak root fungus has to be accepted as part of gardening in the East Bay, along with the occasional mild frost, no rain from May to the end of November and aphids. It is a serious disease, but there are steps that can be taken to prevent it.
Photo by Marla Koss