The Q&A Archives: Oak Root Fungus

Question: I have a well established privet hedge all around my yard for about 12 years. My privets started to die (leaves start to turn yellow, then brown and then the plant dies) a couple of years ago in two areas. I try to replace the dead ones. They seem to be OK for a few months and then just do the same thing and die. Now it is happening in a third area that is very sunny and the privets have done very well until now. I have since cut back the watering to once a week (winter time). The soil is quite compacted. I have been told that I have oak root fungus. I do not understand why it seems to be spreading in different areas of the yard since oak root furgus spreads by infected roots. I want to make sure that is what I have. Any thoughts on this diagnose? I need to pull out the dead privets and replace with new plants. I need a fairly tall (about 10 feet) hedge. This is to plant against a low wall of about 3.5 feet. I need suggestions for oak root fungus resistant plants for a hedge? What can I do to save the existing privets? What do I need to do to the infected areas before replanting? Thank you very much. This is one of the best web sites that I have seen.

Answer: Armillaria root rot or shoestring root rot is caused by Armillaria (Armillariella) mellea, a common and damaging soilborne fungus. Armillaria is used loosely to refer to a group of about 20 genetically distinct fungal species that can be distinguished most readily using serological techniques. Common names for this group include oak fungus, shoestring root rot, honey mushroom, and honey agaric. The latter two refer to the color of the mushroom fruiting structure of the fungus that can sometimes be seen at the base of infected trees.

This fungus spreads through the soil via shoestring mycelium, which can travel 3-8' from the source each year. This explains why only some of your plants are infected and others are not - at least not at the moment. Since the fungus is soil-borne, you'll want to remove the infected soil and replace it. Otherwise, the mycelium present in the soil after removing the dead privets will manage to infect the roots of the new privets you plant.

It may be possible for you to save your existing privets. If the precise source of infection is known and cannot be removed, it should be possible in some cases to prevent the rhizomorphs from reaching the shrubs to be protected by sinking a sheet of heavy polyethylene vertically into the soil between diseased and healthy plant(s), provided it extends far enough laterally (several feet beyond the outer dripline) and at least a meter (3 feet) into the soil. A suitable deep ditch would have the same effect.

If this is not possible, your only alternative will be to plant immune or resistant plants in the infected area. Boxwood is highly resistant; Yew is moderately resistant to Oak Root Fungus.

Best wishes with your landscape!

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