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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