See:
Blackberry (Rubus spp.) - Armillaria Root Rot
Note Out of dozens of samples there have been 3 that were diagnosed with Armillaria root rot by the OSU Plant Clinic. Armillaria occurs on the roots of many different plant species. The fungus can live many years as a saprophyte in soil, on infected stumps, roots, and other organic matter. Diagnostic mycelial fans can be observed between the bark and wood of the root crown. Avoid wounding roots and overwatering. Remove severely infected plants and destroy them, being careful to remove small roots from the soil. If practical, do not replant where infected plants have been removed or consider resistant plants.