Armillaria Root Rot.
Damage: Infected bushes have small, pale leaves and die within 1-2 years. At the late stages of disease, entire foliage turn brown. Root-like fungal growth (called rhizomorphs),dark brown to black when matured or pale white when young, can be found attached to roots or crown or under the bark. White fungal mats can be seen on the stem underneath the bark when scraped away at the crown just below or at the soil line.
Disease cycle: This fungal disease is common on trees and woody species, and may attack blueberries planted on recently cleared of forest land. Wood bark and other woody plant materials used as planting medium for blueberry propagation or as mulch in the field may harbour the fungus.