From your picture and description we believe this to be witches' broom. Witches' broom is caused by a rust fungus that infects both blueberry bushes and fir trees. This disease causes clusters of small branches known as witches' brooms to form at the base of the plant. Branches with witches' broom have small leaves and spongy, reddish bark.

At this time, there are no effective fungicides for managing witches’ broom. And highbush blueberries (V. corymbosum) are most susceptible...

But the good news is that there are methods to manage it and still enjoy a blueberry harvest. Because this disease is systemic, i.e. it moves and spreads internally through the plant, burning or cutting out the brooms will not provide control. However, experience in Maine has shown that the symptoms can be managed with judicious pruning, allowing infected plants to remain productive, often for many years, in spite of the disease. It is suggested that broom growth be pruned out as soon as it appears and that the entire branch or cane on which the broom appears, be removed to slow the spread of the fungus through the plant. Once the infection has developed to the extent that heavy broom growth is emerging from the base of the plant and no healthy new canes are emerging, it is best to remove the plant.

Do you have any balsam fir trees within the vicinity of your blueberries? If yes, removing any and all balsam fir trees within a quarter-mile of blueberry plants can reduce or even eliminate witches’ broom. Even if it is not possible to clear balsam to that distance, it is best to keep blueberries and balsam separated to the greatest extent possible.