Witches' broom is an uncommon fungal disease that 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, instead of the green bark normally found on new blueberry shoots. Witches' broom is most common on blueberries in northern Minnesota planted near fir trees.
Witches' broom is caused by the rust fungus Pucciniastrum goeppertianum, which infects both fir trees and blueberries. In firs, Pucciniastrum primarily grows in the needles, causing yellowing and needle drop. Spores produced on fir needles are carried by wind and can infect nearby blueberry plants. The fungus grows into the bark of the blueberry plants and becomes systemic. The fungus lives in the blueberry plant for many years. Each spring, spores are produced on the witches' broom that can infect balsam fir trees.
Because the fungus that causes witches' broom is both perennial and systemic, infected blueberry plants cannot be cured with fungicides nor can the pathogen be removed by pruning. Infected blueberry plants should be removed or killed with an herbicide.