Witches' Broom

    Witches' Broom

    Arceuthobium spp.; Candidatus phytoplasma; Chrysomyxa spp.

    witches' broom

    Witches' broom on ash caused by phytoplasma (Claudia Nischwitz, Utah State University Extension)

    witches' broom

    Dwarf mistletoe shoots on pine (Brytten Steed, USDA Forest Service, Bugwood.org)

    witches' broom

    Witches' broom caused by eriophyid mites on hackberry (Whitney Cranshaw, Colorado State University, Bugwood.org)

    Hosts, Symptoms & Signs

    • conifers and deciduous trees
    • caused by dwarf mistletoes (Arceuthobium sp.) on conifers or several fungi on pine or deciduous trees; phytoplasma and eriophyid mites on some deciduous trees
    • dense cluster of twigs on one or more branches
    • to determine if the witches’ broom is caused by mistletoe or a fungus, a broom needs to be cut down and inspected for mistletoe shoots; fungi causing witches’ brooms may not produce fruiting structures for identification

    Disease Cycle

    Dwarf Mistletoe

    • perennial, parasitic plants
    • seed is forcefully shot off by the mistletoe, then germinates and colonizes the branch by tapping into the vascular system under the bark
      continues to grow under the bark and after 3 or 4 years sends up shoots through the bark that produce flowers and seeds

    Phytoplasma

    • phytoplasma is introduced into the plant by leafhoppers
    • colonize plant phloem
    • leafhoppers become infected with the phytoplasma by feeding on an infected tree

    IPM Recommendations

    • In many cases, tree removal is the only solution as infected trees provide inoculum to infect healthy nearby trees.
    • For dwarf mistletoes, if only one or two brooms are present, pruning may be an option. However, it takes about 3 to 4 years for mistletoes to emerge and brooms to develop, so asymptomatic branches may already be infected.