Arceuthobium spp.; Candidatus phytoplasma; Chrysomyxa spp.
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
- 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 is introduced into the plant by leafhoppers
- colonize plant phloem
- leafhoppers become infected with the phytoplasma by feeding on an infected tree
- 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.