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Witches' Broom

Welcome back to Ask a Naturalist: your own personal “Google” for information on all things natural on PEI!  Recently, we looked at the phenomenon of fasciation (, and many people asked me: “is that also the cause of Witches’ Broom?”.  Witches’ Brooms are in the same general group of plant deformities as fasciation, but I think they deserve a feature of their own.  So, let’s fly into it!

Photo 1: A Witches' Broom on a PEI Balsam Fir tree.

 Most people recognize a Witches’ Broom – that dense cluster of twigs that looks like a bird’s nest or old-fashioned household broom (Photo 1).  Here on PEI, they are usually found on Spruce or Fir, but Witches’ Brooms can occur on a range of species including Birch, Blueberry, Cedar, Lilac, Oak, and Pine among others.  Our Witches’ Brooms are almost always caused by one of two things: Broom Rust fungi or a plant known as Dwarf Mistletoe (Arceuthobium pusillum).


Photo 2: The beginning of a Witches' Broom caused by Broom Rust Fungus on Balsam Fir.

In my area of Central PEI, Fir Broom Rust (Melampsorella caryophyllacearum) is common, and many Balsam Fir trees show the result.  Airborne spores of this fungus infect Fir buds in early spring, causing them to produce abnormal upright shoots (Photo 2).  As the shoots grow and divide, the fungus produces a new type of spore that attracts insects with a sweet scent. If an insect manages to transport this new spore to another of that type, fertilization occurs and a new, third type of spore is produced.  

Photo 3: The fungus that causes Witches' Brooms in Balsam Fir needs an alternate host from the Chickweed Family to complete its life cycle. It has lots to choose from on PEI, including Mouse-eared Chickweed shown here.

This third spore can’t infect Fir trees; it needs to find an alternate host from the Chickweed Family (Caryophyllaceae; the rust’s long species name is no coincidence!).  Here on the Island, there’s lots for it to choose from, including Common Chickweed, Mouse-eared Chickweed (Cerastium arvense, Photo 3), and several species of Starworts, Stitchworts, and Spurreys.  If the rust successfully infects a Chickweed, it produces a fourth type of spore that allows it to spread to other Chickweeds. There it overwinters and re-emerges in spring to produce a fifth – and final – spore type that can infect the Fir and start the cycle over again.  How’s that for a complicated life cycle?


Over the summer, the pale green needles on infected shoots turn yellow and eventually drop off.  That’s when we usually notice the brown Witches’ Broom among the otherwise green foliage of the conifer.  In most cases, the Broom is not dead but just dormant for the winter and will sprout infected needles again in spring.  (The main body of the fungus lives mostly-unseen inside the tree). 


Spruce Broom Rust (Chrysomyxa arctostaphyli) has a similar life cycle, infecting White and Black Spruce (Picea glauca and P. mariana) with Bearberry (Arctostaphylos uva-ursi) as its alternate host.  Bearberry isn’t all that common on the Island, and the Witches’ Brooms I see on Spruce are more often the result of a plant: the partially parasitic Dwarf Mistletoe. 


Dwarf Mistletoe does make a bit of food from sunlight but is reliant on nutrients it steals from its host.  Trees often produce abnormal growth – including Witches’ Brooms – as a side effect of Dwarf Mistletoe infection.  (And yes, Dwarf Mistletoe is a relative of the famed Christmas Mistletoe, although that’s a European species). Infections from Broom Rust fungi don’t kill the tree but can be problems for commercial operations such as Christmas tree growers; Dwarf Mistletoe is more often fatal.


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