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Rust in the Forest – complicated, but at times sweet

Little yellow fir tree on big green fir tree? First I spotted it—dense yellow growth on a subalpine fir (Abies lasiocarpa). Then I was hit by a powerfully sweet odor reminiscent of evening primroses opening at dusk. Up close, it was truly nauseating! The similarity of the deformity to the tree suggested a pathogen had modified its growth. But why the sickeningly sweet smell?
At home it didn’t take long to learn more. This is a common fungal pathogen of North American true firs—the fir broom rust, Melampsorella caryophyllacearum. Like all rusts, it’s an obligate parasite, unable to live on its own. It causes the fir to produce a dense growth of small shoots, a witch’s broom. Why? For reproduction, of course!
A simple answer, yes, but this is where things get really complicated. Dense small needles on dense small shoots make a witch's broom. Maybe you're like me, with vague memories from your college days of the notoriously complex life cycle of rusts. The fir broom rust fully lives up to this reputation, as it is macrocylic, passing through five different stages, each with a specific type of spore and reproductive structure. Furthermore, fir broom rust is heteroecious—requiring two different plant hosts. Can this really work?!
There’s no need to learn a lot of intimidating terms to appreciate the amazing life of a rust. Just being exposed to its convoluted but successful reproductive strategy—to understand it in a general way—is worth the time and effort. Let’s start at a convenient point in the life cycle. Life of fir broom rust simplified (spore names added for interest). A broom rust spore is carried by the wind (1. in diagram above) to a lower branch of a fir tree. If it happens to land on the growing tissue of a recently-opened bud, it produces a tiny tube that enters the host and causes growth of short disfigured shoots—the broom.
From the needles of the broom grow fungal reproductive structures of two types. The first produces sexual spores (2. in diagram above) in droplets with the distinctive too-sweet odor. This attracts insects, some of which might manage to carry a sexual spore to meet another of its kind for fertilization. If successful, a third type of reproductive structure is produced (photos below), releasing orange spores to be carried off by the wind (3. in diagram). Infected, sickeningly sweet needles. Ruptured aecia (fruiting bodies) of fir broom rust on needles; Bruce Watt photo. This is the part that amazes me. These spores CANNOT infect fir trees; an alternate host is required. In the case of fir broom rust, perennial chickweeds serve as alternate hosts. They are members of  the pink family, Caryophyllaceae, which explains the second part of the rust’s scientific name.
If one of these wind-borne spores happens to land beneath a chickweed AND conditions are suitably moist, infection will occur. After a few weeks, reproductive structures of yet another type grow on the stems, leaves and flowers of the chickweed (photo below). These produce spores that can infect other chickweeds (the spreading infection at 4. in diagram). And so it goes through the end of the summer. Fir broom rust on mouse-ear chickweed; US Forest Service RM Res. Stn. The rust persists through the winter in the base of the chickweed plant; in spring, it infects new growth. But this time a different kind of spore (5. in diagram) is produced from a different kind of reproductive structure, and only on the lower leaves of the chickweed. These spores then divide to produce another kind of spore (actually, a familiar one). Millions of these fly with the wind to maybe land on … can you guess? … a fir branch! We’re back to 1. in the diagram.
It spite of massive numbers of spores, it seems the chance of success in such a life cycle is slim. Though a single broom produces millions of spores in a season, rate of chickweed infection is quite low. An infected chickweed in turn produces millions of spores, but rate of fir infection is equally low (except in monocultures such as Christmas tree farms). Yet the fir broom rust carries on. That’s optimism! Brooms typically are on a low branch (arrow). Field assistant for scale. | Impressum