Juniper hawthorn rust

The juniper hawthorn rusts are amazing organisms that cause many unsightly growths on both juniper and hawthorn plants, saskatoons and sometimes others, but they are not really dangerous unless you are trying to raise saskatoons commercially. In fact, their life cycle is intriguing and makes mammalian biology look simple.

The juniper hawthorn rusts are amazing organisms that cause many unsightly growths on both juniper and hawthorn plants, saskatoons and sometimes others, but they are not really dangerous unless you are trying to raise saskatoons commercially. In fact, their life cycle is intriguing and makes mammalian biology look simple.

Imagine your haploid cells floating around on the breeze and being required to spend some time with a completely different species to complete your life cycle. That’s what is happening here with alternative host rusts.

Rust fungi have a lot of interesting technical names for their various sexual stages and I am going to include those for accuracy, and hopefully to pique your curiosity about this fascinating life form and its cycle through the year.

Persistent woody galls on junipers, usually varieties of Juniperus scopulorum and horizontalis produce teliospores in June. These resemble gelatinous little fingers, usually brown and slimy.    By early summer the spores germinate and produce basidiospores which infect the rosaceous host. These plants are from the rose family and can include saskatoons, hawthorns, pear, mountain ash, apple and crabapple. On the rosaceous host look for yellow to orange spots on the leaves, which later form little spiky galls. As the basidiospores mature they produce spermogonia and aecia. It is the aeciospores that are responsible for infection on the junipers, completing our yearly cycle.

What to do? Seriously, there is not much you can do. Removing all the junipers in your garden would have little effect; there are described instances of infections occurring from 24 km, so that is not going to work. If you have a particularly susceptible plant and it troubles you, remove it. Better still, learn how to keep your plants truly healthy, water properly, accept some fungal damage as part of the game and take on other problems where you stand a better chance of winning.

Of the dozens of Gymnosporangium species in North America the two that concern us are G. clavariiforme and G. globosum. Their similar life cycles are what I have described above.

(Photo credit: “Juniper Rust Fungus (Gymnosporangium clavariiforme) (44489720891)” by Bernard DUPONT, licensed under CC BY-SA 2.0.)

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