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Cedar-Apple Rust

Congratulations to Roseanne Saalfield, the first of several readers who correctly identified the Mystery Photo as a stage in the life cycle of Cedar-Apple Rust (Gymnosporangium juniperi-virginianae Schwein). This rust is a member of the family Pucciniaceae, a group of fungi that contains many species that usually require plants from two different families (usually within a mile of each other) in order to complete their life cycle: one plant from the Cupressaceae family (eastern red cedar, juniper) and the other from the Rosaceae family (crabapple, apple, hawthorn, serviceberry).

The fungus assumes very different forms on each host. On rose family plants, the fungus can be present on the leaves (orange spots on the surface of the leaves and tiny projections beneath them) as well as the fruit. On cedars and junipers, brown spherical galls produce orange, fleshy projections.

For those readers who wish to know the fairly involved details of the life cycle of this fungus, read on: This rust produces four kinds of spores: basidiospores, teliospores, spermatia, and aeciospores. Teliospores are produced on orange, gelatinous telial horns (see bottom photo) which originate from hard, brown galls on red cedars or other junipers, usually in the spring when it’s been raining. Teliospores germinate to form basidia. Basidia produce basidiospores that are released into the air, blown two to three miles potentially to an apple or hawthorn leaf or fruit. They germinate and form a yellow or orange spot on the leaf or fruit (see photo). These spots produce spermogonia that in turn produce spermatia. The spermatia are released into a sticky liquid attractive to many insects. As insects carry spermatia from one spot to the next fertilization takes place. The fungus grows on the fruit or through the leaf and produce aecia on the underside of the leaf (see photo). The aecia produce aeciospores that are windblown back to the red cedars. They then germinate and start the formation of galls that in the following year will produce telial horns to start the process over again. (U.S. Forest Service)

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5 responses

  1. Man that’s some weird looking stuff! Especially the last one (the horns) some crazy looking stuff! Thanks! 🙂 ❤

    September 24, 2021 at 8:35 am

  2. Alice

    Complicated! I’ve seen the ‘horns’ many times. Looks very slimy.

    September 24, 2021 at 8:38 am

  3. For the record, cedar-apple rust doesn’t help the apple fruit, either! In our “wild apple” orchard – trimming away at the abundant red cedar trees (naturally coming in as early succession) makes only a dent in the problem…

    September 24, 2021 at 8:52 am

  4. Bill on the Hill...

    Fascinating Mary… Quite the process this stull goes through only to repeat itself for the next growing season, i.e. apple growers… Imagine having over a 1000 trees in a orchard & having to deal with this problem…This is a classic example on how difficult it must be to be an organic apple farmer… katharinepreston alludes to this in her comment…
    Bill… :~)

    September 24, 2021 at 9:37 am

  5. Mary, This 2-host progression is interesting. We have seen witch’s broom on our blueberry bushes. I read it comes when the bushes are near white pines. I can’t do much about pine trees in Maine. Chickweed is involved, too, so I uproot it from the area like a fiend. It’s a complicated cycle, so I am interested that something similar happens with crabapples. Thanks for lifting it up.

    From Susan Gilpin


    September 24, 2021 at 12:35 pm

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