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Cedar-Apple Rust Galls

Galls are abnormal plant growths caused by various agents including insects, mites, nematodes, fungi, bacteria and viruses.  During the summer spores of a particular fungus cause the formation of brown Cedar-Apple Rust galls (Gymnosporangium juniperivirginianae) on Eastern Red Cedar trees. Members of the fungal family Pucciniaceae are known as rusts because the color of many is orange or reddish at some point in their life cycle.

This fungus requires two hosts, Eastern Red Cedar and primarily apples or crabapples, to complete its life cycle.  The two host trees are usually located within a mile of each other. When the Cedar-Apple Rust galls on cedar trees get wet from spring rains, orange, spore-filled fingers or horns, called telia, emerge from pores in the gall. As the horns absorb water, they become jelly-like and swollen (see inset). When the jelly dries, the spores are carried by the wind to apple trees, where they cause a brownish mottling on apples, referred to as Cedar-Apple Rust, which makes apples difficult for growers to sell, even though it doesn’t affect the flavor or texture of infected apples. The rust produces spores on the underside of apple leaves in late summer, which, if they land on Eastern Red Cedar trees, cause galls to form, thereby continuing the cycle. 

Spores produced on apple trees do not infect apple trees, only cedar; spores produced on cedar trees infect only apple trees. (Photo: Brown winter form of Cedar-Apple Rust gall & (inset) orange spring form of Cedar-Apple Rust gall. Blue “fruit” on Eastern Red Cedar branch is actually a cedar cone.)

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6 responses

  1. Anne Shakespeare

    Do you have a picture of an affected apple, or of the spores on the underside of their leaves? Thanks.

    October 30, 2020 at 8:42 am

  2. Natasha Atkins

    There are quite a few other species of Gymnosporangium that red cedars host, and these infect other apple relatives. Here in Virginia, our serviceberry (Amelanchier) fruit is very susceptible to cedar rust. The trees will set a large fruit crop, but if infected, the fruit will be inedible–a big bummer for the robins, catbirds and cedar waxwings. Other apple relatives such as quince and hawthorn are affected by different fungal species in the same genus.

    October 30, 2020 at 9:02 am

  3. It is bummer when your property exhibits this fungus and it could be the tree interacting with yours is not on your property, which is a real bummer.

    October 30, 2020 at 9:54 am

  4. Cordelia Merritt

    Dear Mary – All your posts are good, of course, but I find this one absolutely fascinating in its details. Thank you so much

    I do hope life at Wake Robin is all you hoped for and more. I would so love to see you BUT I’m not even driving to Shelburne to visit family – ()Jane has come down here a very few times.) I know your visitor protocol is wisely strict.) Anyway, I do think of you often and fondly. Cheers and STAY SAFE! Cordie

    >

    October 30, 2020 at 10:29 am

  5. Well, I’ll be a blue-nosed gopher! Another wonderfully complex and seemingly unlikely interweaving of life cycles! (And is a cedar cone really so blue??!) Thanks, Mary!

    October 30, 2020 at 12:41 pm

  6. Alice

    I’ve seen many of these galls on different cedar trees..and know of the interaction with apple trees. I’m going to look up if it’s harmful to the cedars.

    October 30, 2020 at 1:07 pm

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