Tom Volk's Fungus of the Month for May 1999

This month's fungus is Gymnosporangium juniperi-virginianae, the cause of cedar-apple rust.

Gymnosporangium on juniper trees Gynosporangium telial spore horns
This month's fungus with the long name is Gymnosporangium juniperi-virginianae. It's really a pretty easy name to pronounce if you sound it out. (Jim-no-spore-ANGIE-um jew-NIP-per-e-virgin-e-ANE-eye). The name means "naked spore-bearer of the eastern juniper tree." (My apologies to those of you who will not be able to view this page because your filtering software won't allow you to view a web page that contains the word "naked"....). It's a very descriptive, although probably unnecessarily long name.

This is a heteroecious rust, which means it needs two species of plants to complete its life cycle. It is parasitic on both the eastern red cedar (Juniperus virginianus) and apple trees (Malus sylvestris). This fungus is an obligate plant pathogen, which means (essentially) that the fungus cannot be grown in culture in the absences of its hosts. This particular rust has four different spore stages, which are described below. This is a very common fungus to see in the month of May in Wisconsin and many other areas of North America. You can often see the orange teliospore-covered trees from quite a distance. It's really kind of a pretty sight to see, but it does cause considerable damage to the juniper tree, which is not so economically important as the damage it causes to the apple trees. An infestation such as this means there are probably severe problems with nearby apple orchards. Usually the spore horns are not so impressive as this picture-- most often you can only see a few telial spore horns on any one tree. This particular tree was growing right next to an apple orchard, which offered the spores from one tree the easy opportunity to inoculate the other species. But I'm jumping ahead a bit. Lets take a look at the life cycle of this rust.

Gymnosporangium life cycleLet's start at the top of this diagram of the Gymnosporangium juniperi-virginianae life cycle with the telial spore horns, which are the most conspicuous part of the life cycle. (You're going to be glad you paid attention in school about meiosis and life cycles when you read the rest of this paragraph.) Each gelatinous spore horn is actually composed of hundreds of two-celled teliospores, each of which is dikaryotic (n+n), containing two haploid (n) nuclei from different parents. I'll explain later where the two nuclei came from. Each teliospore usually germinates right there in the spore horn to form a basidium. This germination is shown in the right side of the life cycle diagram. In the basidium the two nuclei fuse to form a transient diploid nucleus (2n), which almost immediately undergoes meiosis to form 4 haploid (n) basidiospores. The basidiospores are shed into the air. By chance, some of them land on an apple tree. On the apple tree the basidiospores germinate to form spermatia (also called pycnidia). These spermatia (not shown) act as sex cells for the fungus and fuse to form a dikaryotic (n+n) mycelium, which begins to grow in the leaf. Eventually it reaches the underside of the leaf and forms aecia, shown in the left bottom corner. These aecia form dikaryotic (n+n) aeciospores, which are shed into the air. By chance some of them land on a juniper tree. Of course there is a better chance for this to occur if the juniper tree is close by. The aeciospores germinate and form a limited dikaryotic mycelium. Round brown galls form and in the spring the teliospores (n+n) emerge in the gelatinous spore horns to continue the cycle. Did I lose you in this explanation? I hope not!

So if you had an apple orchard, how could you prevent infection of your apple trees by this fungus? Certainly you would want to eradicate as many nearby junipers as you could. This would break the cycle-- only the basidiospores produced on the juniper can infect the apple. Neither of the spores produced on the apple can re-infect the apple; the aeciospores can only infect the juniper.

Puccinia graminis on wheatGymnosporangium juniperi-virginianae is a rust, which means it belongs in the class Urediniomycetes in the Basidiomycota. There are a number of other economically important rusts, the most notorious of which is black stem rust of wheat caused by Puccinia graminis (image by Mike Clayton). This is also a heteroecious rust that requires two hosts to complete its life cycle: wheat (or some other grass) and common barberry. This rust has five spore stages!

Another common rust is White Pine Blister Rust, caused by Cronartium ribicola. This heteroecious rust alternates between white pine (Pinus strobus or Pinus monticola) and Ribes species-- gooseberry, currants and their relatives. WPBR has been especially harsh on western white pine. See Jim Worrall's Forest and shade tree pathology web page for his course at SUNY-ESF in Syracuse, New York. He's got quite a bit more information on WPBR and other forest diseases. I highly recommend visiting this page.

May-apple rustThere are also some autoecious rusts, such as May-apple rust, caused by Puccinia podophylli. This rust has only one host, the May-apple, Podophyllum peltatum. You can imagine the advantage of not having to alternate hosts-- the fungus can continue to re-infect nearby May-apples. This rust can be commonly seen in the spring when you're hunting for morels.

Like all rusts Puccinia podophylli is mainly a foliar (leaf) pathogen and does not grow systemically through the plant as smuts do. Smuts (Ustilaginomycetes) such as Ustilago maydis also tend to replace fruits of their host plant with large masses of spores. Smuts also differ in having only two spore stages, teliospores and basidiospores. Many smuts can also be grown easily in culture.

I hope you enjoyed learning something about an interesting and common rust fungus. Rusts are pretty interesting and common parasites of many different species of vascular plants. most of them are very specific to their species of host trees. Be on the lookout for these leaf and stem pathogens. They're pretty interesting under the microscope as well.

If you have anything to add, or if you have corrections or comments, please write to me at

This page and other pages are © Copyright 1999 by Thomas J. Volk, University of Wisconsin-La Crosse.

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