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Serviceberry rust - Gymnosporangium spp.

Serviceberry rust

Good evening, friends,

This week we’re looking at a rust fungus in the genus Gymnosporangium. It is one of likely two species, either the cedar-quince rust (G. clavipes) or the cedar-hawthorn rust (G. globosum), but for all intents and purposes it might as well be the serviceberry rust because that’s where I see this fungus without fail.

Serviceberry (Amelanchier spp.), also known as juneberry or shadbush, are fruiting up at the preserve and the cedar waxwings, rust fungi, and myself are all quite enjoying these delectable little treats. The rains have been plentiful but I haven’t seen the fungal explosion that I’ve been anticipating, so let’s learn about these peculiar orange orbs while we wait for the mycorrhizal fungi to drink up all this water.

Serviceberry rust

Before delving into the fungus, the host tree (the deciduous host tree to be precise since there are two host trees) has some interesting lore surrounding a couple of the common names. Serviceberry, which is the name I use, grow throughout the northeast and across the northern hemisphere. The name “serviceberry” is thought to originate in colonial times when the arrival of the tree’s flowers in May indicated the ground had thawed enough to have a proper service (dig a grave) for folks that died over the winter. Not as fun anymore, huh.

Another name, shadbush, also has phenological (relating to the seasonal time of year) origins. The American Shad (Alosa sapidissima), swims up freshwater rivers on the Atlantic coast of North America to spawn each spring. The flowers on the tree may have been used to indicate when the shad, desirable to eat, would be available in the local river. Lastly, the name juneberry is straightforward enough since the edible berries arrive in June.

Serviceberry (Amelanchier canadensis) bearing fruit. Leaves are ovate to lanceolate in shape and serrated.

Now onto the fungus. Those little orange growths erupting out of the green serviceberry below are the fruiting bodies of a Gymnosporangium fungus during one stage of life. This rust fungus has a lifecycle very similar to the Juniper-apple rust in that it requires two host trees to complete its life cycle (one host is an evergreen, around here usually the eastern red cedar).

Serviceberry rust
Those orange growths of the fungus are fruiting structures called aecia.

As previously mentioned, there are two Gymnosporangium species in the area that infect serviceberries: G. claviceps, the cedar-quince rust, and G. globosum, the cedar-hawthorn rust. It’s easier to differentiate between the two when they are on their juniper host. G. claviceps causes galls that swell the branches of the evergreen whereas G. globosum creates round, golf ball shaped galls.

In early spring, orange gelatinous “telial horns” form from both galls and release spores (basidiospores) that land on the deciduous host (in our case the serviceberry). The spores infect a variety of trees in the rose family (Roseaceae) - around here those would be hawthorns, serviceberries, and apples/crab apples to name a few. Usually the two host trees are near each other but there has been an instance where the spores traveled fourteen miles between hosts (Reference 4).

Serviceberry rust
Not too much going on inside the serviceberry, but you can see the orange basidiospores on the plate.

Two to three months after those basidiospores land on the serviceberry (later in the spring) the orange aecia begin to develop on the leaves and young fruit of infected plants. The aecia release aeciospores throughout the summer to find a new cedar host. The next spring, a gall begins to form on the new cedar host and by the fall is fully grown. The following spring, telial horns grow, release basidiospores, and the whole 24 month cycle is repeated again. For the most part, the infection is cosmetic and the trees aren’t too afflicted by infection - I’d say only one 1 out of 50 berries were infected.

Serviceberry rust
Half healthy and half infected?

In addition to these fascinating, dual-host parasitic fungi, I’ve found a lot of other interesting life forms the past week. Let’s take a look:

Spicebush Swallowtail caterpillar (Papilio troilus) on a spicebush.
Spicebush Swallowtail caterpillar (Papilio troilus) on a spicebush.

Young buck
A close encounter with a curious, young buck (and a discarded fishing knife?).

A preview of next week’s mushroom, the hemlock varnish shelf (Ganoderma tsugae) or more colloquially known as reishi.

Bear corn
Bear corn (Conophilis americana) is a non-photosynthesizing plant (which is why it isn’t green) that parasitizes oak and beech roots.

The New York Mycological Society is doing their annual Gary Lincoff Memorial Walk in Central Park on 7/2. Unfortunately, I won’t be there, but all the recent rain should ensure that it’ll be a great showing for fungi. One of the club’s most-tenured mycologists, Paul Sadowski, is hosting an introduction to mycology zoom session this Thursday, 6/29 to coincide with the walk. All the walks and lectures for a mere $20 a year, the best educational bang for your buck you can find. More info on the club’s website.

Full moon is Sunday night/Monday morning - the first “Supermoon” of the year,




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