Cedar-apple Rust

Cedar-apple Rust (Gymnosporangium juniper-virginianae), a parasitic fungus on

 Eastern Red Cedar (Juniperus virginiana). This wood gall produces orange, gelatinous horns

that produce spores of the fungus.

Walk by a Eastern Red Cedar (Juniperus virginiana) in the spring and you may see something weird.  Looking like an extraterrestrial invader among the cedar needles you could see gelatinous orange horns protruding from a small wooden ball.  This is one of the life stages of, not an alien, but a fungus, Cedar-apple Rust (Gymnosporangium juniper-virginianae).   

As the name suggests this fungus infects two different plant species to complete its reproductive cycle.  Some parasites, like the Small Pox Virus, infect only one host, humans.  An infected person would be the host for the virus and could pass the Small Pox Virus to other people.  Because it has a simple reproductive cycle and humans were the only host, a worldwide vaccination campaign has made this once deadly viral disease extinct in the wild.  Other parasites have a more complex life cycle.  The malaria parasite, the protozoan Plasmodium sp., requires two alternate hosts, a mosquito and a warm-blooded animal (like us) to go through its reproductive cycle.  Parasites with complex life cycles are harder to combat.  As its name suggests Cedar-apple Rust needs both a cedar tree and an apple tree, so its life cycle is more like that of malaria. 

 

Cedar-apple Rust on Eastern Red Cedar. The orange
horns are producing spores.  Rowan County, NC.

Cedar-apple Rust on Eastern Red Cedar. 
The orange horns have tried up on this gall. Rowan County, NC.

Cedar trees can have a long-term infection by the fungus.  Cedar-apple Rust spores, produced from the fungus infecting an apple tree, lands on the cedar.  A spore germinates and enters the stem of the cedar.  There is produces hormones that cause the stem to expand into a wood ball of tissue called a gall.  In early spring, the galls put forth their orange horns that bear spores and released them into the air.  The galls may produces several rounds of gelatinous horns during the spring, usually after rain.  Sometimes you can find the fungus covering a whole stem of a cedar.  Cedar trees may bear multiple Cedar-apple Rust galls.  We have one in our yard that looks like it is covered with Christmas decorations during the spring Cedar-apple Rust season. 

Cedar-apple Rust infecting Eastern Red Cedar stems. 
This rust has not yet induced the tree to make a gall.  Rowan County, NC.

An Eastern Red Cedar decorated with many spore
producing galls of Cedar-apple Rust.  Rowan County, NC.

Some of these spores produced on the cedar trees are lucky enough to land on the bud of an apple tree (Malus spp.) and infect the young leaves.  The Cedar-apple Rust infection on apple makes yellow lesions on the leaves.  During late spring and early summer, the fungus makes spores on the apple leaves that float through the air and can infect other cedar trees.  The spores made on the apple are different from those made on the cedar.  Cedar-apple Rust makes four different spore types during the course of its life cycle, only two of which made it into this blog.  Of course, the different spore types each have different names.  Mercifully, I will spare you these spore names. 

Cedar-apple Rust lesions on the upper surface of
an apple tree (Malus spp.) leaf Mecklenburg County, NC.

Cedar-apple Rust lesions on the lower surface of
an apple leaf.  Mecklenburg County, NC. 

Cedar-apple Rust infections can reduce fruit yield in apple orchards.  Apple growers try to remove cedar trees from near their orchards but it is nearly impossible to take out all the cedars trees in an area.  Most growers spray fungicides on their trees to control Cedar-apple Rust and other fungal diseases.   

We are now in the dog days of summer.  The apple phase of  Cedar-apple Rust is releasing spores. The cedar phase of the fungus is keeping a low profile. The galls on cedar phase will produce their strange orange horns again next spring.  Cedar-apple Rust, with its complex life cycle and bizarre spore bearing structures, is strangely beautiful.   This beauty is appreciated more by naturalists than by the apple growers who struggle to raise their crops in the presence of this fungus.