7/1/2013 - By Marshall Horsman

This month I wanted to look at a very unique disease called Cedar-Apple Rust. I must warn you, this disease is very complex and takes 2 years to complete its lifecycle, so bear with me, you’re about to see a lot of big words!

This strain of rust is in the gymnosporangium family of fungi and is one of the most complex when it comes to lifecycle completion. As you can tell by the name the disease requires 2 different host plants, the cedar and the apple tree to complete its lifecycle. Spores from this pathogen can travel a couple of miles when wind blown, so having these two types of plants in the same vicinity is key to allowing the disease to prosper. This is why most apple orchards will cut down cedar trees and Junipers around their properties.

This disease, like many other foliar diseases, needs the presence of water and high humidity to release its spores. It thrives in temperatures in the mid 60s but can be successful at temperatures from the mid 40s to the mid 70s. The disease starts on the apple tree, crab apples and even quince act as host plants too!

On the upper leaf surface and the fruit lie dormant basidiospores from the cedar tree. As the weather warms in the springtime small yellow rust spots form on the upper leaf surface of susceptible host plants, and soon small black specks or “pimples” will form inside these necrotic leafspots. These structures are called spermogonia and produce a sweet sticky substance called spermatia….I know what you are all thinking, get your mind out of the gutter! This substance helps attract insects which help to move the fungus from one leafspot to the next, which will fertilize the fungus.

Photo of CAR leafspot – photo courtesy of : http://www.caf.wvu.edu/kearneysville/disease_descriptions/omcar.html

CAR on apple fruit – photo courtesy of: http://www.caf.wvu.edu/kearneysville/disease_descriptions/omcar.html

Once fertilization is complete, the fungus grows through the leaf structure and small reproductive fruiting bodies will start to appear on the underside of the leaves. These structures are called Aecia, which produce…you guessed it, aeciaspores. These spores are wind-blown back to the cedar tree where they start their lifecycle on the second host plant. Once these aeciaspores land on the cedar or Juniper host they infect the needles. Most of the time needles past the infection point will die, which can cause some branch dieback. Initially the pathogen starts to produce a jelly like substance. 

Small jelly like mass, infection site second year on Cedar Host – photo courtesy of Matthew Shultz 

After the infection has occurred the pathogen will start to form a brown to green colored gall on the branch the following year. These galls usually grow to about the size of a golf ball but I have seen them as large as a softball in Maryland. The galls take on a smooth appearance and usually look like little brown brains. This hard gall serves as the overwintering structure on the cedar. 

Overwintering CAR gall on Cedar host – photo courtesy of Peggy Sellers Purdue U http://www.ppdl.org/dd/images/rust_cedar_apple.jpeg

Once the weather starts to warm the following year and with the spring rains, the gall starts to come to life. The bumps on the galls start to swell and then they open, and out emerges an alien looking structure.

CAR gall just as it starts to swell in the second spring – photo courtesy of Matthew Shultz 

Long orange jelly like tendrils emerge from the holes in the gall, and these reproductive structures contain teliospores. These 2 spore teliospores emerge as 4 spore basidiospores. These basidiospores then blow and infect the apple or quince host. Wow…told you that was complex! 

CAR “telial horns” often called cedar apples – photo courtesy of Matthew Shultz 

Once the gall is done releasing its spores it will usually dry up and may fall off the plant. 

Exhausted CAR gall on Cedar host – photo courtesy of Matthew Shultz 

As far as control goes, like I mentioned above, removing cedar and juniper plants from around nurseries and orchards can help. If you can’t remove the tree then remove the galls when you see them present to cut down on pathogen inoculum. Bury or burn the galls. Fungicides like Daconil, Banner Maxx, Protect DF, T-methyl and Eagle do well against rust diseases.

Material adapted from http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-1036/EPP-7611web.pdf