If you have any cedar trees on your property, you may have recently noticed several bright orange, slimy balls growing from the branches. These growths are the fruiting bodies of a fungal disease called cedar-apple rust, known to scientists as Gymnosporangium juniper-virginianae.
Cedar-apple rust is one of the most fascinating fungal diseases we encounter. It is interesting because it requires two host plant species, cedar and apple, to fully complete its life cycle each year.
This fungus survives through the winter on cedar trees, having been infected the previous year. Fungal spores from infected apple trees travel to cedar trees by wind and animal movement and infect cedar needles and buds. Small, almost undetectable growths develop on the cedar branches, which enlarge in the spring during moist weather. These galls swell and produce orange jelly-like growths called telial horns. During a very wet spring, like what we are experiencing in 2018, these billiard ball-size galls can seem to cover up cedar trees.
The telial horns, named so because they represent the telial stage of the fungus life cycle, produce spores that are spread by air, water and birds to infect apple and crabapple trees up to several miles away. After infection occurs, symptoms appear as small yellow to orange-color, circular spots on the leaves and fruit. The undersides of the leaf spots exhibit hair-like, raised fruiting bodies, which will send spores out to infect cedar trees. Severe infections of apple or crabapples trees will result in leaves and fruit dropping from the trees.
With any fungal disease, the most important question is how to manage it. Cedar-apple rust rarely causes long-term damage to cedar trees. It is usually not recommended to spray a fungicide on a cedar tree, as most are too big, and the damage is minimal. However, you can lessen the severity of future disease by picking off all of the galls that you can reach. Of course, most people find this a more pleasant task after the gooey orange telial horns have dried up.
Apple trees are a different story. It is important to protect your fruit crop if you expect to enjoy any fruit at harvest time. It is recommended in apple trees that have been previously infected with cedar-apple rust that they be sprayed with myclobutanil, sold under the trade name of Immunox. This chemical can be alternated with Captan for a broader spectrum of fungal control.
Please contact the Dawson County Extension office at (706) 265-2442 with any questions.