As one takes a walk through the woods in spring, they may notice something unusual going on with Eastern redcedar trees (Juniperus virginiana); a brown ball covered with long orange jellylike protrusions. Is this a sign of alien life, an unusual insect nest, or some strange fruit? In fact, it is none of the above, what you are seeing is just one stage in the life cycle of the fungus that causes cedar-apple rust (Gymnosporangium juniperi-virginianae), an interesting plant pathogen that requires 2 host species.
The galls that are produced look unearthly, but while these growths may look alien, they are a naturally occurring fungus native to North America. Hosted on both the Rosaceae (apples) and Cupressaceae (cedars and other junipers) families of plants, the fungus requires both hosts to be present in an area to complete its life cycle. Other hosts of cedar-apple rust can be hawthorn trees and flowering quince bushes. Most commonly found in landscapes that vary between forested and agricultural land, the fungus can be quite common and widespread in both rural and urban landscapes.
Cedar-apple rust requires multiple hosts for the fungus to complete its life cycle. Asking which host begins the cycle is akin to asking, “Which came first, the chicken or the egg?”. For the purpose of this fact sheet, we will begin with the fungus on cedar trees.
When a cedar tree is infected by aeciospores (one of four types of spores produced during the fungi’s life cycle), gall formation begins. Oftentimes cedars are infected in the late fall to winter months, and spores overwinter until temperatures begin to rise in spring. Galls grow the most during spring, after warm wet weather in April to May, producing bright orange gelatinous structures called telial horns. These horns, once fully developed, begin to produce teliospores (the second of the four spore types produced). When weather conditions are hot and dry, these horns dehydrate but can become rehydrated and active again after periods of wet weather. These structures are capable of, after rehydration, new spore growth up to seven times a season. Once the teliospores are matured on the telial horns, they germinate into basidia.
Basidia are a secondary growth characteristic found on certain fungi allowing for the production of different types of spores and act as fruiting bodies. These structures produce basidiospores (the third of the spore types), which are adapted to be spread by wind. These spores can drift up to several miles until they land on a member of the Rosaceae family, where the next stage of the life cycle can begin.
As spores land on the tops of leaves of host species, they start to colonize the leaf surface, showing as yellow-orange to red circles with a dark black or brown center. The basidia produce spermogonia, another form of reproductive structure that, in turn, produces spermatia.
Spermatia (the fourth and final type of spore produced by cedar-apple rust) secrete a sticky, sweet fluid that attracts insects, and as the insects feed and move from spot to spot, they carry spermatia stuck to their bodies along with them. This transport allows for fertilization and prompts the rust spots to grow through to the underside of the leaf. Once on the undersides of leaves, the spots produce aecia in the form of small 1/10-inch fungal tubes, which produce aeciospores that are blown by the wind to cedar trees, starting the life cycle again.
Since this fungus can appear on multiple species of plants, the symptoms of this fungal infection can appear in very different forms on different host species. The easiest of these to see are the galls produced on Eastern redcedar trees. Often appearing on branch tips, these galls appear in early spring, generally after periods of wetter weather. Beginning as small brown growths that are covered in ring-shaped indentations, these galls swell (up to 2 inches in diameter) and, once mature enough, begin to produce the bright orange, gelatinous telial horns. Branch tips and twigs that are beyond where the gall is growing can die back to the gall, leading to a browning of the outer foliage that can make infected trees visible from a distance. Oftentimes, heavily infested trees can look ‘decorated’, almost like a Christmas tree.
In hosts in the Rosaceae family, symptoms are often much less noticeable, as they are similar in appearance to other infections and diseases until the rust infection is severe. As mentioned previously, as the spores produced on cedar galls land on leaves of apple and crabapple trees, they begin to colonize the top surface of leaves, showing as yellow-orange to reddish circles, hence the name ‘rust’. These spots can reach up to half an inch and diameter and will merge with other infected spots to form large patches of rust on the surface of leaves. As the infection progresses, spots will also begin to appear on undersides of leaves, showing as discoloration and small reddish less than 1/10-inch fungal tubes. Fruits of infected apple and crabapple trees will become disfigured and can produce structures similar to telial tubes of cedar galls.
Management of Rust
Cedar-apple rust can be extremely damaging to apple trees, if severe enough to cause leaf drop, and disfiguring to cedar trees. While it will generally not cause the death of the trees, repeated infestations can severely weaken trees, lowering the ability to resist damage from other diseases or pests. As well as weakening trees, apples and crabapples that are infected will have extreme trouble producing fruits, and the fruits that are produced are disfigured and inedible.
Several methods of control are available to prevent and stop the spread of this fungus. Cultural control by removal of cedar trees within a 2 to 3-mile radius is often undertaken by commercial orchards; however, this is not a feasible option for most individuals who have apple trees on their property. Pruning of infected branches and leaves on apple and crabapple trees can help to slow the spread of the infection to the rest of the tree and pruning galls in late winter to early spring from cedars will also assist in slowing the spread.
While fungicides can be used to protect trees from cedar-apple rust, there are no treatments available that can be used on trees intended to produce fruit for human consumption. Any use of fungicidal products on trees to prevent or slow an infection should be for plants intended to serve a purely ornamental purpose. Fungicides containing myclobutanil are most effective in preventing rust. Copper and sulfur-based fungicides are also options but are less effective than myclobutanil. Fungicides will only be effective if they are sprayed before leaves become infected and should be applied when flowers and leaves first begin to appear in early spring. Applications can continue until the weather turns consistently hot and dry. Another aspect to consider before using fungicides is that trees in the Rosaceae family are highly favored by pollinating insects and bird species, and fungicide use can have adverse effects on wildlife. If fungicide application is a route that you wish to pursue, be sure to read and follow all label directions. The label is the law!
Since cedar-apple rust can be such an aesthetically damaging infection, growers have developed many resistant cultivars of trees both in the Rosaceae family and Juniperus genus. These cultivated trees can be planted in areas that are prone to having cedar-apple rust problems, and several of these cultivars are resistant to other types of rust diseases. Resistant cultivars are recommended for individual landscape plantings and not for any large-scale growing operations.
Final Thoughts on Cedar-Apple Rust
Gymnosporangium juniperi-virginianae can be a jarring thing to find on one’s trees but will ultimately, with proper care and management, not be a death sentence. Rust diseases can be quite unsightly but are not as harmful as they may appear. This unique life form’s complex life cycle is an incredibly interesting and marvelous thing to witness for its complexity and multiple forms.
Originally published 05/20