Member of the Juniper family, cedar trees have a clean fragrance that repels insects and makes their lumber useful for lining chests and closets. Highly drought-resistant, cedars grow easily in a wide range of soils. While generally disease-resistant, cedars are susceptible to three forms of fungus, cedar-apple, cedar-hawthorn and cedar-quince rust. Each has a distinctive set of symptoms.
Closely related Gymnosporangium fungi are responsible for the three diseases, according to the University of Rhode Island Landscape Horticulture Program's website. G. juniperi-virginianae is responsible for cedar-apple, G. clavipes, for cedar-quince and G. globosum, for cedar-hawthorn. These fungi attack rose family shrubs and trees as well as cedars.
All three fungi infest eastern red cedar. Apple and crabapple trees are rose family cedar-applerust hosts. Cedar-hawthorn rust affects hawthorns, serviceberry, quince and pear bushes and trees. Nearly 500 rose family plants host cedar-quince rust. Because the fungi spend different parts of their life cycles in cedar and family trees, they must have both to survive.
The rusts infect the leaves of their rose family hosts in the spring. By summer's end, infected leaves have large spots with spore-containing spines. The spines burst, releasing spores to infect cedar trees in the fall, said Iowa State University Extension plant pathologist Christine Engelbrecht. The cedars harbor the spores over the winter before they escape in spring to repeat the cycle.
Cedars infected with cedar-apple rust develop winter and early spring galls that surface as as green leaf swellings. Up to 2 inches in diameter, the mature galls are brown, round or kidney-shaped structures. Their spiny protrusions--telia--release orange, gelatinous spore-containing material. The wind carries the spores long distances to infect apples or crabapples. Active for only one year, the galls seldom damage cedar trees significantly, notes the University of Rhode Island Landscape Horticulture website.
Cedars with cedar-hawthorn rust develop 1/2-inch, reddish-brown galls that can cause serious cedar twig destruction and infect rose family hosts for up to five years. Cedar-quince rust produces branch-encircling swellings on new cedar wood. The spore-releasing swellings often kill branches and twigs. Surviving twigs continue to produce spores each year. Severely infected cedars experience dieback.
Because fungus-hosting rose family plants can infect cedar trees from as far as a mile away, simply separating the plants in the home landscape isn't useful. A better prevention for cedar-apple rust, Englebrecht said, is to plant rust-resistant apples, crabapples and hawthorns. McIntosh, Delicious and Liberty apples all qualify, as do Lisbet and Beverley crabapples Autumn Glory and Winter King hawthorn cultivars. Gall removal on lighty infected cedar trees is another option.