When your Eastern red cedar tree (Juniperus virginiana), an evergreen conifer that typically contains bluish, fleshy, berry-like cones, suddenly develops structures that resemble hard brown apples, you wonder what is happening. The growths, sometimes referred to as cedar apples or incisions, come from the fungus infection known as cedar-apple rust. The fungus can happen anywhere cedar and apples (Malus spp.) grow near each other. It requires both kinds of plants to complete the rust’s life cycle. At times the cedar’s health is not affected, but the disease can damage or kill both trees.
Beginning Period in Cedar
Although Eastern red cedar, which grows in U.S. Department of Agriculture plant hardiness zones 2 through 9, is the most vulnerable to the rust, ornamental junipers can also be hosts. The rust’s life cycle begins in summer when a spore blows from an infected apple to a cedar leaf. The fungus grows slowly at first, developing a small, greenish-brown swelling from the next summer. That fall, the swelling enlarges to the cedar apple, which can be a gall, or abnormal plant growth prompted by substances from another organism. It can be kidney-shaped or rounded, and can be 1/4 to above 2 inches broad. The surface has small depressions like golf ball dimples.
Fruiting Period in Cedar
Another phase is showy, with activity beginning in spring. Warmer temperatures and spring rains prompt growth of orange, gelatinous-looking horns that protrude from every dimple on the gall. Rather ornamental contrary to the green cedar foliage, the horns produce the reproductive spores that fly around the end to an apple tree. After spore release, the horns die as well as the cedar apple dries up, frequently falling to the ground.
Apple Tree Infection
Not just edible apples (Malus domestica), which are hardy in USDA zones 3 through 8, are affected, but crabapples (Malus spp.) Can also host cedar-apple rust. Crabapples grow in USDA zones 4 through 8a. Spores in the cedar tree gall land on an apple fruit or leaf and begin to feed on the tissues, developing yellowish circular spots. The fungi then form black fruiting bodies at the center of these areas, and insects cross-fertilize the dark spots. Then the last life stage grows in the dark spots, producing small, white, tubular constructions. These release the spores that cycle back to the cedars, typically at June through August. The apple tree loses leaves, fewer apples form and apples can be small or misshapen.
Reduce the Effects
If there are not too many cedars growing within a period of an apple tree, remove all of the cedar galls before they form the star-like reproductive phase. Since red cedar is the most susceptible host, plant rust-resistant species of juniper rather than red cedar. Cultivars of apple and crabapple can be found that are immune to cedar-apple rust. If you can, reduce the number of junipers that grow near apple trees, and maintain junipers as far away from apples as you can. For the disease not to be able to transmit back and forth, nevertheless, they would have to be just two miles apart.