In the moist weather of spring, one may notice orange-coloured, gelatinous horn-like projections on the twigs of junipers. Although they resemble ugly flowers, they are actually growths caused by the fungus Gymnosporangium juniperi-virginianae. This cedar-apple rust is so named because it alternates between two hosts: Juniperus and Malus (apple, crabapple).
Cycle: Juniper to apple: Spores initially infect a juniper in the summer. Symptoms appear the following spring, when the fungus stimulates the growth of galls. They are brownish growths which quickly become orange horns in moist conditions. The next spring, spores are released from the ‘horns’. By mid-summer, orange spots appear on the upper surfaces of leaves. Eventually these spots have a black centre. In August, spores are released, carried on the wind, and deposited on nearby junipers.
Damage: Unsightly galls on junipers are long-lasting, and cause branch death. The plant itself does not usually die from cedar-apple rust. Infected apple trees show premature leaf drop and therefore less vigor and reduced yields.
Control: Since junipers and apples are alternate hosts, avoid planting them together when possible. This method is useful but limited because spores can travel long distances on wind. Apply a fungicide such as Funginex (ferbam) for apples. For junipers, ferbam or copper may be used. It is recommended to spray ferbam in August before spores are released.