There are several cedar-rust diseases that spend part of their life cycle on Eastern red cedar (Juniperus virginiana) and other junipers, and another part of their life cycle on apple, hawthorn, and other members of the rose family. Both hosts are required for the fungus to complete its life cycle. The three most common rusts occurring in Illinois are caused by Gymnosporangium juniperi-virginianae (cedar-apple rust), G. globosum (cedar-hawthorn rust), and G. clavipes (cedar-quince rust).
The rust organism spends one full year of its life cycle on junipers. During the second spring, usually around the time crabapples are in bloom, the galls become rain-soaked and swell, producing jelly-like tendrils (spore horns) that project out of the galls. As the spore horns begin to dry, the spores are released and carried by the wind to young, newly developing leaves of hawthorns and other susceptible plants. Dispersal of spores can range up to 5 miles from a juniper but most infections develop within several hundred feet. About a month after crabapples have bloomed, the spores are exhausted and most leaves are no longer susceptible.
Ten-to-14-days from initial infection, small yellow spots can be seen on upper surfaces of infected leaves. Several weeks later, the fungus appears as orange or brown spots with hairlike appendages on the underside of the leaf. In late summer, the rust spots release the spores and are carried to nearby junipers.
Cedar-apple rust is the most common of the three fungal rust diseases and attacks susceptible cultivars of apples and crabapples. It infects the leaves, fruit, and, occasionally, young twigs. The alternate host plant, Eastern red cedar (Juniperus virginiana), is necessary for the survival of the fungus. Spores produced on rose family plants only infect juniper plants, and those originating on the evergreen host only infect rose family plants. Repeated infections of cedar-apple rust can be unsightly and seriously weaken and destroy the ornamental value and health of susceptible plants.
On leaves: Bright yellow/orange spots develop on the upper surface of the leaves in late spring. These spots gradually enlarge, becoming evident on the undersurface of the leaves as small bulges. In midsummer, these rust lesions develop hairlike, cylindrical tubes (hyphae), which release spores into the air that are blown to the juniper host. Infected leaves of apples and crabapples may drop, with defoliation more severe in dry summers. Galls that form on the juniper host do not become evident until July the next year, requiring two years for the fungus to complete its life cycle.
On twigs: The rust appears as a swollen corky gall on the current year’s growth, usually no more than 1 inch in length. The swelling eventually develops the characteristic cylindrical fruiting bodies. Seriously affected twigs are stunted and may die.
On fruit: The rust causes yellow to orange spots similar to those found on the leaves, but the spots are usually much larger. Fruit infection causes an inferior fruit quality or premature fruit drop.
In mid-spring, swellings or galls develop on juniper needles that were infected with spores during the previous year. These galls are brown to dull red in color, globular in shape, and may vary from pea-sized to an inch or more in diameter. As they mature, circular pits or depressions are found over the surface of the galls. After spring rains and damp weather, yellow gelatinous tendrils or spore horns form in these pitted areas. The tendrils elongate rapidly and release spores during dry, windy weather that follows the spring rains. Spores produced on the juniper host are then blown to the apple, crabapple, and hawthorn hosts as their new growth emerges.
Eventually, the galls dry out but remain attached to the tree for several years, resulting in some small twig and tip dieback.