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.
Symptoms on Apple and Crabapple
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.
Symptoms on Juniper
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.
Cedar-hawthorn rust is very similar to cedarapple rust, both in appearance and occurrence, but infects a broader range of plants within the rose family. The severity of the disease is usually minor on crabapples and apples (Malus sp.), mountain ash (Sorbus), and pears (Pyrus), but can be quite serious on many hawthorns (Crataegus sp.).
Symptoms on Hawthorn
On leaves: Large yellow spots appear on cedar-hawthorn rust the upper surface of the leaves turning yellow orange to gray-brown as the spores mature. When rust is severe, all the foliage may turn bright yellow and drop prematurely. The orange leaf spots are smaller on apple and crabapple.
On fruits and twigs: Deformation of fruits and young twigs is particularly severe on hawthorns, but this damage is usually caused by the cedar-quince rust fungi and not cedar hawthorn rust fungi.
On juniper: Cedar-hawthorn rust galls are smaller in size than cedar-apple rust galls, less symmetrical, and more chocolate-brown in color. Galls remain on the twigs of branches of junipers for several years, where they continue to produce spores, compared to the one season spore production of cedar-apple rust.
Cedar-quince rust affects quince (Chenomeles), serviceberry (Amelanchier), hawthorn (Crataegus), mountain ash (Sorbus) and many other plants in the rose family. Though generally not as prevalent as cedar-apple rust, it causes the greatest amount of damage to the fruits, twigs, and thorns of susceptible plants. During extended periods of wet weather, when temperatures range between 50 degrees F and 75 degrees F, severe infection can occur just four hours after initial leaf contact.
Symptoms of leaves: Basically none, occasionally veins or petioles will be swollen.
On twigs and thorns: Elongated swollen cankers appear on twigs and thorns. In damp weather, orange to brown spores are visible.
On Junipers: Spindle-shaped swelling occurs on twigs and branches of junipers. Young branches are usually girdled, then die. In damp weather, older galls are covered with masses of gelatinous, orange to brown spore horns. Galls can produce spore horns for 4 to 6 years, sometimes longer.
Because this disease requires two hosts, the separation of the hosts for a distance of one mile will help reduce infection. Ideally, to minimize disease host availability, plant trees and shrubs that are resistant to rust diseases. There are many apples, crabapples, hawthorns, and junipers that exhibit resistance to these diseases. The Morton Arboretum publication Crabapples for the Home Landscape provides information on selecting crabapples.
Protective fungicides can be applied to help minimize infection. A minimum of three applications should be done. These applications protect the new leaves from spores that are dispersed from the juniper host in mid-spring. Spraying apple, crabapple, and hawthorn foliage after symptoms develop has no controlling effect.
Apples and Crabapples: Begin spraying when new growth appears and flower buds show color but are not yet open. Repeat three to four times at 10 to 14 day intervals.
Spray as new growth appears and flower buds begin to open. Repeat 3 to 4 times at labeled intervals. Washington hawthorns are very susceptible to quince rust and form noticeable stem cankers that should be pruned out.
Begin spraying susceptible plants in early July and continue at labeled intervals through August. Remove galls and cankers to reduce infection of alternate hosts.
Contact the Plant Clinic (630-719-2424 or firstname.lastname@example.org) for current recommendations.