Gymnosporangium juniperi-virginianae

On the apple tree, the infections occur on leaves, fruit and young twigs.[2] The brightly colored spots produced on the leaves make it easy to identify. Small, pale yellow spots appear on the upper surfaces of the leaves, usually during late April or May on the eastern seaboard of the United States. These spots gradually enlarge and turn orange or red and may show concentric rings of color. Drops of orange liquid may be visible on the spots. Later in the season, black dots appear on the orange spots on the upper leaf surface. In late summer, tube-like structures develop on the undersurface of the apple leaf. Infected leaves sometimes drop prematurely, particularly during drought conditions or when the tree is under additional stress. Infections on fruit are usually near the blossom end and are somewhat similar to the leaf lesions.

On the eastern red cedar host, the fungus produces reddish-brown galls from ¹⁄₄–1 inch (6.4–25.4 mm) in diameter. These galls can be mistaken for cone structures by the uninitiated. After reaching a diameter of about ¹⁄₂ inch (13 mm), they show many small circular depressions. In the center of each depression is a small, pimple-like structure. In the spring these structures elongate into orange gelatinous protrusions or horns. The spore-bearing horns swell during rainy periods in April and May. The wind carries the microscopic spores to infect apple leaves, fruit and young twigs on trees within a radius of several miles of the infected tree.

On other species of juniper more common in landscaping and bonsai, the sizes of the infections are reduced. Early in the infection, the galls are small bumps on the woody portions of the plant. They maintain the orange gelatinous form after the first warm rains of spring but generally on a greatly reduced scale.

Depiction of the life cycle of cedar apple rust.

Understanding of the disease cycle of this rust fungus is necessary for proper identification and control. Cedar apple rust is caused by the fungi Gymnosporangium or more specifically Gymnosporangium juniperi-virginianae that spend part of their life cycles on Eastern Red Cedars growing near orchards. The complex disease cycle of cedar apple rust, alternating between two host plants, was first delineated by Anders Sandøe Ørsted.[3]

On the eastern seaboard of the United States, at the first warm rain of spring, the spore horns become gelatinous masses and produce their teliospores.

Gall on eastern red cedar (Juniperus virginiana) before rain.

Wind carries the spores to apple leaves at about the time that apple buds are in the pink or early blossom stage. Upon reaching apple buds or leaves, the spores attach themselves to the young leaves, germinate, and enter the leaf or fruit tissues. Infection takes place in as little as four hours under favorable conditions. Yellow lesions develop in one to three weeks.

Gall on eastern red cedar (Juniperus virginiana) after rain

In July and August, spores from the apple leaves (aeciospores) are produced. The wind carries the spores back to eastern red cedars, completing the infectious cycle. The spores land on cedar needle bases or in cracks or crevices of twigs. There, they germinate and produce small, green-brown swellings about the size of a pea. Galls do not produce spores until the second spring. However, mature galls usually are present every year. This fungus produces four out of five of the spores known to be produced by the class Urediniomycetes during its life cycle. (These include teliospores, basidiospores, spermatia, and aeciospores. The type of spore it does not produce is urediniospores.) Rust fungi have a complicated life-cycle with up to five types of spores (each borne on a different type of structure) in its life cycle and often an alternate host, and an "alternate alternate host" as well. Basidiomycetes that have all 5 spore stages and those with less are said to be "macrocyclic" or "microcyclic" respectively.

Because apples are an economically important crop, control is usually focused there. Interruption of the disease cycle is the only effective method for control of the cedar apple rust. The recommended method of control is to “remove cedars located within a 1 mile (1.6 km) radius” of the apples to interrupt the disease cycle,[4] though this method is seldom practical. For those doing bonsai, it is common to have the trees within feet of each other and on the central eastern seaboard of the United States, Eastern Red Cedar is a common first-growth conifer along roadsides.

There are differences in the susceptibility of various apple varieties. 'Jonathan', 'Rome Beauty', 'Wealthy' and 'York Imperial' are susceptible. 'Grimes Golden', 'Narragansett', 'Red Delicious', 'Winesap', 'Staymans', 'Redfree', 'Jonafree' and 'Priscilla' are resistant. Crabapples are generally more susceptible than apples. Resistant crabapples include 'Adams', 'Beverly', 'Candied Apple', 'Dolgo', 'Donald Wyman', 'Eleyi', 'Inglis', 'Indian Summer', 'Liset', 'Mt. Arbor', M. persicifolia, 'Red Jewel', 'Robinson', 'Robusta', 'Royalty', M. sargentii, 'Tina', 'Snowdrift', and 'Special Radiant'. Resistant Crataegus (Hawthorn) include C. crus-galli, series Intricatae, C. laevigata, 'Autumn Glory', C. phaenopyrum, C. pruinosa, C. viridis, and 'Winter King'. The resistant varieties are less susceptible to attack, but that does not mean that they are free from an aggressive attack.

Fungicide sprays applied in a timely manner are highly effective against the rust diseases during the apple cycle.[5] Most sprays are applied four times at 7- to 10-day intervals, starting with pink bud on crabapples. These applications are to protect the apples from spores being released from the cedar host in mid-spring. If cedar apple rust disease is diagnosed on apple fruits and leaves it is far too late to spray. Systemic fungicides are available.

Fungicides listed for use on apples can be used in July and August on the cedars to reduce infection. Application of fungicides to the junipers before and while they are in the infectious orange gelatinous state seems to reduce the severity of the outbreak.

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  Another gall on eastern redcedar (Juniperus virginiana)
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  Close-up on a telial horn: It is full of teliospores (visible as tiny orange spots)
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  Two-celled teliospore
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  Cedar apple rust on a crab apple showing stomata
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  Cedar apple rust on a crab apple showing stomata
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  Cedar apple rust on a shimpaku juniper
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  Cedar apple rust on a J. procumbens nana)
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  Gall with telial horns

• Cedar-Apple Rust, Gymnosporangium juniperi-virginianae