Gymnosporangium juniperi-virginianae is a plant pathogen that causes cedar-apple rust.Gordon Grice, “Pondering a Parasite,” Discover, July 2008, 54-56. In virtually any location where or crabapples ( Malus) and eastern red cedar ( Juniperus virginiana) coexist, cedar apple rust can be a destructive or disfiguring disease on both the apples and cedars. Apples, crabapples, and eastern red cedar are the most common hosts for this disease. Similar diseases can be found on quince and Crataegus and many species of juniper can substitute for the eastern red cedars.
Symptoms
On the eastern red cedar host, the fungus produces reddish-brown galls from to 2 inches (6 to 50 mm). After reaching a diameter of about , the galls show many small circular depressions. In the center of each depression is a small, pimple-like structure. In the spring these structures absorb water during rainy periods and elongate into orange gelatinous telial horns that are 10–20 mm long. The wind carries the microscopic spores to infect apple leaves, blossoms, 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.
Disease cycle
When exposed to the first warm rain of spring, the small bumps on the galls absorb water, swell, and produce telial horns –gelatinous masses that produce . When swollen, teliospores will germinate and produce which are forcibly discharged and travel along air currents to infect apple trees and other alternate hosts. The telial horns will dry out once the rain passes and will lose their gelatinous appearance, instead resembling dark brown threads. When the rain returns, the horns will swell again. This process can repeat eight to ten times during the spring. It can take as little as four hours for basidiospores to form inside the telial horns under optimal conditions. '') 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 covered by films of water, the spores attach themselves to the young leaves, germinate, and enter the leaf or fruit tissues. Light infection can take place in as little as two hours under favorable conditions. Heavy infections take at least four hours to develop. Lower temperatures delay infection. Yellow-orange lesions develop on the upper sides of leaves or on fruit one to two weeks following infection. These lesions contain pycnia and pycniospores. These lesions will produce a sticky honeydew like substance to attract insects that assist in the transport of the pycniospores to different lesions, allowing for sexual recombination.
One to two months later, in July and August, orange-yellow aecia are produced in concentric rings on the bottom of the apple leaves or surrounding the pycnia on the fruit. The aecia produce . 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. 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 , , spermatia (also called ), and . The type of spore it does not produce is .) 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.
Control
There are differences in the susceptibility of various apple varieties. 'Jonathan', 'Rome Beauty', 'Wealthy', 'Stayman', 'Jonafree' and 'York Imperial' are susceptible. 'Grimes Golden', 'Red Delicious', 'Winesap', 'Redfree', 'McIntosh', 'Liberty', 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', Malus 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. Most protective fungicide 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. Although curative fungicides also exist for cedar apple rust, they must still be applied before trees begin to develop symptoms. Systemic fungicides are available as well, which require fewer sprays during the season. However, there are no fungicides available to home gardeners that can be used on trees that produce fruit which will be eaten by people.
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