Forest Health > Insects and Diseases publication
Diseases of... Conifer Foliage
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CEDAR APPLE RUST
 Brightly-colored telial horns of the cedar-apple rust fungus Gymnosporangium juniperi virginianae
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| Pathogens: |
Fungus Scirrhia acicola |
| Common Host: |
Eastern Redcedars Juniperus virginiana
Southern Redcedars Juniperus silicicola |
| Alternate Host: |
Rosaceae family members including: Apple Malus spp.
Hawthorn Crataegus spp.
Pear Pyrus spp. |
Significance: |
Cedar-apple rusts are not diseases of serious magnitude in Florida. They are, however, fairly common in parts of the state and may, under certain circumstances, represent a potential threat to redcedars being grown as ornamentals or Christmas trees. In other situations, the problem may be the damage caused by cedar-apple rust fungi on the leaves and/or fruit of apples, pears, hawthorns, or other pomaceous (apple-like) trees which serve as alternate hosts in the pathogens' life cycles. Damage on redcedars is usually a disfiguring of infected foliage and branches, generally involving various types of swellings - galls (hypertrophies) and witches brooms on infected tissues. The fungus G. juniperi-virginianae causes the distinctive and rather well-known cedar apples on infected redcedars. Damage on the alternate, pomaceous hosts includes the spotting, yellowing, or deformation of infected foliage and the blemishing or loss of infected fruits. See Figure 12- Generalized life cycle of the cedar-apple rust fungi. |
Recognition: |
Infected redcedars typically display some type of swelling or hypertrophy and occasionally witches brooms in infected wood tissues (branch, twig or stem) or the typical gall-like structures called cedar apples which sometimes reach 5 centimeters in diameter. Immature, first-year galls, or cedar apples, are typically greenish brown and have small pit-like depressions on their surfaces. Mature, second-year and older inactive galls are reddish brown to dark brown. During rainy weather, particularly during the spring and early summer, mature second-year galls give rise to gelatinous, yellow-orange to orange-brown finger-like projections (tendrils) called telial horns, a spore stage typical of rust fungi. Telial sori (masses or heaps) of Gymnosporangium spp. causing branch or stem swellings are similarly colored, but appear as irregular or elongated gelatinous masses, ridges or flaps on the surface of the woody galls. At this time infections are showy and unmistakable. In dry weather telial horns shrivel, become dark brown in color, harden and remain inactive until sufficient moisture is available for their revival. Infected foliage on the pomaceous, alternate hosts appears during the summer or fall and is characterized by spots or large areas of yellow-orange discoloration and varying degrees of deformation. Small blackish fruiting bodies (less than 1 mm) called pycnia (sing., pycnium) may often be seen embedded in the discolored leaf tissue on the upper surface of the leaves. On the lower leaf surfaces small yellow-orange blisters or pustules called aecia (sing., aecium) are often readily visible, but these generally appear after the formation of the pycnia on the upper leaf surfaces. Infected fruits are blemished and disfigured, and may sometimes shrivel completely and fall from the tree. |
Infection Biology: |
Microscopic spores called sporidia or basidiospores are produced during the spring and early summer by the gelatinous telia horn or sori (sing., sorus) on infected redcedar tissues. Those spores are windblown to the surfaces of susceptible alternate host leaves or fruit where the pathogens initiate their alternate infections by direct penetration. Later, during the summer and fall, another type of spore (aecispores) is produced in the aecia on the underside of infected foliage of the pomaceous, alternate hosts. These spores are again wind-carried to susceptible redcedar tissues where new infections are then initiated. Rust fungi survive perennially in the bark of infected redcedars (stem and branch infections) and are capable of producing telial horns and sporidia year after year under suitable weather conditions G. juniperi-virginianae, the primary cause of the typical cedar apples, survives in the gall tissue only two years. After its second and spore-producing year, this fungus dies in the galled tissues. On the alternate, pomaceous hosts the fungi survive for only a matter of months, long enough to generate spores for new redcedar infections. Cedar-apple rust fungi cannot complete their life cycle without passing through both types of hosts. Spores produced on redcedars cannot reinfect redcedars, and spores produced on pomaceous hosts cannot reinfect pomaceous hosts. |
Control: |
The best method of controlling cedar apple rust infections is to avoid growing redcedars and the pathogen's pomaceous alternate hosts in close proximity to one another. By eliminating either redcedars or the broad-leaved alternate hosts (choice dependent upon the tree crop preferred) the pathogens are unable to complete their life cycles. Hence there can be no intensification or spread of the disease. Both redcedars and the alternate, broad-leaved hosts of cedar-apple rusts can be protected with the carefully timed application of certain fungicides if alternate host eradication is not feasible or desired. Infected and deformed branches of redcedars can simply be pruned and discarded where cosmetically acceptable and economically justified.
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| Branch swelling and telial sporulation on red cedar resulting from infection by Gymnosporangium nidusavis. |
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Bulletin No. 196-A | Printed October, 1983 | Contact the Forest Health Section |
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