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Cedar Apple Rust, Quince Rust

Gymnosporangium juniperi-virginianae, G. clavipes

Fig 1 - Eastern red cedar

Fig 1. Eastern red cedar

Cedar apple rust (CAR), caused by Gymnosporangium juniperi-virginianae, and quince rust (QR), caused by G. clavipes, occur commonly on apples in the southeastern United States. The eastern red cedar (Juniperus virginiana (Fig 1)) is the alternate host for both of these fungi. G. juniperi-virginianae infects leaves and fruit; G. clavipes infects only fruit.

Symptoms

In late May or early June, apple leaves infected with the cedar apple rust fungus develop pale yellow spots on the upper surface which enlarge and become yellow to orange with age (Fig 2). Small fruiting structures, called pycnia, are produced in the centers of these spots. In July and August, numerous tube-like protuberances known as aecia appear on the bottom of the leaves (Fig 3 and Fig 4). If infections are numerous, extensive defoliation can occur. On some cultivars, infections result in a hypersensitive reaction that results in circular necrotic lesions (Fig 5) resembling frogeye leaf spot. Infections on fruit often occur on the calyx end but may occur on the sides or even the stems. Lesions on fruit are shallow and orange to yellow (Fig 6) or green with a yellow to orange border. Late in the summer, aecia sometimes are visible on lesions on fruit (Fig 7).

Quince rust infections usually occur on the calyx end of fruit and result in dark green lesions with necrotic tissues that often extend to the core (Fig 8). Fruit affected with quince rust are usually distorted and drop prior to harvest (Fig 9).

Fig 2 - Cedar apple rust leaf spots Fig 3 - Cedar apple rust aecia on leaf Fig 4 - Cedar apple rust aecia on leaf Fig 5 - Cedar apple rust necrotic lesions
Fig 2. CAR leaf spots Fig 3. CAR aecia on leaf Fig 4. CAR aecia on leaf Fig 5. CAR necrotic lesions
Fig 6 - Cedar apple rust lesions of fruit Fig 7 - Cedar apple rust aecia on fruit Fig 8 - Quince rust on fruit Fig 9 - Quince rust damage
Fig 6. CAR lesions on fruit Fig 7. CAR aecia on fruit Fig 8. QR on fruit Fig 9. QR damage

Disease Cycle

Spores produced in the aecia on the apple leaves (Fig 10 and Fig 4) or fruit infect leaves of the cedar in the late summer. Dry summer weather can reduce the number of infections on cedar and reduce the inoculum level. Over the next 18 to 20 months the leaf infections on the cedar enlarge into galls (Fig 11) that are capable of producing spores during favorable weather. Once galls have produced spores, they dry into woody-like structures that may remain on the cedar tree for several years (Fig12a and Fig 12b).

Quince rust infections result in perennial spindle-shaped galls on cedar branches or twigs that vary from several inches to several feet in length (Fig 13 and Fig 14). During rainy periods in the spring, spores are produced on gelatinous orange-colored protuberances (called "telia" and often referred to as horns), which extrude from the galls during wet weather in the spring (Fig 15, Fig 16). On drying the telia shrivel (Fig 17), but rehydrate and elongate when it rains again. These spores germinate and give rise to a second set of spores, basidiospores, which are airborne and initiate infections on apples. Temperatures favoring infection are similar to those of apple scab. The optimum temperature for infection of fruit is about 65 F with little infection occurring below 55 F. Fruit are most susceptible to infection to cedar apple rust from tight cluster to petal fall; leaf infections can occur as long as the telial horns produce spores.

Fig 10 - Cedar apple rust spores in aecia Fig 11 - Cedar apple rust galls Fig 12a - Mummified cedar apple rust galls Fig 12b - Mummified cedar apple rust galls Fig 13 - Quince rust canker
Fig 10. CAR spores in aecia Fig 11. CAR galls Fig 12a. Mummified CAR galls Fig 12b. Mummified CAR galls Fig 13. QR canker
Fig 14 - Quince rust canker Fig 15 - Cedar apple rust gall with telia Fig 16 - Quince rust gall with telia Fig 17 - Cedar apple rust gall with telia - old
Fig 14. QR canker Fig 15. CAR gall with telia Fig 16. QR canker with telia Fig 17. CAR gall with telia - old

Control

Where rust diseases are severe enough to require control, remove all cedars, if possible, within 1/2 mile of the orchard. Removal of all cedars will usually provide sufficient control in areas of the mountains in the Southeast where the eastern red cedar is not abundant. Fungicides should be applied from early pre-pink through petal fall for fruit infections and from pre-pink through second cover for leaf infections. Of the cultivars planted in the Southeast, Rome Beauty, Gala, Mutsu (Crispin), Braeburn, Ginger Gold, Cameo and Jonagold are most susceptible to cedar apple rust. Delicious is not susceptible to cedar apple rust but is very susceptible to quince rust.

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Format updated March 29, 2011