Beginner

Scientific Names
Gymnosporangium juniperi-virginianae and Gymnosporangium clavipes

Identification
On apple trees:

• Small, pale yellow spots appear on the upper surface of leaves and on fruit shortly after bloom
• The lesions grow larger and more orange-coloured, often with a red margin.
• Eventually, small black spots (spermagonia) appear within the centre of the lesions.
• In late summer, cup-like structures appear on the undersides of leaves and on fruit.
• Apple cultivars vary in their level of susceptibility to rust diseases

On red cedar trees (and other alternate hosts):

• Overwinters as mycelium within rounded, brown-coloured galls 10-30mm in size.
• Galls produce orange telia horns 10-20mm long that grow from the galls and become orange-yellow and gelatinous during warm rains in the spring.

Often Confused With

• Apple scab - Apple scab lesions can be on both fruit and leaves. The lesions on the fruit and leaves are green and velvety and as they mature they become black and corky. Cedar apple and quince rust lesions appear as orange lesions on the fruit and leaves.
• Frog’s eye leaf spot - Frog’s eye leaf spot an be distinguished from cedar apple and quince rust lesions on the leaf by the characteristic tan center or brown with a light center giving the lesion a frog’s eye appearance. Cedar apple and quince rust lesions appear as orange spots on the leaves and later as cup-like structures on the underside of the leaf.

Period of Activity
Cedar-apple and quince rust overwinters on their alternate host, red cedar, or other hosts in brown coloured galls. These galls produce spores that infect apples during warm, wet conditions in the spring. Symptoms begin to appear on the upper surface of apple leaves and on the fruit shortly after bloom. Leaves that are three to four weeks old are fairly resistant to infection. Symptoms begin to appear on the alternate hosts at about the same time apple trees are in the pink stage, after a wet period caused by a rain or heavy dew.

Cedar-apple and quince rust cannot spread from apple to apple or from red cedar to red cedar – the fungus must go through the two-year life cycle, alternating between hosts.

Scouting Notes
Regular scouting is the best way to assess disease levels. When scouting susceptible varieties such as Empire, Golden Delicious, Mutsu, Northern Spy, Red Delicious and Russet, note infected leaves and fruit when scouting for other pests.

Estimate the potential for rust fungus infections of apple by examining alternative hosts near the orchard from early May to mid June. Galls producing the orange spore-bearing masses indicate that infection is probable.

Thresholds
None established.

Advanced

Scientific Names
Gymnosporangium juniperi-virginianae and Gymnosporangium clavipes

Identification
On apple trees: Small, pale yellow spots appear on the upper surface of leaves and on fruit shortly after bloom. The lesions grow larger and more orange-coloured, often with a red margin. Eventually, small black spots (spermagonia) appear within the centre of the lesions. In late summer, cup-like structures appear on the undersides of leaves and on fruit. Apple cultivars vary in their level of susceptibility to rust diseases.

On red cedar trees (and other alternate hosts): Overwinters as mycelium within rounded, brown-coloured galls 10-30mm in size. Galls produce orange telia horns 10-20mm long that grow from the galls and become orange-yellow and gelatinous during warm rains in the spring.

Often Confused With

• Apple scab - Apple scab lesions can be on both fruit and leaves. The lesions on the fruit and leaves are green and velvety and as they mature they become black and corky. Cedar apple and quince rust lesions appear as orange lesions on the fruit and leaves.
• Frog’s eye leaf spot - Frog’s eye leaf spot an be distinguished from cedar apple and quince rust lesions on the leaf by the characteristic tan center or brown with a light center giving the lesion a frog’s eye appearance. Cedar apple and quince rust lesions appear as orange spots on the leaves and later as cup-like structures on the underside of the leaf.

Biology
Cedar-apple and quince rust overwinters on their alternate host, red cedar (Juniperus virginanae), or other hosts, as mycelium within rounded, brown-coloured galls 10-30 mm in size. Symptoms begin to appear at about the same time apple trees are in the pink stage, after a wet period caused by a rain or heavy dew. Galls on the eastern red cedar produce orange telia horns 10-20 mm long that grow from the galls and become orange-yellow and gelatinous. The telia produce teliaspores on the horns that release windborne basidiospores only capable of infecting susceptible apple leaves and fruit during spring rains.

Most basidiospores are discharged in the period from tight cluster of apple through to petal fall. The galls of cedar-apple rust die following the release of the spores, but horns can swell and dry several times, releasing spores during intermittent rains. Spores are discharged as soon as rainfall begins. A short wetting period (compared to scab) of four to six hours at 10°-24°C can result in severe infection.

Generally, long wetting periods around petal fall are particularly damaging. While spores are released during periods of high humidity (>85%), infection only occurs if free water is available. Spores travel a maximum distance of 6-8 km, but most infections occur when alternate hosts are within a few hundred metres. In late summer, cup-like structures appear on the undersides of leaves and on fruit. These structures release spores carried on the wind to red cedars. Spores infect red cedar leaves from mid summer into autumn. Greenish galls are produced the next spring, but these do not mature to release the spores to infect apple until the following spring.

Cedar-apple and quince rust cannot spread from apple to apple or from red cedar to red cedar – the fungus must go through the two-year life cycle, alternating between hosts.

Period of Activity
Cedar-apple and quince rust overwinter on their alternate host, red cedar, or other hosts in brown coloured galls. These galls produce spores that infect apples during warm, wet conditions in the spring. Symptoms begin to appear on the upper surface of apple leaves and on the fruit shortly after bloom. Leaves that are three to four weeks old are fairly resistant to infection. Symptoms begin to appear on the alternate hosts at about the same time apple trees are in the pink stage, after a wet period caused by a rain or heavy dew.

Scouting Notes
Regular scouting is the best way to assess disease levels. When scouting susceptible varieties such as Empire, Golden Delicious, Mutsu, Northern Spy, Red Delicious and Russet, note infected leaves and fruit when scouting for other pests.

Estimate the potential for rust fungus infections of apple by examining alternative hosts near the orchard from early May to mid June. Galls producing the orange spore-bearing masses indicate that infection is probable.

Thresholds
There are no established thresholds for cedar-apple and quince rust.

Management Notes

• Apple cultivars vary in their level of susceptibility to rust diseases.

• Theoretically, it is desirable to remove alternate hosts of rust diseases and wild apple trees from the vicinity of the orchard. In reality, this is usually not practical. Remove and burn rust galls from valuable ornamental junipers, and plant rust-resistant cultivars of ornamental species.
• Most of the fungicides registered for scab control also give good control of rust diseases.
• However, in orchards with a history of rust problems, a protectant fungicide program applied specifically for apple scab control may not be adequate during the period from tight cluster/pink until after petal fall. For information about the activity of different fungicides available for the rust diseases. see OMAFRA Publication 360, Fruit Crop Protection Guide.
• Scab-resistant cultivars susceptible to rust infection require a specific fungicide program to control these diseases.