Cedar-Apple Rust

Understanding Cedar-Apple Rust and Related Rust Diseases

Cedar-apple rust (Gymnosporangium juniperivirginianae), cedar-hawthorn rust (G. globosum), and cedar-quince rust (G. clavipes) are closely related fungal diseases that require two hosts—junipers and a member of the rose family(such as apple or crabapple)—to complete their life cycle.

Symptoms & Diagnosis

🔴 On Junipers:

• Brown, perennial galls form on twigs.

• Galls mature in two years, swelling and producing orange, gelatinous telial horns during rainy spring weather.

• Galls range in size, with cedar-apple rust galls often over 2 inches in diameter.

• Occasionally, twigs beyond the gall may die, but no significant damage occurs.

🍏 On Apples & Crabapples:

• Yellow circular spots appear on upper leaf surfaces shortly after bloom.

• In late summer, brownish clusters of threads or tubes (aecia) form beneath the yellow leaf spots, appearing on fruits and twigs.

• Severe infections may lead to premature leaf drop, especially during dry summers.

Life Cycle

• On Junipers: Galls develop 7 months after infection and release spores after 18 months.

• In Spring: Telial horns form, elongate, and turn bright orange in wet weather, releasing spores that infect apple and crabapple trees.

• On Apples & Crabapples: Wet spring conditions promote twig, leaf, and fruit infection.

• Late Summer: Rust-colored spores on apple/crabapple leaves spread to junipers, completing the cycle.

Management Strategies

✂️ 1. Prune – Remove galls from junipers, though wind-spread spores make complete control difficult.

🌳 2. Co-exist – The disease rarely kills trees, though it may cause disfiguration.

🛡 3. Preventive Fungicide – Apply fungicides labeled for apples when juniper galls are active (orange and gelatinous). Once dry, fungicide use is unnecessary.

🌿 4. Strategic Planting – Avoid planting junipers near rust-susceptible apple/crabapple trees.

🍏 5. Use Resistant Apple Varieties – Replace susceptible trees with resistant varieties:

• Resistant: ‘Redfree,’ ‘Liberty,’ ‘William’s Pride,’ ‘Freedom’.

• Susceptible: ‘Jonathan,’ ‘Rome,’ ‘Wealthy,’ ‘York Imperial,’ ‘Golden Delicious’.

Organic Management Strategies

• Pruning, co-existing, strategic planting, and using resistant varieties are all organic-friendly approaches.

By managing plant selection and environmental conditions, you can reduce the impact of cedar-apple rust and maintain healthier trees. 🌿🍏