Cedar-Apple and Related Rusts on Ornamentals: Complete Host Range and Management

Understanding Gymnosporangium Rusts on Ornamentals

Cedar-apple rust and related diseases (cedar-quince, cedar-hawthorn, Japanese apple rust) are caused by Gymnosporangium species. These fungi require TWO different living hosts to complete their life cycle—if either is absent, the fungus dies. Juniper (cedar) is one host; alternates include apple, hawthorn, quince, shadbush/serviceberry depending on fungus species. Japanese apple rust (G. yamadae) introduced from Asia in 2004, nearly indistinguishable from cedar-hawthorn rust on Malus.

Symptoms by Host

Juniper (Juniperus)

• Smooth, round, golf-ball-size galls on twigs (cedar-apple rust)
• Elongated swollen branch areas (cedar-hawthorn rust)
• Prolific branching/witches' brooming

Apple (Malus)

• Yellow to bright orange leaf spots
• Fruit spots

Hawthorn (Crataegus)

• Yellow leaf spots
• Distorted fruit with horn-like protrusions
• Galled twigs

Shadbush/Serviceberry (Amelanchier)

• Brownish-orange leaf spots
• Distorted fruit with horn-like protrusions

Life History

Rust overwinters as mycelium in juniper gall tissue. Warm rainy spring: gelatinous horns emerge from galls. Teliospores on horns germinate → basidiospores → wind to broadleaf host leaves/fruit. Infection requires leaf moisture and adequate temperatures. Pycniospores on broadleaf upper surface allow mating. Aeciospores (same spots, leaf underside) wind back to junipers in summer. Spring after juniper infection: gall formation begins. Second spring: spores released from mature gall. 18-20 months for gall formation, maturation, spore release.

Management

Landscape: Damage usually not severe enough for fungicides. Susceptible broadleaf varieties can be heavily infected/defoliated; fruit severely deformed. Remove galls from junipers during dormancy before spore horns develop.

Nursery: Fungicide on crabapples spring/early summer. Hawthorns: fungicide from flower opening at 10-day intervals. Resistant crabapple/apple cultivars available. Junipers: summer fungicide if broadleaf host nearby. Avoid planting alternate hosts close together. Remove susceptible non-crop plants from vicinity.

Host Combinations by Rust Species

Cedar-apple rust (G. juniperi-virginianae)

• Apple, crabapple
• Eastern red cedar, Chinese, prostrate junipers

Cedar-hawthorn rust (G. globosum)

• Apple, crabapple, hawthorn, quince, pear, shadbush
• Eastern red cedar, Rocky Mountain juniper, prostrate junipers

Cedar-quince rust (G. clavipes)

• Apple, crabapple, mountain ash, cotoneaster, hawthorn, quince, pear, photinia, shadbush
• Eastern red cedar, Rocky Mountain juniper, savin, prostrate junipers

Japanese Apple Rust (G. yamadae)

• Malus spp. (13 listed)
• Juniperus chinensis and varieties

Integrated Rust Management

The alternate host requirement is the key management lever: spatial separation of juniper and broadleaf hosts breaks the cycle. In nurseries, targeted fungicide timing (spring for broadleaf, summer for juniper) protects crops. Resistant cultivars reduce inputs. The 18-20 month gall development means dormant-season gall removal on junipers is highly effective for reducing inoculum.