Cedar-apple rust

Gymnosporangium juniperi-virginianae

also called apple rust, juniper-apple rust

Bright orange-yellow spots on apple leaves and fruit, part of a rust that shuttles each year between apple trees and nearby junipers.

Fungus Serious Airborne spores Spreads to neighbors

What you'll see

Vivid orange-yellow spots on the upper surface of apple and crab-apple leaves.
The spots enlarge, and small tube-like or cup-shaped growths form beneath them, on the leaf underside or on the fruit.
Fruit is spotted and distorted, and heavily spotted leaves drop early.
On juniper hosts, hard brown galls form on the twigs.
In spring those galls swell and push out gelatinous, bright orange horns after rain.

Why it happens

A rust fungus with a two-host life cycle: it needs both an apple (or crab-apple) and a juniper to complete its year. Spores from the orange horns on juniper blow to apples in spring, and later spores from the apples blow back to junipers, so the two hosts keep reinfecting each other.

Wet, mild spring weather triggers the orange horns on juniper and lets their spores infect nearby apples. Trees within a few hundred meters of an infected juniper are most at risk.

Treating it

Prune the brown galls out of junipers in winter before they can produce spores in spring.
Pick off and destroy the worst-spotted apple leaves and fruit.
Keep apple trees vigorous with mulch, feeding and watering to tolerate the leaf loss.
Where feasible, remove nearby wild or ornamental junipers to break the cycle.
On susceptible orchards, an approved fungicide from bud-burst through spring gives protection.

Keeping it away

Don’t plant susceptible apples and junipers close together.
Grow resistant apple and crab-apple varieties in areas where the rust is common.
Inspect junipers each winter and cut out galls before spring.
Keep good distance between the two hosts wherever practical.

Who gets it

Apples and crab-apples on one side, and junipers, especially eastern red cedar, on the other — the disease needs both.