Fungal blisters on apple leaf


As the summer progresses into fall gardeners start noticing these blisters on their apple tree leaves.  It’s a fungus !!! This disease is one of several caused by the genus Gymnosporangium. Its called Cedar Apple Rust.


Life cycle on broadleaf plants


The disease  has a complex life cycle.  It spends part of its life on a juniper or cedar and part on one or more hosts in the rose family, requiring both to complete it’s life cycle. If prefers broad leaf plants like Malus (apple, crab apple), Amelanchier, pear, roses, and evergreens ones like cedars and junipers.


Damaged Crabapple fruit


In the broad leaf plants, spores infect leaves, fruit, and soft stem tissues in early spring.  They grow and produce bright yellow or orange lesions by late summer.  Heavily infected leaves and fruit fall off prematurely.


Hawthorne leaf with blisters


The fungal genus also contains species that damage pear (above)


During dry weather, these lesions on the broad leaf trees disperse spores which spread by wind and infect nearby junipers and cedars.


Lesions dispersing new spores


Lifecycle on evergreen plants


Galls form on the evergreens where it survives the winter.  It may take more than a year for the galls to reach maturity. Once ready, in the early spring, bright, orange, gelatinous, horn-like growths 1-2 cm long emerge from the galls. These structures produce spores, which spread by wind to infect the broad leaf host in the spring, repeating the two host cycle.


Cedar Apple Rust gall on Arborvitae




CAR is extremely difficult to eradicate and can only be controlled by applying several treatments of fungicide during Spring and early Summer on both broad leaved and evergreen plants.  Because the spores are so prevalent in the environment, treatment has to be repeated every year.  Specific synthetic fungicides are available to treat CAR in the apple family but these must be applied as soon as the flowers fade and be repeated 4 times about 3 weeks apart. As this can become extremely expensive, we suggest moving towards  using CAR resistant varieties.


If you see any on your trees – Call Plant Specialists TODAY !

We have experts that know what to do !

They are NYSDEC licensed applicators and can treat them too !!!

Don’t delay – the sooner the better !







Article written by our Staff Horticulturist, Peter B Morris, BSc, MSc, MBA

All photographs used with permission @SHUTTERSTOCK