Missoula County Weed District & Extension

Cedar Apple Rust

 

 

 

Plant Diagnostic Database

Cedar Apple Rust (Gymnosporangium juniperi-virginianae)

                       

cedarapplerust

cedarapplerust2005

cedarapplerustleaves

 

 

  • Hosts
  • Life Cycle

 

 

  • Description
  • Controls

 

                                                                                                                                                                                              
Hosts:

Rust occurs anywhere that perennial hosts (juniper, cedar, etc.) grow near their deciduous hosts(apple, hawthorn).

Description:
Orange leaf spots, defoliation and fruit and twig deformities. Juniper infections appear as swellings on deciduous host or galls on twigs, these form gelatinous orange fruiting bodies in the spring.
Life Cycle:
The rust fungi (following infection) overwinters on juniper and cedar.  Galls start to form during the spring and for the next 20 months.  Rainsplash or wind to nearby hawthorn disperses the spores.  Under conditions of high humidity, dispersal may be for several miles.  Infection of the broadleaf hosts results in formation of orange-yellow spots and later developement of hair like structures(aecia) on the leaf underside.  Spores are dispersed by wind and infect the juniper during late summer or early fall.
Controls:
Cultural: Mineral:

Optimal temperatures for rust infections are 64-70° F. High temperatures (>85° F.) and dry weather discourage rust disease.


Irrigate early enough in the day so that plant surfaces have time to dry before the cooler temperatures of evening. Keep irrigation off of leaves, branches, and trunk.

Prune plants for good air circulation.

Plant resistant varieties.

Sulfur or copper soap sprays can be effective if sprayed preventatively in the spring when rust infection periods occur(wet with temperatures 60-70ºF).

 

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