Fact Sheets no. The fungus that causes juniper-hawthorn rust spends 18 to 20 months of its life on the juniper. It requires two different hosts to complete its life cycle.
Infections typically are more harmful to the alternate host than to juniper.
Repeated infections over several years can cause reduced fruit size on apple and premature defoliation of hawthorn, apple, crabapple or mountain ash. Defoliated trees may suffer winter injury and often fail to set fruit the following season. The rust galls become active following rainy spring weather, when an orange gelatinous material exudes out of the pocket-like depressions on the galls. The telial horns eject billions of spores that are carried by the wind to the alternate host. These cup-like lesions also can appear on immature apple fruit and cause dwarfing and malformation.