Our comon friends, the Juniper and the Hawthorn, share a fungal pest which requires both these plants be present and near enough to each other to allow transport of fungal spores from one to the other.
Gymnosporangium spore forming bodies on Juniper seen each spring at Ardgay.
Gymnosporangium juniperi-virginianae is the fungal pest which in turn attacks Juniper, then Hawthorn. Juniper is infected in mid to late summer by spores created within galls on Hawthorn. After over-wintering spring moisture causes Gymnosporangium galls to form on the Juniper. Spores from the Juniper infect the Hawthorn. This heteroecious cycle repeats.