Spores (called aeciospores) are produced from the fungal lantern-shaped growths which protrude from the blisters on the underside of the pear leaf which become airborne and infect junipers.

In the spring after a rain or heavy dew, the galls on the juniper produce tiny dark horn-like growths that become covered with an orange-brown gelatinous mass called telia.

The telia and aecia release wind borne resting or hibernating spores (called teliospores and aeciospores) capable of infecting susceptible pear leaves and Juniper respectively.

The fungus has, however, extended its range in Europe as far north as Scandinavia[4] and the British Isles[5] through the frequent use of savin and other junipers as ornamental garden shrubs.

If there is a chance of infection, spraying pear trees with a fungicide in spring and summer (typically a systemic one that is certified as capable of dealing with rust) may help, although this is often not considered worthwhile.