Pear Trellis Rust
Gymnosporangium sabinae
Also called european pear rust, juniper-pear rust, pear rust
A two-host rust that paints pear leaves with vivid orange spots backed by tiny lattice-like tubes, while producing gelatinous orange horns on juniper stems in spring.
Overview
Pear trellis rust is a microfungus in the genus Gymnosporangium, a group of rusts (Pucciniales) that famously split their life cycle between a juniper and a member of the rose family. It never forms a fleshy mushroom; instead the fungus appears as coloured spots, blisters, gelatinous horns and minute tube-like structures on living plant tissue.
The species is best known from garden and orchard pears (Pyrus communis), where the upper leaf surface develops rounded spots that begin yellow-green and ripen to intense orange with a red-brown centre. Turn the leaf over and the matching patch on the underside carries a cluster of pale, finger-like or cage-like projections. These are the aecia, technically roestelia, and their open, latticed sides give the fungus its English name.
On the other side of the life cycle, savin and related junipers (Juniperus sabina, J. chinensis, J. scopulorum) carry perennial spindle-shaped swellings on twigs and small branches. In wet spring weather these swellings extrude bright orange-brown gelatinous horns, a sight quite unlike anything produced by the pear stage.
How to identify it
Fruiting structures on pear
Spots on the upper leaf surface are circular to slightly irregular, 3-10 mm across, glowing orange with a darker centre and often a paler halo. Held to the light they are translucent. Heavily marked leaves may carry dozens of spots, and the same colouring can appear on green fruit and young shoots.
Trellis tubes (aecia)
Directly beneath each orange spot, the underside swells into a brown, blister-like cushion from which 5-20 tubular projections emerge, each 3-6 mm long, pale buff to grey-brown. As they mature the walls split lengthwise into strands, leaving an open lattice or miniature bird-cage through which dusty orange-brown aeciospores escape. A hand lens makes the trellis structure obvious and is the single most reliable field character.
Structures on juniper
Infected juniper twigs develop elongated, spindle-shaped swellings that persist and enlarge for several years, the bark eventually cracking. In spring rains these galls push out gelatinous, tongue- to horn-shaped telial masses 5-15 mm long, orange to brownish-orange and jelly-soft when wet, shrivelling to an inconspicuous crust when dry.
Spores
No spore print can be taken. Aeciospores from pear are globose, thick-walled, warty and orange-brown in mass; the teliospores in the juniper horns are two-celled, brown and borne on long gelatinous stalks that swell dramatically in water.
Where and when it grows
The fungus requires both hosts within dispersal range, typically a few hundred metres to a few kilometres. The telial (gall) stage occurs on savin juniper and several ornamental junipers, especially in gardens, parks and hedges; the aecial (trellis) stage occurs on cultivated and wild pear, and occasionally on quince and related pome trees. It is common across Europe, extends into western and central Asia and North Africa, and has become established in parts of North America and Australasia where ornamental junipers and pears grow side by side.
Spring rain triggers the juniper horns, which release basidiospores that travel to expanding pear leaves. The pear spots appear from late spring, and the trellis tubes ripen through mid to late summer, sending aeciospores back to juniper in autumn.
Ecological role
Gymnosporangium sabinae is an obligate biotrophic parasite: it draws nutrients from living cells and cannot be grown on dead tissue. Its heteroecious, demicyclic life cycle needs two unrelated hosts to complete, and unusually for a rust it produces no urediniospores, so the pear stage cannot spread from pear to pear.
On juniper the fungus is perennial, living in the bark and cambium of the gall for many years and fruiting each spring. On pear the association is annual and ends when the leaf falls. Because it kills no host outright, the fungus persists as a long-term resident of juniper populations, with the pear phase acting as a seasonal bridge that regenerates the juniper infections.
Similar species
Other Gymnosporangium species produce similar orange leaf spots on pome hosts. Cedar-apple rust (G. juniperi-virginianae) forms globose, golf-ball-like galls on juniper rather than spindle swellings, and its aecia on apple leaves are shorter, cup-like and less obviously latticed. Gymnosporangium clavariiforme makes long, unbranched, tongue-shaped horns on juniper and its aecia occur on hawthorn, where the swellings are on stems and fruit rather than leaves.
On the pear side, orange to rusty leaf spotting can also come from Entomosporium leaf spot, which produces small dark red-purple spots with a pale centre and no projections underneath, or from ordinary autumn leaf pigmentation. The presence of split, cage-like tubes on the lower surface is decisive: no other pear leaf fungus builds them.