White Pine Blister Rust
Cronartium ribicola
Also called blister rust, currant rust, pine blister rust
A two-host rust that raises orange, blister-like sacs on the bark of five-needle pines and clothes currant leaf undersides with yellow pustules and hair-like brown columns.
Overview
White pine blister rust is caused by Cronartium ribicola, a member of the Cronartiaceae within the rust order Pucciniales. Like most Cronartium species it is heteroecious, requiring a pine and an unrelated broadleaf host to complete its life cycle, and it produces no cap or stalk of any kind.
On pines with needles in bundles of five, the white pines and stone pines (Pinus strobus, P. monticola, P. lambertiana, P. albicaulis, P. flexilis, P. cembra and relatives), the fungus lives perennially in the bark and inner tissues. The affected zone swells into a spindle-shaped canker with roughened, discoloured bark, and each spring the bark ruptures to expose crowded white to cream sacs that split and shed brilliant orange-yellow spore powder.
The alternate hosts are currants and gooseberries in the genus Ribes, with some Pedicularis and Castilleja also acting as hosts in western North America. Their leaves develop yellow-orange powdery pustules on the underside in summer, later replaced by dense clusters of stiff brown bristles.
How to identify it
Signs on pine
Branches and stems carry fusiform swellings, often with a slightly sunken, roughened surface and a margin of resin flow. In spring the swollen bark cracks and pushes out clusters of blisters (aecia) 1-3 mm high, covered by a white or pale cream membrane. When the membrane tears the blisters release copious orange-yellow aeciospores, and after spore release the area is left pitted and roughened. Small honey-coloured droplets of spermogonial exudate often appear on the same bark a season earlier.
Signs on Ribes
The upper leaf surface may show faint yellow flecking. The underside carries scattered to abundant yellow-orange uredinia, each a small powdery pustule under a millimetre across, appearing from early summer and able to spread from leaf to leaf all season. Later in the year these are replaced or accompanied by telia: dense clusters of stiff, hair-like brown to reddish-brown columns 0.5-2 mm long, giving the leaf a bristly, felted texture under a lens.
Spores
No spore print applies. Aeciospores from pine are orange, thick-walled and coarsely warty; urediniospores on Ribes are echinulate and orange-yellow; teliospores are one-celled, arranged end to end into the characteristic brown columns.
Where and when it grows
The rust occupies cool, moist temperate forests wherever five-needle pines grow within a few kilometres of currants or gooseberries. Native to Asia, it is now widespread through Europe and, since introductions in the early twentieth century, across northern and western North America from the Great Lakes and Appalachians to the Rockies, Sierra Nevada, Cascades and Pacific Northwest, reaching high-elevation whitebark and limber pine stands.
The pine phase is perennial and fruits in spring, aeciospores travelling to Ribes leaves. The Ribes phase runs through summer and produces the telial columns in late summer and autumn; the short-lived basidiospores from those columns move back to pine needles under cool, humid, foggy conditions, entering through the stomata and growing down into the branch.
Ecological role
Cronartium ribicola is an obligate biotroph and a true parasite of living tissue on both hosts. Its full macrocyclic life cycle involves five spore stages spread across two host genera, and the pine infection is perennial, expanding a few centimetres a year along the branch and toward the main stem.
Ecologically the fungus is a major structuring force in high-elevation white pine forests, thinning cone-bearing crowns and altering stand composition and regeneration. The Ribes stage is annual and comparatively mild, with the leaves shed each year, so the shrub acts as a summer amplifier rather than a long-term reservoir.
Similar species
Other stem rusts of pine can look similar at a distance. Western gall rust (Endocronartium harknessii) attacks hard, two- and three-needle pines and forms rounded woody galls rather than elongated cankers, and it needs no alternate host. Comandra blister rust (Cronartium comandrae) also makes spindle cankers with orange blisters but occurs on hard pines and alternates with Comandra. Pine-pine gall rust and Peridermium species on jack and lodgepole pine similarly favour hard pines with needles in twos and threes.
On currants and gooseberries, the orange summer pustules resemble those of Puccinia ribesii-caricis, but that species produces cup-shaped aecia with a fringed rim on Ribes and lacks the hair-like brown telial columns. Anthracnose and leaf-spot fungi of Ribes leave flat dark spots without powdery orange pustules.