Western Gall Rust
Endocronartium harknessii
Also called pine-pine gall rust, peridermium gall rust
A rust of hard pines that swells branches into round woody galls which split each spring and release clouds of bright orange spores.
Overview
Western gall rust, Endocronartium harknessii (long known as Peridermium harknessii), is a rust fungus of the Cronartiaceae that differs from most of its relatives in needing only one host. It completes its life cycle entirely on pines, spreading from pine to pine without an alternate broadleaf host, a condition called autoecious and, in this case, endocyclic.
The fungus is visible only through the deformity it creates. Infected branches and young stems swell into globose to pear-shaped woody galls, from a couple of centimetres to well over ten across on old stems. The galls are perennial, enlarging with the branch for many years and persisting long after spore production has ceased.
Each spring the gall surface blisters and flakes, and the powdery orange-yellow spore mass beneath is released so abundantly that a shaken branch can raise a visible cloud. After sporulation the gall surface is left rough, cracked and grey, often colonised by other fungi and by insects.
How to identify it
Galls
Rounded to slightly elongated woody swellings on branches and small stems, usually roughly spherical and centred on the branch axis. Young galls are smooth and green-brown; older ones are corky, cracked, grey and may be several times the branch diameter. Branches beyond a gall are frequently stunted, and repeated stem galls on young trees produce crooks and brooms of distorted growth.
Spore stage
In spring the outer bark of the gall separates into papery flakes and lifts to reveal a continuous, powdery layer of bright orange to orange-yellow aeciospores covering much of the gall surface. This is the only conspicuous fruiting stage; there are no blisters with persistent white membranes as in blister rusts, and no gelatinous horns.
Spores and hosts
No spore print can be taken. The spores are thick-walled, warty and orange in mass, and unusually they germinate directly to infect new pine shoots rather than needing an intermediate host. Confirmation of identity relies on host type: only hard pines with needles in bundles of two or three are used.
Where and when it grows
The fungus is widespread through the pine forests of western and central North America and extends into the Great Lakes region and eastern Canada. Principal hosts are lodgepole pine, ponderosa pine, jack pine and shore pine, with Scots pine and other planted hard pines also affected in plantations and shelterbelts.
Spore release coincides with the flush of new shoot growth in spring, and infection requires wet, humid weather while the young shoots are still elongating and thin-barked. Galls become visible one to two years later and then persist for the life of the branch. Dense young stands, plantations and regeneration after fire are where galls are most abundant.
Ecological role
Endocronartium harknessii is an obligate parasite of living pine tissue. Because it recycles directly from pine to pine, populations can build up rapidly within a single stand without any dependence on a second host species, in contrast to Cronartium relatives.
Inside the branch the mycelium is perennial, stimulating the cambium to produce extra woody tissue, which is what creates the gall. Old galls are important microhabitat: they crack and rot, admit wood-decay fungi and insects, and provide nesting and feeding sites for birds. Branch breakage at galls also contributes deadwood to the forest floor.
Similar species
White pine blister rust (Cronartium ribicola) forms elongated spindle cankers rather than round galls, occurs on five-needle pines, and produces blisters with a white membrane. Comandra and sweetfern blister rusts (Cronartium comandrae, C. comptoniae) also swell hard pine stems but produce elongate, more diffuse cankers and require alternate hosts. Eastern gall rust (Cronartium quercuum) makes similar round galls on hard pines but alternates with oaks, and separating the two often relies on geography and host association rather than the gall itself.
Non-rust swellings can also mislead. Insect galls and stem burls are woody but never shed orange spore dust, and mistletoe brooms produce clustered shoots and visible parasitic plant tissue rather than a smooth spherical swelling.