White Pine Blister Rust Identification Guide

The setting and the first impression

This fungus produces no cap, gills or stem, so identifying it means reading a tree rather than picking a mushroom. You are standing in a stand of five-needle pine, in the mountains or in the northern forest, looking up at a crown in which one or two branches have turned rust-red while the rest is green. That pattern, called flagging, is what draws the eye from a distance, and it is the usual starting point.

The second half of the picture is at knee height and often a hundred metres away: a currant or gooseberry bush whose leaves are stippled underneath with orange. White Pine Blister Rust cannot complete its life cycle on either host alone, so a confident field identification means finding both signs in the same landscape, or finding one of them well enough developed to read on its own.

Working through the pine host

Check the host first. The pines involved are five-needle pines: eastern white, western white, sugar, whitebark, limber, and some high-elevation relatives. Pull a fascicle and count the needles. Two- and three-needle pines carry different rusts, and a count settles the question before you look at anything else.

Follow the flag back to the stem. Pick the dead or dying branch and trace it inward toward the trunk. The infection did not start where the needles died; it started further in, and everything beyond the infection point died because the conducting tissue was girdled.

Find the swelling. At the point where the branch or trunk is infected you will find a spindle-shaped or diamond-shaped swelling, thicker than the wood above and below it and often noticeably asymmetric. On a small branch this can be a neat fusiform bulge; on a trunk it becomes a broad, elongated, roughened zone.

Read the canker margin. The bark over an active canker is discoloured, often with a yellow-orange to brownish tinge at the advancing edge, and the surrounding tissue may be roughened, cracked or resin-soaked. Sticky resin flow is common and old cankers are frequently blackened with dried pitch.

Look for animal work. Squirrels and mice chew the sugary bark of active cankers, and gnawed, ragged patches of stripped bark around a stem swelling are a familiar secondary sign in whitebark and limber pine country.

The spore stages on pine

The pine phase produces two visible spore structures, and the season determines which one you find.

Pycnia. In late summer and autumn of the year after infection, and again in following seasons, the canker face exudes small droplets of a sticky, sweetish, honey-coloured to orange fluid. These ooze from tiny openings in the bark, dry to a varnished amber crust, and are easy to overlook unless the light is right.

Aecia. The following spring, usually from late spring into early summer, the canker erupts. Blister-like sacs push out through the bark, each a fragile white to cream membrane a few millimetres across. As they mature they split and release masses of powdery orange to orange-yellow spores, so an active canker looks as though someone has dusted it with paprika. After the spores are shed the ruptured membranes remain as papery white flecks. This blistering is the origin of the name, and it is the single most diagnostic sign on the pine host.

Needle spots. The very earliest stage, visible only with close inspection, is a scatter of small yellow or reddish flecks on individual needles where the fungus entered through the stomata. From there the fungus grows down the needle into the twig, so needle spots on a shoot leading to a young swelling are good confirmation.

Working through the currant and gooseberry host

Turn the leaves over. The undersides carry the summer stages.

Uredinia. From early to mid-summer, small yellow-orange pustules erupt on the lower leaf surface, each shedding powdery orange spores that can be smeared onto a fingertip. On the upper surface these show as diffuse yellowish blotches. Because this stage re-infects other bushes repeatedly through the season, a heavily infected bush becomes uniformly orange-speckled beneath.

Telia. Later in the summer and into autumn the same spots produce a completely different structure: short, stiff, brownish, hair-like or bristle-like columns, roughly one to two millimetres long, projecting straight down from the leaf underside. Viewed with a lens against the light they look like a coarse brown stubble or a miniature felt of spines. These columns are the most distinctive of all the currant-side signs; nothing else on a currant leaf looks quite like them.

Taking material and reading it

A spore print in the ordinary sense is not possible, but you can collect readable spore material easily. Press a strip of clear tape sticky-side down onto an erupting aecium or onto an orange pustule on a leaf, then stick the tape onto a slide or a card. In the field this gives you an immediate record of spore colour and abundance; under a microscope the aeciospores are orange, coarsely warted and broadly ellipsoid, and the telial columns are built of stacked, thick-walled, pigmented cells.

For a canker, cut a shallow slice with a knife at the swelling margin and look at the inner bark. Infected tissue is discoloured, often yellow-brown to orange-tinted, and the boundary between colonised and uncolonised bark is usually sharp.

Season and life cycle

The cycle runs on a fixed calendar that you can use as an identification aid. Orange spores released from pine cankers in late spring travel to currants and gooseberries. Through the summer the fungus multiplies on those leaves in the orange pustule stage. In late summer the brown columns form, and these produce a further spore stage that travels back to pine, entering through needle stomata in cool moist autumn weather. The fungus then grows slowly down the needle into the twig over one to two years before a swelling becomes visible, and pycnia and then aecia follow on that swelling in subsequent springs. Because of this lag, a young infection is invisible for a year or more after it happens.

Separating it from its closest look-alikes

Western gall rust. Occurs on two- and three-needle pines, and produces a round or pear-shaped globose gall rather than an elongated spindle. The whole surface of the gall erupts with orange spores, and the fungus needs no alternate host.

Comandra blister rust and stalactiform rust. Also produce elongated stem cankers with blistering aecia, but on hard pines such as lodgepole and ponderosa, not on five-needle pines. The needle count on the host separates them immediately.

Dwarf mistletoe. Causes swellings and witches brooms on pine stems, but the swelling carries visible jointed shoots or, after they drop, small basal cups, and there are no blisters and no orange spores.

Mechanical and weather injury. Old branch stubs, lightning scars, sunscald and animal rub produce discoloured, resinous stem lesions, but they lack the spindle swelling with a spreading margin, the amber pycnial droplets and the spring blistering.

Bark canker fungi such as Atropellis. Produce sunken cankers with dark-stained wood beneath the bark and small black fruiting structures, never white blisters or orange powder.

On currants: powdery mildew and leaf spot fungi. Powdery mildew is a superficial white bloom that wipes off, and common leaf spots are angular brown or purple lesions. Neither produces powdery orange pustules or brown bristle-like columns beneath the leaf.

Field checklist

1. Host is a five-needle pine; count needles per fascicle.
2. Flagged rust-red branches in an otherwise green crown.
3. Spindle or diamond-shaped swelling on branch or trunk, with a discoloured, often orange-margined, resinous canker.
4. Amber sticky droplets in late summer; white blister sacs shedding orange powder in spring.
5. Rodent gnawing around the canker.
6. On nearby currant or gooseberry: orange pustules under the leaves in summer, brown hair-like columns later.