Wheat Stem Rust Identification Guide

What You Are Actually Looking For

Most fungal identification starts with a fruit body pushed up out of soil or wood. Wheat stem rust gives you nothing of the kind. There is no cap, no stipe, no gills. The entire visible fungus is a series of pustules that erupt through the outer skin of a living grass plant, and recognition is a matter of reading those pustules: where they sit on the plant, what shape they are, what colour they are at that moment in the season, and how violently they have torn the host tissue on their way out.

First Impression and the Setting

You will be standing in a stand of wheat, barley, rye, or in rough ground among wild grasses such as Elymus, Agropyron or Bromus, in the warm, humid middle of summer. From a few paces the stand looks faintly rusty, as if someone had dusted the stems with brick powder. That transfer test is the single most reliable first impression: brush a palm firmly along a stem, and if the hand comes away the colour of powdered terracotta, you are dealing with a cereal rust, and the question narrows to which one.

Reading the Uredinial Stage

The reddish summer pustules are called uredinia, and four characters do the work.

Position first. Wheat stem rust concentrates on stems and leaf sheaths, and it also takes the glumes and awns of the head and both surfaces of the leaf blade. That preference for the stem and sheath is the primary field separator from the other cereal rusts, which are largely leaf-blade fungi.

Shape second. Uredinia of this species are elongate, oval to linear, running lengthwise with the axis of the stem. Individual pustules commonly reach 3-10 mm long, and in a heavy infection they run together into long broken streaks that can extend for several centimetres.

Colour third. Brick red to reddish-brown, sometimes almost cinnamon, and always powdery rather than matt or waxy.

Texture last, and this is the character people underuse. As the pustule expands it ruptures the host epidermis and pushes the torn tissue back into conspicuous ragged flaps on either side of the opening. Run a fingernail along the edge and the tissue lifts as papery shreds. That shredded, blistered look, felt as much as seen, is characteristic.

The Telial Stage

Late in the season the same lesions change character completely. The brick-red powder is replaced, often in the same pustule, by a matt to shiny black crust. These are the telia. They are hard, they do not smudge onto the fingers the way the summer spores do, and they stay welded to the dead straw right through winter, so blackened stem lesions on last year's stubble are worth examining in early spring.

Scrape a little of a black lesion onto a slide. The teliospores are two-celled, thick-walled, dark chestnut brown, rounded to slightly pointed at the apex and tapering into a persistent brown stalk. A two-celled, stalked teliospore is the defining character of the genus Puccinia.

The Barberry Half of the Life Cycle

This is a heteroecious, macrocyclic rust, meaning it needs two unrelated hosts to complete its cycle, and the second host looks nothing like a grass. In spring, on common barberry and a handful of related Berberis and Mahonia, you get the other two spore stages. The upper leaf surface develops small yellow-orange spots peppered with minute flask-shaped spermogonia, visible with a lens as pinprick dots that sometimes exude a sticky droplet. Directly beneath, on the lower surface, the leaf erupts in dense clusters of pale orange, cup-shaped aecia with recurved, white, fringed margins. The old name for these is cluster cups, and the description is exact: they look like a bed of tiny torn paper cups filled with orange dust. Aecia also appear on petioles, young twigs and occasionally the fruits.

Nothing about the barberry stage suggests a grass rust, so most people meet the two halves as separate organisms before connecting them. Find cluster cups on barberry in May and brick-red stem streaks on grasses in July in the same landscape, and you are watching one fungus.

Spores Instead of a Spore Print

You cannot make a conventional spore print here, because there is no cap to lay face down. The equivalent procedure is a scrape. Hold a sheet of white paper under a pustule and flick it with a fingernail, or draw a slide with a drop of water directly across a lesion, then cover and look at x400.

Urediniospores are oval to ellipsoid, roughly 20-40 micrometres long, orange-brown, thick-walled, and covered in short blunt spines, with germ pores set around the equator. Spiny orange summer spores plus stalked two-celled black winter spores from the same lesion is as close to a diagnostic pairing as this group offers without molecular work.

Substrate and Season

The fungus is an obligate biotroph, so it only occupies living tissue and you will never find it on dead plant matter that it killed outright. Its grass hosts sit in the Poaceae and include wheat, barley, rye, occasionally oats, and a long list of wild grasses. Specialised host-restricted forms exist, but they are separated by which hosts they colonise rather than by anything you can see.

Timing runs as follows: spermogonia and aecia on barberry in spring, uredinia repeating on grasses from late spring through summer with a new generation roughly every one to two weeks in warm, humid weather, and telia forming as the grasses ripen and dry. Urediniospores travel enormous distances on the wind, so the rust turns up every year in regions with no barberry at all, arriving on airborne spores or overwintering as uredinia on green grass wherever winters stay mild.

Separating It from the Other Cereal Rusts on Morphology

Stripe rust, Puccinia striiformis, forms small bright yellow to orange-yellow round pustules arranged in narrow lines that are confined between the leaf veins, so the leaf looks striped. It sits almost entirely on the blade, barely disturbs the epidermis, and favours cool weather.

Leaf rust, Puccinia triticina, forms small round to slightly oval orange-brown pustules scattered at random across the upper surface of the blade. The epidermis is lifted but not shredded into flaps, and stems are rarely involved.

Crown rust, Puccinia coronata, on oats and with buckthorn as its alternate host, is bright orange on leaves; its teliospores carry a crown of finger-like projections at the apex, which is unmistakable at x400 and settles the matter alone.

Stem rust, then, is the one with large, elongate, brick-red pustules concentrated on stems and sheaths, with the epidermis torn conspicuously back.

Two non-rust confusions are worth a note. The smuts, such as Ustilago, replace the whole grain head with a mass of black powder rather than forming pustules along a green stem. Fungal leaf spots such as Septoria make flat, sunken, bordered lesions with no powdery spore mass to rub off.

A Practical Field Sequence

Rub a stem and check whether orange dust transfers. Note whether the pustules sit mainly on stems or mainly on blades, and whether their outline is elongate or round. Check whether the epidermis is torn into flaps. Revisit late in the season for the black telial stage. Finally, look for barberry nearby, and in spring check its leaves for cluster cups.