Wheat Stem Rust
Puccinia graminis
Also called black stem rust, black rust, cereal stem rust
The classic cereal rust, forming ragged brick-red pustules along wheat stems that turn coal-black late in the season, with orange cluster-cups on barberry in spring.
Overview
Puccinia graminis is the most studied rust fungus in the world and the type example of a macrocyclic, heteroecious life cycle: five spore stages divided between a grass host and barberry. It belongs to the genus Puccinia in the Pucciniaceae, an enormous group of obligate plant parasites.
On wheat, barley, oats, rye and many wild grasses the fungus erupts through the stem and leaf sheath surface in elongated, ragged pustules that shed rusty brick-red powder onto fingers and clothing. The torn epidermis around each pustule gives the stem a characteristically shredded appearance, quite different from the neat round pustules of the leaf rusts.
Late in the season the same pustules darken and are replaced by hard black telia that adhere firmly to the straw, the source of the name black stem rust. Straw carrying these black streaks over winter, and in spring the teliospores germinate to infect barberry (Berberis) and Mahonia, where the fungus makes bright yellow spots above and crowded orange cluster-cups below.
How to identify it
Uredinial stage on grasses
Pustules (uredinia) are elongate, 3-10 mm long, oval to linear, and run parallel with the stem. They break through the epidermis violently, leaving a torn, papery fringe around each pustule. Colour is brick-red to cinnamon-brown, and the spore mass rubs off readily. Pustules occur mainly on stems and leaf sheaths but also on leaf blades, glumes and awns, and can be found on both leaf surfaces.
Telial stage
From late season the same lesions become telia: elongate, hard, shining black streaks that do not rub off and remain firmly embedded in the straw. Teliospores are two-celled, thick-walled, dark brown with a rounded to pointed apex and a persistent stalk.
Stage on barberry
Infected barberry leaves show bright orange-yellow thickened spots, often slightly raised, with tiny dark spermogonia on the upper surface exuding sticky droplets. The underside carries dense clusters of cup-shaped aecia with a white, ragged, outward-curled rim, packed with orange aeciospores. Fruit and young shoots can be affected too.
Spores
No spore print applies. Urediniospores are oval, echinulate and rusty-orange; a hand lens plus the torn stem epidermis is usually enough to separate stem rust from other cereal rusts in the field.
Where and when it grows
The grass stage occurs wherever wheat, barley, oats, rye and wild grasses grow, from temperate plains to tropical highlands, and the fungus is effectively global. Warm days and heavy dew favour rapid pustule development, so outbreaks are most explosive in humid, warm summers.
The barberry stage occurs in hedgerows, woodland edges and gardens where common barberry and related Berberis and Mahonia grow, mainly in temperate regions. Where winters are mild the fungus can skip barberry entirely and survive year-round as urediniospores on green grasses, riding long-distance wind currents between regions in what is often called a rust corridor.
Ecological role
The fungus is an obligate biotroph: it feeds through haustoria inserted into living host cells and dies with the tissue it occupies. The complete life cycle passes through spermogonia and aecia on barberry, uredinia and telia on grasses, and basidiospores from germinating teliospores that return to barberry.
Barberry serves as more than a bridge; sexual recombination happens there, generating new genetic combinations in the fungus. Where barberry is absent, populations reproduce clonally through urediniospores and rely on wind transport and green grass hosts to persist between seasons. The rust is a significant driver of grass population dynamics in natural grassland as well as in cultivated cereals.
Similar species
Wheat leaf rust (Puccinia triticina) makes small, round to oval orange-brown pustules mostly on the upper leaf blade, with the epidermis only slightly ruptured, and its telia stay under the epidermis as grey-black patches. Stripe rust (Puccinia striiformis) forms narrow yellow pustules arranged in long stripes between the leaf veins, a pattern no other cereal rust makes. Crown rust (Puccinia coronata) on oats produces bright orange pustules and teliospores with a crown of apical projections, and it alternates with buckthorn rather than barberry.
On barberry, the orange cluster-cups can be confused with those of other Puccinia species using the same host, but stem rust aecia are densely crowded on markedly thickened yellow-orange spots with conspicuously frayed, recurved white rims.