Hollyhock Rust
Puccinia malvacearum
Also called mallow rust, malva rust
A short-cycle rust of the mallow family that studs hollyhock leaf undersides with orange-brown wart-like pustules, each matched by a yellow spot above.
Overview
Puccinia malvacearum is a rust of the mallow family (Malvaceae), best known on garden hollyhock (Alcea rosea) and on wild mallows such as Malva sylvestris and Malva neglecta. It is one of the simplest rusts to identify because it has an abbreviated life cycle: it produces only teliospores, skipping the aecial and uredinial stages entirely, a condition called microcyclic.
Because it needs no second host and no summer spore stage, the same pustules that appear in late spring keep generating new infections all season on the same plant and its neighbours. Colonies build steadily from the lowest leaves upward, and by late summer a hollyhock may carry pustules on leaves, petioles, stems, bracts and even the green parts of the flowers.
The fungus originated in the Americas on wild mallows and has spread with cultivated hollyhock to almost every temperate region where the plant is grown, making it one of the most familiar garden rusts worldwide.
How to identify it
Pustules
The defining structures are telia: rounded to slightly conical, wart-like pustules 1-4 mm across, packed on the underside of leaves and along stems and petioles. They begin pale buff to yellow-orange and darken through orange-brown to dull cinnamon or greyish-brown with age. Unlike many rusts the spore mass is not freely powdery; the pustules feel firm and granular and remain intact for weeks.
Upper leaf surface
Each pustule corresponds to a bright yellow to orange spot on the upper surface, typically 2-5 mm across and often with a slightly sunken centre. On a heavily colonised leaf these spots merge into irregular yellow blotches, and the tissue between them browns and dies from the leaf margin inward.
Stems and petioles
On stems the pustules are elongate rather than round and can be raised several millimetres, giving a distinctly bumpy, roughened outline to the stalk. Petioles frequently carry more pustules than the blade.
Spores
No spore print applies. Teliospores are two-celled, brown, smooth-walled and tapering to a point at the apex, borne on colourless stalks. They germinate in place on the living plant, which is why the fungus needs no other spore stage.
Where and when it grows
The rust occurs on cultivated hollyhock in gardens, allotments and borders, and on wild and weedy mallows along roadsides, waste ground, field margins and coastal banks. It also uses Lavatera, Althaea officinalis, Sidalcea and other Malvaceae.
It appears from late spring, builds through humid summer weather and can continue into late autumn while green tissue remains. Development is favoured by extended leaf wetness, crowded plantings and still, sheltered sites. Distribution is essentially worldwide in temperate and warm-temperate regions, following hollyhock cultivation, with a native centre in the Americas.
Ecological role
The fungus is an obligate biotroph confined to the Malvaceae and completes its life cycle on a single host, so no alternate plant is involved. It overwinters as mycelium and teliospores in overwintering rosette leaves, stem bases and plant debris, and on the seed coat surface, restarting on the first new growth of spring.
Ecologically it is a persistent, chronic parasite rather than a rapid killer. It reduces leaf area progressively over the season, weakening the plant and lowering seed production, then survives the winter on the plant's own perennating tissue. Wild mallow populations act as long-term reservoirs from which nearby garden hollyhocks are colonised each year.
Similar species
Other rusts of Malvaceae exist but are far less common in gardens; Puccinia heterospora and Puccinia sherardiana occur on wild mallows in the Americas and differ in teliospore shape, requiring microscopy. On hollyhock, leaf spots caused by Cercospora or Ascochyta are flat, grey to brown, and lack raised pustules on the underside, while downy mildew forms a fine greyish felt rather than discrete warts.
Spider mite stippling can also produce yellow flecking on the upper leaf surface, but turning the leaf reveals fine webbing and mites instead of firm brown pustules. The combination of yellow upper spots exactly matched by raised orange-brown warts beneath is unique to this rust among common hollyhock problems.