Hollyhock Rust Identification Guide

What You Are Actually Looking For

Puccinia malvacearum is a rust, so the field routine is different from that for a cap-and-stalk mushroom. There is no cap, no stalk and nothing to pick up. The whole fungus lives inside the leaves and stems of a living mallow-family plant, and the only part you can see is the erupting spore layer breaking through the plant's own skin.

The setting is a garden border, an allotment, a cottage front path, a derelict lot or a coastal waste ground - anywhere hollyhock (Alcea rosea) is grown or common mallow (Malva sylvestris), musk mallow, tree mallow, marsh mallow or checkerbloom is growing wild. It appears from late spring, builds through summer, and is at its heaviest in humid late-season weather.

The first impression is of a plant looking speckled and tired: leaves stippled with small pale-yellow or orange dots on the upper side, lower leaves browning and hanging, and the whole rosette looking as if it has been peppered with shot.

Reading the Upper Leaf Surface

Start with the top of a leaf. The lesions here are small, roughly circular, pale yellow to bright orange-yellow spots, typically 1-4 mm across. Early in an infection they are faint and translucent - hold the leaf up to the light and they glow. As they age, the centre of each spot often becomes a dull tan or dead brown, sometimes with a narrow reddish or purplish rim.

Note carefully that these upper-surface spots are flat. Run a fingertip across them: the leaf feels smooth. This discolouration is the plant's reaction to the fungus, not the fungus itself. On a heavily loaded leaf the spots run together and the whole blade goes chlorotic and then brown, and severely affected leaves distort, curl and drop, leaving bare lower stems.

Turning the Leaf Over - the Diagnostic Surface

Now turn the leaf. This is where the identification is actually made, and it is unambiguous.

Directly beneath each yellow spot, on the underside, sits a raised, wart-like pustule. These are the telia - the spore-producing structures - and they are what makes this a rust rather than any of the ordinary leaf spots.

Read them with a hand lens:

• Shape: hemispherical to conical, blister- or pimple-like, standing clearly proud of the leaf surface, usually 1-4 mm across. Run your finger over the underside and you can feel them as distinct bumps.
• Colour: this is the key. They are dull brown, cinnamon-brown, tan or greyish-brown - never the bright orange dust that most rusts produce. Very young pustules can be paler and creamier; old ones darken and become dry and crusty.
• Texture: erumpent. The pustule begins under the leaf's epidermis, swells, and splits it open, so a ruptured pustule is often surrounded by a small ragged white collar of torn plant tissue.
• Content: firm and somewhat waxy or crusty rather than a loose powder. Rub one and it smears; it does not puff away as a cloud.

Pustules are not confined to the leaf blade. Examine the petioles, the green stems, the flower bracts and the outside of the calyx - on a well-established infection, all of these carry elongated or streak-like pustules, and heavily infected stems can become roughened, swollen and distorted.

Reading the Spores Without a Spore Print

You cannot take a spore print from a rust, because there is nothing to lay face down. Substitute a tape mount.

Press a short piece of clear sticky tape firmly onto a mature pustule on the leaf underside, peel it away, and stick it to a glass slide over a drop of water. Alternatively scrape a pustule with the point of a knife onto a slide.

Under the microscope the spores are the confirmation:

• Two-celled teliospores, divided by a single cross wall, the two cells often unequal.
• Pale golden-brown to cinnamon, thin- to moderately thick-walled, with the apex of the upper cell often thickened and drawn out into a short point.
• Each spore carries a long, colourless, persistent stalk (pedicel) - noticeably long relative to the spore body, which is one of the most distinctive features of this species.
• Size roughly 35-60 microns long by 15-25 microns wide including the apex, plus the pedicel.

You may also see spores that have germinated in place, sending out a short tube that has divided into a four-celled basidium bearing tiny stalked basidiospores. That is normal for this fungus and diagnostic in its own right.

Crucially, look for what is absent: there are no orange, single-celled, echinulate urediniospores, and no cup-shaped aecia with white fringed rims. This rust skips those stages entirely.

Life Cycle and Ecology

Puccinia malvacearum is microcyclic and autoecious - the whole life cycle runs on a single mallow-family host and involves only the telial stage plus the basidiospores it produces on germinating. There is no alternate host, and there is no repeating summer spore stage of the kind that lets most rusts sweep through a stand in days.

That has practical consequences for identification. Because there are no urediniospores, you will never find the bright orange, powdery, dusty pustules typical of cereal or poplar rusts. The pustules you find on hollyhock are brown, waxy and comparatively few - but they persist a long time.

The fungus overwinters as mycelium and mature telia in infected basal leaves, in old stems and on the crown of the perennial plant. In spring, sustained leaf wetness triggers the overwintered teliospores to germinate, releasing basidiospores that are carried a short distance by air and splash to infect new leaves. Infection needs several hours of free water on the leaf, so the disease follows wet weather, dense planting, overhead irrigation and shady, still corners. Visible pustules typically break through one to two weeks after infection.

Spread through a garden is short-range - from the lower leaves upward on the same plant, and from plant to neighbouring plant - which is why infections so often look worst at the bottom of a stand and on plants that were infected the previous year. It is now essentially cosmopolitan wherever hollyhocks are grown.

Separating It From Look-alikes on Appearance Alone

Fungal leaf spots (Cercospora, Alternaria, Ascochyta, Colletotrichum) on the same plants produce spots that are flat on both surfaces. There is no raised pustule and no spore mass you can rub off. Lesions tend to be larger, more angular or zoned, and often grey-centred with a dark border.

Septoria-type spots carry tiny black dots - pycnidia - embedded within the dead tissue, visible with a lens as pinpricks in the spot centre. Those are sunken, black and hard, quite unlike a raised brown blister.

Downy mildew produces a soft, fuzzy, greyish or violet growth on the leaf underside beneath a pale angular blotch on top. It is a felt, not a blister, and it wipes away leaving nothing raised behind.

Powdery mildew is a superficial white bloom sitting on top of the leaf, easily rubbed off with a finger and leaving green tissue beneath. There is nothing erumpent about it.

Spider mite feeding gives fine pale stippling on the upper surface and webbing beneath, with no discrete pustules; the underside is dusty and silky rather than bumpy.

Other rusts on Malvaceae are much rarer. Puccinia heterospora and Puccinia sherardiana occur on other mallow-family genera in some regions; microscopic examination of teliospore shape and pedicel length is the practical way to distinguish them, but on hollyhock and common mallow the brown, long-pedicelled, two-celled telia of P. malvacearum are what you will find.

Field Sequence

1. Identify the plant as a member of the mallow family.
2. Note pale-yellow to orange flat spots on the upper leaf surface.
3. Turn the leaf over and confirm raised brown pustules aligned with those spots.
4. Check petioles, stems, bracts and calyces for the same pustules.
5. Confirm the pustules are erumpent, with torn epidermis at the rim.
6. Tape-mount a pustule: two-celled brown teliospores on long colourless stalks, no orange urediniospores.