Hollyhock rust

Puccinia malvacearum

The disease that comes with the plant: orange pustules under every hollyhock leaf by midsummer, working up from the bottom until the stems are bare.

Fungus Serious Airborne sporesRain splash Spreads to neighbors

What you'll see

Bright orange-brown raised pustules crowding the underside of the leaf, especially along the veins.
Matching yellow-orange spots showing through on the upper surface.
Lower leaves yellowing, browning and hanging dead on the stem while the top still flowers.
Pustules on the stems and on the green flower bracts as the season goes on.
A plant that is bare-legged and ragged by August every single year.

Why it happens

A rust that lives only on the mallow family and spends the whole of its life cycle on it — no alternate host, so it goes round and round on hollyhocks and on the wild common mallow in the hedge. It overwinters on the old leaves, on the crown, and on the seed, and infects the new leaves in spring from a few centimeters away.

Warm and damp: it needs several hours of leaf wetness to infect. It builds from the bottom of the plant up because that is where the humidity and the splash are. Almost universal wherever hollyhocks are grown, and a wet summer makes it much worse.

Treating it

Pick off affected leaves as they appear, starting with the lowest, and bin them.
Strip the bottom 30 cm of leaves off the plant early in the season to break the splash route upward.
Cut the whole plant to the ground and remove it once the flowers are over rather than leaving it standing.
Water at the base in the morning and never over the foliage.
A sulfur or myclobutanil spray from early in the season helps, but it has to start before the first pustule, not after.

Keeping it away

Grow hollyhocks as biennials from fresh seed and pull them after flowering rather than keeping old plants.
Remove common mallow and any other malva from the boundary; it is the reservoir.
Clear all hollyhock debris and fallen leaves in autumn and do not compost them.
Space plants well and grow them where the morning sun dries them quickly.
Try the figleaf hollyhock, Alcea rugosa, which is markedly more resistant than the common one.

Who gets it

Hollyhocks, common mallow, lavatera, sidalcea and other members of the mallow family.