Orange Powder on the Underside of Leaves

Rust announces itself more clearly than most plant diseases, which makes it one of the few worth identifying from a photograph.

What it looks like

Small raised pustules on the underside of the leaf, orange to rust-brown, powdery when rubbed. The powder transfers to a finger, which is the confirming test.

Above the pustule, the upper leaf surface usually shows a yellow spot. Someone looking only at the top of the leaf sees yellow blotching and reaches for a fertiliser.

Later in the season the pustules often darken to brown or black as the fungus switches to its overwintering spore stage on the same leaves.

What it is not

The powder test settles nearly all of it. Rub the underside with a white tissue: rust leaves an orange smear and nothing else on the list does.

How it spreads

Wind-borne spores landing on a wet leaf. Infection requires several hours of continuous leaf wetness, which is the lever worth pulling.

Overhead watering in the evening provides exactly those conditions. Watering at soil level in the morning removes them, and that alone changes rust pressure substantially in a garden.

Many rusts are host-specific and several need two different host species to complete their life cycle, which is why removing a nearby unrelated plant occasionally solves a persistent problem.

Managing it

Remove affected leaves as they appear, and take them out of the garden rather than composting them. Home compost heaps do not reliably reach temperatures that kill spores.

Clear fallen leaves in autumn. Most rusts overwinter on debris, and a clean bed in spring starts the season with far lower pressure.

Improve airflow. Thinning congested growth shortens the time leaves stay wet, which is the single most effective cultural measure.

Water at the base, in the morning.

Fungicides work preventively rather than curatively, and they need to be on the leaf before the spores land. On an ornamental with a bad annual problem that can be worth doing; on a vegetable crop late in the season it usually is not, for the reasons in when not to spray.

Resistant varieties

The most durable answer for anything grown repeatedly. Rust-resistant roses, beans and hollyhocks exist, resistance is well documented by breeders, and it removes the problem permanently rather than annually.

Worth choosing at the point of buying, since replacing a susceptible plant that will get rust every year is easier than treating it every year.

When it does not matter

Late-season rust on a plant that is about to die back anyway is doing very little harm. Beans in September, a herbaceous perennial in October — the leaves have already done their work.

Tidying up the debris matters more than treating the leaves at that point, because it is next year's infection that is being decided.

Rust on lawns

A common and confusing case, because the symptom people notice is orange dust on their shoes rather than anything on the grass itself.

Lawn rust follows slow growth — low nitrogen, drought stress, or mowing during a dry spell. The treatment is to get the grass growing again with a feed and adequate water rather than to apply anything fungicidal, and it usually clears within a few weeks of the grass recovering vigour.

What the yellow spots mislead people into

Fertilising. The yellow blotching on the upper surface looks like a nutrient problem, and nitrogen gets applied to a plant that does not need it.

Soft new growth encouraged by a nitrogen feed is more susceptible to rust than mature tissue, so the response makes the problem worse. Turning the leaf over first is what prevents it, and it is worth making a habit of before diagnosing any leaf discolouration, as yellow leaves sets out.