Rust
Raised pustules on the underside of leaves that release orange, brown or dark powder when rubbed, with pale spots visible on the upper surface above them. Rust fungi are highly host-specific — leek rust lives on alliums and rose rust on roses — so the rust on one plant is not a threat to an unrelated one beside it.
Caused by: Pucciniales · also called leaf rust
What brings it on
Damp still air, crowded plantings and soft growth from heavy feeding. Most rusts need a period of leaf wetness to infect.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Rust diseases
Checked 2026-08-13
Usually seen: June, July, August, September, October
Preventing it
Ordered by how much difference each measure actually makes, not by how often it is repeated.
- Space plants and avoid wetting the foliage. Clearing affected debris at the end of the season reduces what carries over to the next one.
source
Sourced guidancemoderate confidence
- Royal Horticultural Society: Rust diseases — prevention
Checked 2026-08-13
Dealing with it once it is there
- Pick off affected leaves early. A light infection late in the season on a mature plant is generally tolerable and does not warrant treatment.
source
Sourced guidancemoderate confidence
- Royal Horticultural Society: Rust diseases — control
Checked 2026-08-13
Signs that lead here
Each of these lists the other things that produce the same sign, and how to tell them apart.
On sprays and treatments
GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.
Plant-specific guides
Where this is one of several causes that look alike on a particular plant, GrowingHQ publishes the differential rather than the general answer.