Treating Rose Rust: A disease guide for nurseries and wholesale growers

Identification

Bright yellow to orange pustules on the undersides of leaves. This is the easiest rose disease to recognize, and there is little chance of confusing it with anything else. From the top of the leaf, the same spots read as light yellow flecks. Long, narrow orange spots can also show up on young canes and sepals, not just leaves. By fall, the pustules darken to near-black as the fungus shifts to its overwintering stage.

The pathogen is causes by fungi in the Phragmidium family.

Rust triggers

Rust loves cool, damp weather. It really hits its stride when daytime temperatures are around 60–70 degrees, nights are cool, and leaves stay wet for 2–6 hours or more. Clouds, heavy dew, light rain, and poor airflow all give the fungus exactly what it wants. Once it gets going, the spores hitch a ride on even the slightest breeze or fan, so an isolated spot can quickly become a greenhouse- or field-wide problem.

Spray program

Several brands carry the same active ingredient under different names, so the active is what to look for. A reliable rotation of actives for rust are Myclobutanil, Azoxystrobin, Mancozeb, and Chlorothalonil. Rotate between them across the season to keep the fungus from building resistance to either one.

If you prefer a more natural approach, neem oil, horticultural oil, hydrogen peroxide, copper octanoate, and sulfur can all be effective on early, light rust infections before the disease really takes hold.

The rate and application method depends on which brand you use, always refer to the label and instructions on the bottle.

Rose Rust (early infection)

Rose Rust (established infection)

Cultural support

Disease control and prevention works best when plants are kept fertilized and spaced apart for consistent air flow. Water at the base of the plant or at the time of day that lets the leaves dry fastest. Because airflow spreads rust spores as readily as it dries leaves, pair good air movement with a strong spray program when needed.