Rose rust is a fungal disease of roses causing orange spots (pustules) on the undersides of leaves and on distorted stems.
What is rose rust?
Rose rust is a disease caused by the parasitic fungus Phragmidium tuberculatum and some other closely related species. It is specific to roses, and appears in spring and persists until the leaves fall.
Susceptibility to rust varies widely among rose cultivars, and most modern roses should be resistant to rust.
Rose rust is the least serious of the common rose diseases; black spot and rose powdery mildew are far more prevalent.
You may see the following symptoms:
- On stems: In spring, distorted young green stems, with large pustules of bright orange dusty spores
- On leaves: Yellow spots on the upper surfaces, corresponding to pustules of dusty orange spores on the lower surface. In late summer, the orange pustules turn black. Infected leaves may fall early
- Prune out spring infections as soon as they are detected, to prevent the spread of spring spores
- Collect and destroy fallen leaves in autumn to reduce the number of overwintering resting spores
- If infections are persistently troublesome, it may be that the cultivar concerned is unusually susceptible, so consider replacing it with a different one
The fungicides difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra, Roseclear Ultra) are approved for rust control on ornamental garden plants.
Some formulations also contain insecticides but these are best avoided if no insect pest problem is specifically identified. For example, some formulations of myclobutanil (Westland Rose Rescue) contain cypermethrin, some formulations of tebuconazole (Bayer Garden Multirose Concentrate 2) contain deltamethrin, and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid. Spray in spring and summer according to the manufacturers’ instructions.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
The fungus causing rose rust is, like all rusts, a biotroph: it infects the host tissues for extended periods without killing them, feeding on the living cells. Like all rusts, it is not able to survive on dead plant material, so must either alternate with a different, perennial host, or produce a resting spore to pass the dormant season.
Phragmidium tuberculatum and several other very similar species which infect roses do not have an alternate host; that is, they only attack roses and pass the winter as resting spores.
The first formed spores (spring spores) infect young stems, causing distortion and the production of bright orange pustules. These in turn infect the leaves to produce dusty orange spores (summer spores) which are spread by wind and initiate further infections. In late summer, the pustules producing summer spores switch over to produce the dark, tough resting spores. These spores survive the winter often adhering to stems or trellises. And then the infection starts over again in spring.
Infections may be severe enough to cause serious damage, but this is relatively rare and most infections are light enough not to require control.