Advice
RHS Help & Advice
Antirrhinum rust & other problems
Hosts
Antirrhinum majus.
Symptoms
Small dark brown pustules on the undersides of the leaves, with a corresponding pale indentation on the upper surface. The pustules shed dark brown spores. In a severe attack the leaves shrivel and die and the plants are severely damaged. Pustules may also be produced on stems and seed pods.
Biology
The disease is caused by the fungus Puccinia antirrhini. The airborne brown spores spread the disease. There is also an overwintering spore, but this is rare and most carry-over is on plants kept from one season to another. Early reports that the rust is seed-borne have not been confirmed.
Control
Cultivars with claimed resistance to rust are available. However, there are probably a number of different strains (‘races’) of the fungus, and new races may develop, so rust still sometimes affects ‘rust-resistant’ cultivars.
Regular early sprays with myclobutanil (e.g. Systhane Fungus Fighter) or penconazole (Scotts Fungus Clear) before the disease gets a hold will help control rust. Destroy plants at the end of the season to avoid carry-over of fungus spores.
Other problems
Antirrhinums are occasionally affected by powdery mildew, which can be controlled by the same fungicides as rust. They are also prone to foot and root rot diseases caused by soil-borne fungal pathogens. Chemical soil sterilisation is no longer an option for home gardeners, and rotation or changing the soil are the only controls available if the problem is severe.
There are no significant pest problems.