Antirrhinums can sometimes be affected by rust, and other fungal diseases.
It is thought that the disease originated in the UK through the importation of related species, and there has also reports that the disease is seed-borne, but this has not been confirmed as fact.
Symptoms;
Rust symptoms are seen as small, dark brown pustules on the undersides of leaves, and pale dimples on the upper surface.
The rust fungi grows within the living tissues of the plant and extract nutrients from the cells over an extended period.
Under wet conditions the disease can build up rapidly and often go unnoticed in the early stages because it usually starts on the lower leaves.
In severe attacks leaves shrivel and die.
It is caused by a plant specific fungus (Puccinia antirrhini) that spreads by airborne spores which over winter on plants kept from one season to the next.
Control;
Prevention is better than cure, so grow cultivars that are reputed to be rust resistant, and destroy plants at the end of the season to avoid the carryover of fungus spores.
Regular spraying with a suitable fungicide to the manufacturer’s instructions or removing affected leaves can sometimes control the problem.