Antirrhinum rust is the most serious disease of antirrhinums (snapdragons). It is a fungal disease that causes dark brown spots on the undersides of leaves. Severely affected leaves shrivel and may die.
What is antirrhinum rust?
Antirrhinum rust is a disease caused by a fungus (Puccinia antirrhini) that spreads by airborne spores. It is specific to antirrhinum cultivars, although some claim to be resistant (see Control section below). Expect to see it from late spring to autumn.
You may see the following symptoms:
- On leaves: Initially, pale yellow spots appear on the upper leaf surface, corresponding to dark brown, dusty pustules on the lower surface. Later, leaves shrivel and vigour is greatly reduced
Cultural methods are of little value, though prompt removal of infected leaves early in the season may delay the build-up of the disease.
Resistant varieties are available, although the rust can adapt to and overcome this resistance. ‘Coronette’, ‘Monarch’ and ‘Tahiti’ series have all been claimed to be resistant.
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 the control of rust on antirrhinums and other ornamental plants. The following four products contain a combination of both pesticide and fungicide enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Westland Rose rescue); tebuconazole containing deltamethrin (Bayer Garden Multirose Concentrate 2), and triticonazole containing acetamiprid (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun). When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated. These products will also give incidental control of powdery mildew. The products permitted under organic regimes have little effect on rusts.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
The fungus originates from western North America on related species, but proved highly infectious to the European Antirrhinum majus when accidentally introduced to the UK and mainland Europe in the last century. It releases dark brown spores from the pustules on the lower leaf surface and these are spread by the wind to initiate new infections on leaves.
The rust fungi are described as biotrophs; that is, they grow within the living tissues of the plant and extract nutrients from the cells over an extended period. However, although they do not kill tissues rapidly, heavy attacks by rusts can cause tissues to collapse and die prematurely and this is the case for antirrhinum leaves. This leads to a great loss of vigour and an unsightly plant.
Occasionally, overwintering resting spores are produced but these are relatively unimportant and it principally survives from one year to the next on infected plants. Under favourable, wet conditions the disease can build up rapidly and is often unnoticed in the early stages because it usually starts on the lower leaves. Like many rusts, the full life cycle requires alternation between two unrelated plant hosts, but in Europe only the stages on antirrhinums occur.
Antirrhinums may also be affected by powdery mildew and by root and foot rots, particularly on wet soils, but these problems are of relatively minor importance compared with rust.