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.
Scientific name Puccinia antirrhini
Plants affected Antirrhinum
Main symptoms Yellow spots on the upper leaf surface, dark brown pustules on the undersides
Caused by Fungus
Timing Late spring to autumn
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
- On stems: In severe infections pustules can also be produced on stems, and dieback may occur
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
Prompt removal of infected leaves early in the season may delay the build-up of the disease slightly, although weather conditions have the greatest influence over the speed of spread. Dispose of plants at the end of the season.
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 tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are approved for the control of rust diseases on ornamental plants.
The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Resolva Rose 3 in 1, Rose Shield Bug & Fungus Killer, Roseclear Ultra Gun 2, Rosegarde) and triticonazole containing acetamiprid (Roseclear Ultra, 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.
Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
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 the fungus 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, particularly on wet soils, by root and foot rots, but these problems are of relatively minor importance compared with rust.
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