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
- 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.
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