Primula leaf spots
Leaf spots on primulas are yellow spots, often with dead, brown centres, and are caused by several different fungi and a bacterium.
Scientific name: Various
Plants affected: Primula spp. (primulas)
Main symptoms: Yellow spots, sometimes with mould growth on undersides of leaves
Caused by: Fungi and a bacterium
Timing: From Spring until Winter
What are leaf spots?
Leaf spots on primulas are caused by several fungi including Ramularia interstitialis, R. primulae, Phyllosticta primulicola and Puccinia primulae, and appear during wet weather from spring until winter, persisting on leaves all year. The bacterium Pseudomonas syringae can also cause leaf spots.
Leaf spots can occur on primulas, polyanthus and auriculas, although are less common on auriculas.
You may see the following symptoms:
- Yellow spots on the upper surface of the leaves, sometimes with a brown, dead centre. On the corresponding under surface there may be a white fungal growth (Ramularia spp.), minute black fungal structures embedded in the dead tissue (Phyllosticta), or with the rust fungus Puccinia very small orange cup-shaped structures. No growth or structures are visible with the bacterial leaf spot
- Eventually, infected tissues die completely, go brown and may drop out, leaving holes in the leaves
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.
- Remove and destroy infected leaves as soon as infections are seen
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. They would probably also give some control of the other primula leaf spot fungi (and the term 'leaf spot of ornamentals' is mentioned on the label of the products containing tebuconazole with trifloxystrobin).
The following products contain a combination of both insecticide and fungicide, enabling the control of both damaging invertebrates and disease: 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 invertebrate damage is not a problem on the plants treated.
No products with activity against bacterial leaf spot are available to amateur gardeners.
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)
Species of Ramularia and Phyllosticta cause leaf spots on many plants. Ramularia infections are sometimes called white moulds because of the white fungal outgrowth from infected tissues. Infections caused by rust fungi are not strictly classified as leaf spots, but the symptoms caused by Puccinia primulae on primulas will be seen by gardeners as spots.
Ramularia and Phyllosticta produce spores in the infected tissue and these are spread by water splash to form new infections. They can survive over winter in dead tissues in the form of resting structures. Since most primulas are perennial, there is probably also some survival of the pathogens in lesions on living leaf rosettes from one year to the next.
Puccinia primulae spreads by means of airborne spores produced from the small orange structures on the undersides of infected leaves. It also has a type of resting spore which carries the fungus through the winter although it, too, can also survive over winter as infections on the leaves of perennial rosettes. The whole life cycle is completed on primulas, there is no alternate host. Puccinia primulae seems to be confined to Primula vulgaris (primrose) and P. veris (cowslip).
Pseudomonas syringae is also spread by water splash and thus favoured by wet conditions.
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