Leaf spots on primulas are yellow spots, often with dead, brown centres, and are caused by several different fungi.
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.
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, fuzzy fungal growth (Ramularia spp.), minute black fungal structures embedded in the dead tissue (Phyllosticta) or very small orange cup-shaped structures (Puccinia)
- Eventually, infected tissues die completely, go brown and may drop out, leaving holes in the leaves
- Remove and destroy infected leaves as soon as infections are seen
Products containing 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 use on ornamental plants for control of rusts. They would probably also give useful control of leaf spot fungi, although this is not claimed by the manufacturers.
The formulation of tebuconazole (Bayer Garden Multirose Concentrate 2) contains deltamethrin and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid to control insect pests. Avoid these unless an insect pest problem is specifically identified.
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 in Puccinia are not strictly classified as leaf spots, but the symptoms caused by P. primulae on primulas will be seen by gardeners as spots.
Ramularia and Phyllosticta produce spores in the infected tissue and these spread by water to form new infections. They can also survive over winter in dead tissues in the form of resting structures. Since most primulas are perennial, there is probably some survival of the pathogens in lesions on 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 can also survive 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).