Viola Flowers: Protecting them from disease
December 23, 2019
Nurtured in Norfolk’s gardening expert Martyn Davey answers all your questions about viola flowers.
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Please help I have grown some viola flowers in the garden from plants that I bought back in October from a reputable garden centre. The plants started well and have started to flower however the leaves have started to get lots of black spots on them and are looking like they are going to die. What should I do I don’t know if this is normal or not, in the past I have only grown them in pots and not had any problems can you please advise me on what I should do to save them?
G. Barnes, Thetford
Pansies and viola flowers may be attacked by various leaf spot diseases that disfigure the plants. Plants in containers and those in the ground can be equally affected. Spots and blotches on the leaves of pansies are caused by the fungal pathogens Ramularia lactea, R. agrestis and Mycocentrospora acerina. Pansies also suffer from downy mildew caused by the fungus-like (Oomycete) organism Peronospora violae.
Symptoms vary, depending on the disease that is to blame;
Ramularia lactea: Dark greasy spots appear on the leaves, becoming brown in the centre.
Ramularia agrestis: Pale tan spots and blotches similar to chemical scorch appear on the leaves.
Mycocentrospora acerina: Very dark blotches appear on the leaves and stems, and the crown of the plant may rot.
Peronospora violae: Yellowish blotches appear on the upper leaf surface, corresponding to a grey, fuzzy, fungus-like outgrowth on the lower surface.
All of these pathogens produce spores on the leaves that are spread by rain over short distances (leaf spots) or by wind over long distances (downy mildew). Mycocentrospora acerina and Peronospora violae also produce tough resting structures in the dead tissues which can contaminate the soil for several years, and the Ramularia spp. probably also do this.
The Ramularia spp. and Peronospora violae only infect pansies and violets, but Mycocentrospora acerina affects a wide range of plants and also causes storage rot in carrots.
Cultural controls include avoiding repeat planting on the same site to prevent build-up of these pathogens in the soil. Spores of the fungal spots are not wind-borne or seed-borne, so be aware that the most likely source of infection is plants which are already infected when bought in. Do not buy plants that show any signs of leaf spot.
Wild pansies and violets may be sources of infection for garden pansies.
The fungicide tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect) has a recommendation for the control of leaf spots on ornamental plants. Additonally, the fungicides tebuconazole (Provanto Fungus Fighter Concentrate) and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) are labelled for the control of other diseases on ornamental plants, and could therefore be used legally on pansy (at the owner’s risk) to try and control the leaf spotting. None of these fungicides will control downy mildew.
Jobs for this week:
Prune acers, birches and vines before Christmas to avoid bleeding.
Deciduous trees and shrubs can still be planted and transplanted.
Cover up outside taps and protect tender plants from the worst of the winter to come.
Martyn Davey – Head Grower