If you're looking for a plant that will provide your garden with rich, vivid colour fifty two weeks of the year, well that's quite a tall order. We have just such a plant however: the perennial wallflower Erysimum 'Bowles Mauve'. It is an extraordinary plant, flowering continuously from the height of summer to the darkest days of mid-winter and back again. Perhaps this explains why the RHS chose to award the species its Award of Garden Merit (AGM)?
Recently (late March) I noticed something strange about this normally most trouble-free plant. At first I couldn't quite believe my eyes. Strange white blobs appeared on the foliage looking for all the world like spilt emulsion! Not only that but the affected stems were somewhat stunted, older leaves being yellowed and newer growth tinged purple. At first glance I was struck by the pattern of the growths. They are in discrete clumps, rather than the overall, diffuse pattern you might expect to see from a fungal condition such as a mildew. I reached the conclusion they were therefore probably a scale insects of some kind. I yearned for a decent microscope so I could look more closely to get a positive ID!
Unsure of what we were dealing with I decided to leave well alone to amass more evidence. I'm so glad that we did, because I think we now have a diagnosis. Yesterday Jo was rummaging away in the undergrowth as she sometimes does. When I went over to see what she was up to she described rubbing and affected leaf.......and a fine white powder being released into the air. Well that signed, sealed and delivered it. Clearly we were dealing with a fungus and the powder is the wind-borne spores. So much for the theory about scale insects.
A bit more research revealed that Erysimums, as improbable as it seems, are closely related to Cabbages and other members of the Brassica family. I suddenly realised that searching for diseases of Brassicas might finally lead us to our culprit. And so it turned out to be; I believe what were are seeing is the disease 'Brassica White Blister' which is caused by the wind-borne, fungus-like pathogen Albugo candida. I consulted Buckzacki & Harris' definitive book on Garden Pest & Diseases and found this description of Brassica White Blister:-
"Small white lesions, at first smooth but later powdery as the spores are produced, make affected plants look as if they have been splashed on the above ground parts with white paint. Distortion of the affected organs or of the entire plant results if the fungus permeates the tissues." Buczacki, Stefan. Pests, Diseases and Disorders of Garden Plants . HarperCollins Publishers. Kindle Edition.
Sound familiar? So we have our culprit at last, but what can we do about it?
Unfortunately the advice of Buckzacki and Harris is that the plant should be destroyed. I'm not sure if we have the heart do do that! There is another important lessons to be learned here about the importance of good garden practise. The weed Capsella bursa-pastoris (aka 'Shepherd's Purse') is able act as an 'alternate host' to the White Blister pathogen. Of course we can't prove that Shephard's Purse was implicated in our diseased wallflower. But we do have that weed in our garden and its early eradication would have done more good than harm.
To find out more about Shepherd's Purse see the excellent Wikipedia entry:-
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