I have several Purplelicious Veronica plants that were planted last summer. They survived the winter and looked fine for the first several weeks but now the first growth looks yellow and unhealthy. Last summer I had a problem with mould (white dust) forming on the leaves. I try to water only the soil and not the leaves. The plants are in full sun. How can I prevent more mould? I don’t want to get the surrounding plants infected.
Thank you for contacting the Toronto Master Gardeners with your inquiry.
It can be very frustraating to see a favorite perennial fall victim to a disease. You mention that you see “white dust” on the leaves. You do not mention if you see this on the upper surface of the leaf or on the underside of the leaf. Veronica is susceptible to Powdery Mildew.
Powdery mildews are common diseases on ornamental plants. A variety of microscopically distinct fungi cause these diseases. In most cases, the fungus is host specific which means that the fungus that is affecting your Veronica cannot infect plants of different genera. Powdery mildew outbreaks are more apparent during years that are humid or damp and we certainly have had a lot of wet weather this spring.
Powdery mildew rarely causes serious damage to the plant, but this disease can discolor leaves, disfigure shoots, and deform flowers. The best control for powdery mildew is to prevent it from occurring in the first place. Powdery Mildew spreads by spores so water drops that splash onto infected leaves cause spores to spray onto healthy foliage, making the problem worse. Watering the soil and not the leaves as you are doing is a very good way to stop the spread of the disease. Good air circulation between plants by thinning and dividing every couple of years also helps in preventing the spread.
When an outbreak does occur, pick off the infected leaves and throw them in the garbage instead of the composter. Garden fungicide sprays available at your local garden centre work to prevent the spread. As always, be sure to read and follow the directions on the label.