This faint, powdery dust is the fungal disease powdery mildew, a very common disease of lilacs, particularly if the lilacs are in an area with shade and poor air circulations (and it rains a lot like this year). However, I find the disease even on lilacs in open, windy sites.
The disease is not serious enough on this plant to warrant control but if [you] do decide to spray a fungicide labeled for powdery mildew and containing chlorothalonil as the active ingredient, [you] should make at least two applications of the chemical, spaced 10 days apart.
The information above is an excerpt from the July 25, 2019 | Vol. 17, no. 23 Pest Update published by the South Dakota Department of Agriculture and South Dakota State University.
Read the entire report here
Topics covered in this Pest Update:
- Ash substitute – the Kentucky coffeetree
- Japanese beetle and roses
- Spotted wing drosophila update
- Treating chlorosis on oaks
- Cedar-hawthorn rust
- Golden buprestid – emerald ash borer look-a-like
- Pear sawfly (slug)
- Picnic beetles in raspberries
- Davison County (tree-of-heaven ID)
- Douglas County (western sand cherry ID)
- Grant County (maple anthracnose)
- Haakon County (powdery mildew on lilac)