Hosts
The genus Rosa and also Prunus laurocerasus (laurel). A different form attacks peach.
Symptoms
On roses a white powdery deposit appears on flower buds and on leaves, which may become distorted.
Biology
Caused by the fungus Podosphaera (syn. Sphaerotheca) pannosa.
It spends the winter either as a resting structure on fallen
leaves or as infections on young stems and in buds. In spring
spores from these sources infect young leaves, leading to
the production of white powdery spores that are spread by
wind and can re-infect even at low humidity. Like rust, this
fungus can only grow on living host tissue.
Non-chemical control
Powdery mildews are worse when plants are water stressed, so mulch to reduce this. Prune hard to remove overwintering sources on the stem.
Chemical control
Organic gardeners can use sulphur dust to control powdery mildews.
The following fungicides can be used:
Myclobutanil (Systhane Fungus Fighter, Fungus Fighter Ready-to-Use Spray, Multirose *, Multirose Ready-to-Use *, Roseclear 3 *, Roseclear 3 Gun! *, Doff Systemic Fungus Control)
Mancozeb (Dithane). Black spot and rust only.
Penconazole (Scotts Fungus Clear and Fungus Clear Gun!)
Sulphur dust (Vitax Green or Yellow Sulphur). Powdery mildew only.
* Note that some products also contain insecticides and should
not be used unless pests are also a problem.
RTU = Ready to
Use.