Identifying Powdery Mildews

It is no exaggeration to say that every sycamore (Platanus) tree in my city is heavily infested with powdery mildew. A few days ago, I also noticed powdery mildew on crape myrtle (Lagerstroemia indica), although not to the same extent as the sycamores. In my search for information, I found an article by Texas A&M Extension titled Powdery Mildew on Crape Myrtles, with pictures that look a lot like mine, and a species mentioned: Erysiphe lagerstroemiae. I noted that atricle as my source in revising my identification on the observation.

The specific epithet lagerstroemiae suggests that this mildew is host-specific on crape myrtle; which further suggests that the one on the sycamores is a different one. So my question is, does this pattern hold? Can I identify powdery mildews by identifying their host plants?

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Yes, in many cases powdery mildews are species-specific or genus-specific, but you need to be aware that there are some powdery mildew species that attack a wide range of hosts.

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In addition to Susan’s comment, I would also add there are some host species with multiple host-specific powdery mildews (Phlox is one example that comes to mind). You will probably have to take a look at the literature for each host to have an idea of the current situation.

I did quick scan through the literature for Platanus and it looks like Erysiphe platani is a pretty safe ID for Platanus powdery mildew on both Sycamores and London Planetrees

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Reliably reported mildews on Lagerostroemia seem to be Erysiphe australiana, E. lagerstroemiae, Phyllactinia lagerstroemiae and Pseudoidium yenii. Some sources report that E. australiana and E. lagerostromiae are synonyms. If so then the correct name is E. australiana (1899) and not E. lagerstroemiae (1933). The opposite assertion has been made but I don’t see why.