is this powdery mildew?

[Nesmani]

this lilac bush has been at my mom's as long as I can remember. I just noticed this bout a month ago.
(nevermind the weeds.. they will be dug out)

https://farm4.static.flickr.com/3485/3855904137_eb8e978e2b.jpg

https://farm3.static.flickr.com/2638/3855904153_38078b39fa.jpg

https://farm3.static.flickr.com/2674/3855904165_9ed8d0e7ed.jpg

please help! is this powdery mildew? what can I do?

[Kisal]

It certainly looks like powdery mildew to me. There isn't much you can do to 'cure' it, but you can spray to prevent it from spreading to uninfected parts of the plant. Try spraying with a solution of milk and water, anything from a 1:10 to a 50:50 dilution rate ought to work.

Cutting the weeds might help prevent further spread of the disease, by allowing better air circulation.

This article has some good information, as well as a suggestion for an alternative organic spray you can use, if you prefer not to use milk.

https://www.uri.edu/ce/factsheets/sheets/powderymildew.html

[luis_pr]

You may also want to evaluate what is making the fungus thrive and see if there is anything you can do about it. If your area is simply experiencing a lot of rain then consider delaying the application of fungicides until the rains diminish, otherwise you will end up re-applying often and that can get tiresome.

[Nesmani]

thanx! that helps
I will prolly dig out the weeds tomorrow
I would like to prune a lil, I live in zone 5. the days are still 70's 80's here. can I still prune at this time of year?

[Kisal]

You can prune them now without causing actual harm to the plant, but you will be sacrificing some of next year's flowers. Lilacs set their buds shortly after they finish flowering. The best thing to do, IMO, would be to wait until next year, and then prune immediately after the flowers are done.

[rainbowgardener]

It is indeed powdery mildew. Kisal is right that if you prune all that off now, you will be cutting off most of next year's blooms. Lilacs are especially prone/ vulnerable to powdery mildew, especially big old ones like that, that were planted before they had created resistant hybrids. The good news is that it is mainly a cosmetic issue and doesn't particularly harm the lilac bush. Next year it will come back fine. The worst thing about it other than the cosmetic issues is that it may cause the leaves to drop earlier in the fall.

It won't spread to most other plants in your garden, e.g. roses, because the powdery mildew on roses is a different species than the one on lilacs.

I have a big old lilac like that in my yard and it gets powdery mildew that shows up late in the season every year. This year strangely enough, even though we had so much rain (humidity generally favors fungal infections), it had a milder case than usual, I think because the lilac really liked the extra water. The best thing you can do is maintain your tree in as vigorous health as possible. Then it can fight the infection off better.