Just last year, I cut back this large shrub of lilac which I believed was somewhat diseased. So this season it seemed to really do well up until just a week ago. The new growth appeared to be thriving well so I don't know what happened. It seems like the alleged disease reappeared. Please advise.

A few of the lilac varieties in my collection do this a bit at this time of the year, but this year, with considerably more rain than usual, they are all looking really good. Is it possible your weather over the years, plus maybe the location of this lilac (shady, dry/hot, poor air circulation or drainage etc.?) is the cause here?
When you cut it back last year did you remove all the cut material from the site and dispose of it?

I did quite a cut back of the shrub and I may have just left a few leaves in the area, but as I said earlier, the thing was really thriving well during the summer months up until about two weeks ago and then I noticed it's starting to change, so do you recommend that I cut it back again and dispose of of all the clippings?? What ever you can offer me with advice would be greatly appreciated. Thanks much.!!

Here is some information about powdery mildew. Cleaning up fallen leaves is critical to control.

https://extension.unh.edu/blog...

https://hortsense.cahnrs.wsu.e...

If there is also the telltale white dusty bloom on the leaves, then yes, probably powdery mildew that is rarely this severe on lilacs. Possibly other leaf diseases that are becoming more and more common on lilacs with climate change.

Whatever happened, it seemed to happen all at once. Think back 2 weeks before you noticed it. What was the weather like? Humid, rainy? Good conditions for disease spores or bacterial infections(1 week before) to occur?

The attack is incredibly uniform. Could it be some pesticide drift/application, something gotten into the soil?

Having switched from my phone to my desktop, I agree with Leftwood that it's unlikely powdery mildew. Apologies for jumping the gun on that. I don't have an answer.

Clearer close-up photos of individual leaves, front and back without sharp shadows will be helpful. Pull them off and place on a plain background in open shade. When finished, place them securely in trash bin and wash your hands and the surface well.

Thank you all for all your positive input regarding the issue with my lilac shrub. I can always depend upon this most informative website relative to all yard and garden matters. Thank you dearly.

it looks like a fungal disease lilacs get, mine had that, every year it starts out normally, blooms sparse, and then by mid July it turns brown and most of the leaves fall off. I treated with a few things but never had any luck. It was planted out all by itself so plenty of air flow. I ended up just digging them out.

Thr crud whatever it is even got on my white lilac. Thanks all.

Powdery mildew has become devastating due to climate change in SE Iowa. 6 days out of seven now in summer, are total cloud cover often with spitting rain, unless we actually get a spell of 90+ heat. We're lucky even on hot days if we get a few hours of actual sunlight in the morning or late afternoon. When it's actually warm, then we have extreme drought until the heat passes. I've lost fields of monarda, and mint, and most of my other ground covers. Everything is being attacked by fungus. Virtually lost 3 giant red honeysuckle vines as a result. 40 year old house plants I've always kept on the patio in summer are etiolating outdoors from lack of sunlight. I've never seen weather like this in my 65 years in this region, and it keeps getting worse. We're in it again now. Weeks of rain in the forecast with no sunlight whatsoever. I've had to start spraying in spring just to keep shrubs alive, before the summer monsoons destroy them. My peonies and everything else look like your lilacs by June when the first week of 10" of rain usually starts. We've had about 30" of rain here this summer, and that was including two month long drought periods. This should be our driest month and we're at about 5" already with 4 more days of rain in the forecast. I need to relocate to a much drier locale. This really sucks!

I have had the same problem with my lilacs for a few years now. Finally researched it and learned of "lilac leaf blight" - see info here: https://www.uvm.edu/extension/...
I will be taking special care this year to clean up and dispose of all leaves and debris. Fingers crossed!

No answers from the so-called experts here. Been gardening for 60+ years in the Midwest, and am now dealing with fungal attacks on plants that were NEVER affected before, from weeks of endless spitting rain in summer, and constant 100% cloud cover. The constant wetness and lack of sunlight produce the conditions the fungus thirves in. After several seasons of attack, most plants succumb. You can try to start spraying fungicides in spring, but there's no guarantee it will work. Here, as this climate worsens, new pathogens are arriving annually, and different plants are affected each year. Now my daylilies which were pretty much immune to everything are showing signs of being attacked by something too. I've lost about 1/3 of what was planted in my yard due to fungus in the last few years. It moves from one herb, shrub, or ground cover to another, slowly spreading to, and wiping out everything. Heirloom roses with super disease resistance are looking real sad. You fertilize and get little new growth because the plants are SO stressed. I clean up yard waste like never before to no avail. That's your problem!

It's so discouraging to read these posts from various parts of our nation. It seems the issue that I had last year is has also got to my white lilac bush, which which has been most lively for the past few years so I have come to the conclusion that global warming with this fungus issue and all the rain that we've had as well there's no doubt the fault is there any fungal spray that is available to deal with these problems or is it wise to just cut them back and dispose thoroughly of all the infectious residue. Please ADVISE!!

YOU NEED A FUNGICIDE THAT WILL CONTROL Pseudocercospora

Copper is a general purpose one that is usually available, but not as effective as others. Applying a treatment BEFORE the disease is obvious is how it is done. A better fungicide may be needed once symptoms appear.
I have been growing and producing lilies and other plants in the wet PNW for decades, it cannot be done without fungicides during the wet season. Which is 9 months of the year. And anytime there is rain in the summer. My schedule for lilies and certain others plants was every 2 weeks during the rainy season. Since it can rain endlessly here in the spring it was not that easily done.
You will have a hard time finding anything if your nursery supply places have become extinct as they are here. Best to know what chemical you are looking for and go try to find it. You will not get any help from big box stores or supply & "feed" stores.
Best to do an internet search and get a list of chemicals to search for. My go to is Daconil, chlorothalinal is the chemical name. Do not keep pruning down the plant, it will just keep weakening it and you will have no blooms. Treat the disease, don't punish the plant for having it.

My "arboricultural specialist" was here yesterday to evaluate my hemlocks for hemlock scale. He also inspected my lilacs and recommended a copper-based foliar fungicide treatment to be applied at "budbreak" in the spring and regularly thereafter depending upon the weather. If you have no knowledgeable nurserymen/women in your area, I'd suggest googling "arboricultural specialist" and/or following up with your local cooperative extension for assistance. Good luck.

Just yesterday, I spoke with an individual at the Connecticut Agricultural center in New Haven regarding my lilac issues. It was basically the same advice that I obtained last year from a different individual at the center of just don't bother cutting it back. Just make sure all the leaves which have fallen from the limbs are policed up and disposed of properly. He mentioned to do it in about another two or three weeks and to just shake the limbs of the remaining affected branches and dispose of properly. And thank you again for the advice of the chemical application as well.