Without photos of the affected plant showing the problems, there's not much we can offer by way of advice.

I suspect that the flowers are showing signs of stress. In my experience, lilac blooms seem to be a kind of 1st level warning signal, maybe because the plant’s depleted energy results in sacrificed blooms before other more essential functions start shutting down. Smaller leaves often accompany flower issues in my experience. Later, there is dieback of one kind or another. What you want to know, though, is the cause, and I am afraid I am not sure about that key piece of info: too much water (poor drainage) or too little water, blight, borers,…. Are the affected lilacs all in the same area of your garden? You mentioned another nearby. Photos of the blooms, foliage etc. would help. Is your daughter around to take some pics? I am SO sorry your lilacs are suffering! They are usually tough and dependable fragrant beauties and such a magical part of spring, but even the steadiest of garden stalwarts seem to be having their moments these crazy days of global warming and other challenges. Carol

Melissa, since the leaves don't seem to be affected it probably is NOT a bacterial blight. It's the most common cause of lilac death here in the southern US. The various blights cause leaf spotting, yellowing, browning (looking burnt, even) and leaf drop. One blight causes the newest twigs to blacken and curl. The flowers will also usually turn brown while still in bud.

You might check for borers by cutting one of the affected branches and looking for obvious holes drilled in the bark, and for tunnels running the length of the branch. But, again, I don't think that's it, because you usually see signs of them in wilting leaves and drooping inflorescences.

I know your summers have been excessively hot lately, but I'm less clear if you've gotten less winter chill? Some varieties of lilac are much more touchy about lack of winter chill. But it usually doesn't affect vigor so much as preventing bud set so that they don't bloom well the Spring after an abnormally warm winter.

Your particular lilac symptoms are a bit of a head-scratcher for me, as the only thing I've ever seen affect the size and color of the blooms is either lack of fertility or sun, and I'm sure those aren't the problem in your garden! lol.

I do hope someone else can help, as mostly I can only tell what it is not.

John

Edit: I see Carol posted while I was typing, lol.

Lilacs want good drainage I know. We are hotter here than Melissa and vulgaris lilacs do fine.

are lilacs a thing in z8?...

just a quick link.. says lilac 5 to 7.. with significant winter chill requirements.. and they dont work in z9 ... so im wondering if you are pushing the tolerance of its native range ... or else you are more of a 9 than an 8 ...

i also wonder how USDA zone info translates to italy ?????

here is the easy question.. do you see much if any other lilacs around the area.. ????

ken

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-9-10-11/zone-9-lilac-growing.htm

LOL!! Don't believe everything you read :-) Common lilacs - Syringa vulgaris and hybrids - abound in my area (zone 8b/9a), bloom heavily and other than powdery mildew or the occasional incidence of lilac blight (Pseudomonas syringae), are pretty much trouble free!! These are a typical May Day flower here and are just about to pop into bloom.

There is a botanical garden dedicated to lilacs not far from here, developed by a lady involved in lilac breeding: Hulda Klager Lilac Garden

Ken, I worried about that too, but lilacs abound here and Melissa is in the mountains in Italy. She is not in Southern Italy.

Ah, the Hulda Klager Lilac Garden! Back around 1995 I was driving down the interstate in southwest Washington when I saw signs advertising the lilac festival there in Woodland. I turned off the road and went to visit it, and there began my passion for lilacs, which is unslaked to this day.

Ken, I know about the USDA hardiness ratings, but the Italian climate is different from that of most of the U.S. Lilacs are a traditional, popular plant locally and do extremely well. The weather pattern here is that temperature lows aren't very low--many winters it never drops below 20F--but there's abundant winter chill, that is, temperatures in the low forties, thirties, and high twenties. This is why we can grow both Tea roses and olives on the one hand, and tulips, apples, and peonies on the other. Summers are traditionally dry and hot, and the lilacs may like that as well. I got into conversation with a nurseryman once who told me that lilacs had disease problems in the plains (the Po Valley), where summer atmospheric humidity is greater than here; but in the hills they're great. Our fabulous weather patterns may be changing, though, with climate change, so I'll see what happens.

Carol and John, thanks for your ideas. The lilac in question is right by two other lilacs that are doing fine. The other two sufferers (and one was definitely diseased, with foliage damage that this one doesn't show) are located well away from this plant. I find it hard to imagine drought stress, drainage, or nutritional inadequacy being a problem, as the two neighboring lilacs are fine, and since the conditions are the same the lilac has been happily growing with for several years. They may not be great conditions in themselves, but lilacs like them.

(N.b. I find this bizarre, but the spring flowering this year, for all plants, has been amazing, in spite of low precipitation since last summer. We seem to be getting enough rain to keep the plants happy....groundwater recharge is a different question. And it has been raining lately, with hints of more starting in a few days. In late years I seem to notice a trend toward drier, sunnier winters and wetter springs, Or is it just a fluke, or my imagination?)

Carol, one of the baffling aspects of this lilac's behavior is that some inflorescences on the plant are normal, others look wrong; while the rest of the growth looks okay. I'm considering cutting out all the branches with the bad flowers, even though they otherwise look healthy. I did look up lilac diseases online before posting my question here, finding nothing that looked applicable.

If we can maintain our gardening activity, perhaps we'll undertake more lilac planting in a few years, say, when DH is ninety (this is sarcastic! although he is in his upper eighties). I was down in the newer part of the big garden, close to the road. Seven or eight years ago, perhaps, we planted a hedge of lilacs, mixed varieties, along the southern boundary of the garden, and property. There had been a sizeable landslide on the land below ours, and the slide was propagating up into our property. So DH, who was seventy-nine at the time, went out with planks, heavy rebar, and a maul as big as he is, and got busy shoring it up, then planted hazelnuts in the upper part of the slumping area, and lilacs along the boundary. There were seedling oaks there, too, which have since grown well. The ground slumped a bit but is holding. The lilacs look great--they love the slope, clay soil, and full sun and wind--only we need a few more dark varieties to set off all the lavenders, pinks, and whites. I'm considering continuing the hedge along the line where the garden bounds neighboring fields below it. I'd like some new lilac varieties, too, so need to go looking for other suppliers. All this naturally assuming we can keep on gardening.