trimming lylac bushes

I have some bushes that were trimmed early spring,new growth is covered in PM.Was wondering rather then fight it just trim off all the new growth.Im in Canada so winter isnt far away.There pretty hardy so I dont see a problem,just normally do in Feb,March.Thx
Frost is about 2~3 weeks away.

The energy going into the next harvest happens right after the blooms die. The best time to trim them back is right after the flowers die. That being said, lilacs are very resiliant. I moved into a house that had a 'tree'. We cut it back by about 1/3 taking the largest branches as close to the ground as we could. The next year, same thing. Repeated yearly until we got it under control and nice healthy branches and blooms. If your growth is covered in powdery mildew you gotta get it off because it will continue to do damage and it will spread. You might have crappy blooms this year, but at least it won't be all dead. You probably need to thin the bush anyways. If it's got PM, then there's no airflow within the bush.

What I read about lilac trimming is you trim the blooms only after the flowers die. For hard pruning you do like early or mid March on a nice day. This march hard prune the left 1/3, next march the middle 1/3 and last 1/3 in year 3. This way you still get blooms each year and you can see what's what. But you can hard prune the whole thing if you have to I guess if you don't have time. Saw a youtube of that.

Thanks. My problem is half of the bushes live under a patio cover. So air flow on the half under doesn’t get the air flow as it’s a 3 walled patio.I trim new growth every spring so I don’t get flowers on that half. Gets pretty messy.this year we had a cold wet July and the PM went to town.Maybe a big box fan or similar will keep it back not sure. The half that’s out in the sun/wind didn’t have a problem. Here’s a pic.

Here’s the outside half.

oh yes for sure that's very pretty. I see with like only 3 main trunks per plant it would be hard to do what I was thinking. Maybe try cutting like 1/3 of what's under the roof on a plant down to where the leaves start there about head height but I'm not sure. Just a thought.
Cute dog too.

well lilacs are decidious and dont bleed (like a maple) so spring time is your best bet IMO, gives the plant a whole season to get solid new growth out. Im unsure of your climate though. In michigan if i were to prune now it wouldnt do much, all the engery of the tree is stored for the year, so it wont be healing much until spring, if at all. Some trees like oak though are okay to prune after first frost, due to their smelly sap that leaks and attracts beetles causing oak wilt and other diseases. Killer patio btw. I could live there, who even needs a house.

Edit: If it were me, i would do a straight line chop a foot or two above that red flower pot on the left. Cut straight across. Start the branch ramification over since its kinda a mess. They grow like crazy so in 2-5 years youll have really nice treciary branching and definite pads of foliage if done right. Highly recommend watching this
I dont know if he covers deciduous in that vid, but you get the idea of how the foliage should look, pads with negative space so air can flow through better.

Ya the outer half gets out of control but its my afternoon shade so ill keep the hieght .The inner half Ive already cut off all new growth,goes about 4'in one season.Ive removed every leaf and vacuum my lawn twice.Ill be ready next year for the PM attack.