Lilac Bush Leaves turning brown

Question: I live in Central MN  I have a row of lilac bushes running the length of my property on the North and East sides. They were likely planted in the 1970s, not trimmed in recent years and are about 10 feet tall. We put cedar mulch down along the bottom of the plants last spring to give a border between the bushes and the lawn. 
Last summer about this time, the leaves began to turn brown around the edges then all over. They dried up and fell off. This spring the bushes came back nicely and bloomed very well. In the last week or so, all the leaves have again turned brown and are falling off.  The branches appear to be healthy – not brittle.  The leaves are turning all over the plant from top to bottom, EXCEPT one area on the northwestern corner where they are shaded by a large tree.  
Can you please tell me what I need to do to ensure the health of these bushes. One area garden center said it was blight and to spray them. Another center advised calling you and perhaps giving them a good trim.

responder: Deb Kroon  Replied July 22, 2024, 5:02 PM EDT
response: Thank you for reaching out to ask2.extension. You could very definitely have blight. Conditions have been excellent for fungus this year.  I can’t get close enough in the picture to see for positive. They need to be sprayed earlier in the season to prevent blight, not to treat it. A copper fungicide spray is recommended, but there are quite a few conditions to using it. You can consider cutting them back, but unfortunately that may not be the answer.  If you cut trim them or let then go, read up on treating blight over the winter so you can treat them next spring. I dealt with this a couple years ago and ended up removing the bush.  I only had one infected though. If you cut them back, be sure to clean and disinfect your tools so you don’t spread it to something else. I’m sorry this happened.

responder: The Question Asker Replied July 23, 2024, 9:53 AM EDT
response: Thank you for your quick response. I have attached a photo showing the progression of the leaves turning.  To be clear, there is not much to do right now. Can I cut them back this fall? Then vacuum up all the leaves? I assume I would wait until the leaves are out in the spring before spraying. When I spray, do I spray like I am washing the bushes?  There is quite a lot to do as you can see.  Should I spray the portion that is not infected now, to prevent spread to them? Thank you Dianne Gideon    From: ask=<personal data hidden> <ask=<personal data hidden>> On Behalf Of Ask Extension Sent: Monday, July 22, 2024 4:03 PM To: Gideon, Dianne <<personal data hidden>> Subject: Re: Lilac Bush Leaves turning brown (#0148161)     You don't often get email from <personal data hidden>. Learn why this is important

responder: Deb Kroon  Replied July 23, 2024, 5:48 PM EDT
response: Unfortunately there isn’t much right now.  The copper based fungicide can’t be sprayed after the temps have reached a certain point.  That’s why it’s suggested in the spring.  I’m not sure if another fungicide would help or not. If these were my bushes I’d cut them back now to prevent further spread.  It’s not the best to trim in the heat of the summer, but I don’t know how much more things will spread before fall.  If you trim them back, be sure to water them good.  This will be a shock to them, but watering them should help.  A drip to the roots would be better than a sprinkler.  If you water with a sprinkler do so early in the morning so they have all day to dry out the leaves. I hope this will help.