Good Morning, 

Thank you for contacting the U of M Extension Service. 

We have had many questions from residents about their lilac shrubs. The problem is a bacterial leaf spot disease. Here is a link from the U of M Extension Yard and Garden website about leaf spot diseases: https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#bacterial-leaf-spots-and-blight-1156513

If you scroll through the information there is a section about management. Hopefully the shrubs will recover. 

Lilacs should be pruned in the spring AFTER they bloom, to encourage more blooms. 

Here is a link about pruning shrubs to rejuvinate them: https://extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs

Good Luck!

OK, So is now a good time to spray with "copper or streptomycin as an active ingredient"?

Is there a better time of year for this treatment?

Thank you!

No, at this point, fungicides will not work. Please read through the information I previously sent to you.