Thanks for your question.

Two general remarks.

1). In most cases powdery mildew does no harm to your lilacs. The infection is only cosmetic. You could just ignore it. Many lilac growers do so.

2). I don’t think that the lack of blooming is related to the presence of powdery mildew.

Here are two thoughts about powdery mildew and lilacs:

1). While a combination of milk + water is effective against certain fungal diseases, there is no solid evidence that this combination can prevent the spread of powdery mildew in lilacs. I’m not surprised that you also found it ineffective. See:

https://www.growveg.com/guides/using-milk-to-prevent-powdery-mildew/?utm_source=chatgpt.com

https://wpcdn.web.wsu.edu/wp-puyallup/uploads/sites/403/2015/03/milk-and-roses.pdf?utm_source=chatgpt.com

2). Once a powdery mildew infection is seen in your lilacs, there is relatively little that can be done to make it “go away”. Preventative measures are better. See:

https://extension.umn.edu/plant-diseases/lilac-issues-and-diseases - :~:text=Managing fungal diseases,the first spots are seen.

https://extension.umn.edu/plant-diseases/powdery-mildew-trees-and-shrubs - :~:text=Here are some tips for managing powdery,or plant parts with little air movement.

https://hgic.clemson.edu/factsheet/powdery-mildew/ - :~:text=Sanitation and Pruning for Powdery,Pruning suckers as they develop

https://extension.unh.edu/blog/2018/09/what-should-i-do-about-powdery-mildew-my-lilacs - :~:text=Planting resistant cultivars is one,location with good air circulation.

With respect to your lilac not blooming for the last few seasons, this is usually due to improper pruning techniques and/or environmental influences. See:

https://extension.umn.edu/trees-and-shrubs/lilacs - :~:text=Causes for a lack of,so prune immediately after flowering.

https://yardandgarden.extension.iastate.edu/faq/can-i-do-anything-encourage-lilac-flower

Good luck. Please get back to us with any further, related questions. Thanks for using our forum.