lilac problem

Asked May 12, 2020, 8:55 PM EDT

I have several varieties of lilacs. The last 2 years they have started showing signs of stress. I believe I may have more than 1 problem going on. I have attached 3 pictures. One problem I believe is powdery mildew so I bought chlorothalonil fungicide but then was told I shouldn't use this product on lilacs instead use tritconazole instead. (I'm not sure why, neither was the guy from ortho who told me that information.)

Barry County Michigan

3 Responses

Hello. About the fungicides you mention I could find no research based info on the efficacy of one active ingredient over the other. Your best option is to read the label carefully and see if lilacs/powdery mildew are listed. The challenge with using fungicides this early in the season is that they need to be re-applied every couple weeks (the label should say) and that the plant needs to be thoroughly covered. Some cultural notes for your shrubs are to provide full sun, good air circulation and move the mulch away from the trunks by 3-4”. If possible only irrigate in the mornings. One other thing, remove and destroy any fallen debris. Hope this helps!

I have read the labels on the products and found no answer, that's why I turned to you. But since you didn't find anything either I will try the product I purchased if the powdery mildew strikes again.
I failed to mention these pictures are from last year. I was trying to be proactive in treating my lilacs for this season.
You didn't mention anything about the 1st two pictures of my lilacs. I don't think either picture represents powdery mildew. Do you have diagnosis and advice for the lilacs in the 1st two pictures?
I will move my mulch away from the lilac trunks, I had not seen that advice in the literature for powdery mildew, thank you.

Hello. Sorry about the omission. The photo with the spots and green islands could be a virus and I suggest you email the photo to the diagnostic service at MSU. The email is (When you receive a diagnosis I would appreciate if you could let me know what it is.). The other leaf photo is not as clear and I suspect nutrient problems or a leaf spot. Using the previous cultural suggestions would be helpful. If the problem develops again please send this year’s photos to AaE. Good luck.