Hi Pamela,

The pictures you submitted indicate that it could be herbicide damage or it could be something else. Right after your lawn was sprayed did the lilac leaves look distorted, curled, cupped or puckered? That would indicate that the herbicide accidently got on your lilac or possibly too much was applied to the soil. The leaves eventually turn brown as your pictures show. It would help the diagnostic process if I knew what that intermediate stage looked like. There are a couple ways your lilac could have been exposed to the herbicide sprayed on your lawn. The first way is spray drift or volatilization. With even the slightest breeze any herbicide applied as a liquid spray can drift onto nearby plants and cause damage. Some herbicides such as 2,4-D and dicamba may volatilize after application. These herbicides can evaporate for hours after application and move as an herbicide cloud onto other plants. Herbicide volatiles can be just as injurious as liquid sprays. Herbicides are most likely to volatilize when air temperatures are in excess of 80F.

The second way is by the herbicide leaching through the soil or running off the soil. Many herbicides are active in soil, which means that they can injure or kill susceptible plants whenever roots come into contact with the herbicide. Most injuries involving soil-active herbicides are the result of the chemical moving from the original site of application into the root zone of another plant. Movement occurs by leaching (water moving an herbicide through the soil), or by run-off of treated soil into an area where it is not wanted. Injury is often the result of both types of movement. Here are a couple articles that have pictures of herbicide damage.

Herbicide Damage to Trees and Shrubs | University of Maryland Extension

Unintended herbicide injury on trees: A growing concern | Pesticide News | Illinois Extension | UIUC

Herbicide Injury | USU

The symptoms I see in your pictures could also indicate your lilac may have some sort of fungal leaf disease like septoria leaf blight. If you decide that your lilac’s symptoms indicate you have leaf blight here are some ways you can get it under control.

• Clean up fallen leaves to help reduce the likelihood of reinfection next season.
• Renewal pruning to decrease density and consistent watering will help support the stressed plant. Opening up the shrub canopy by removing a third of the largest branches allows more light to penetrate the canopy and better airflow to reduce humidity and moisture. This reduces favorable conditions for fungal growth.
• When the time comes to plant new shrubs, look for those that are resistant to leaf diseases. Note that “resistant” doesn’t mean immune, just less susceptible. Here is a link to a publication that lists some resistant varieties Lilac: An old-fashioned, favorite shrub - Gardening in Michigan

• Implement renewal pruning and proper plant care (watering, mulching, fertilizing) to help the plant recover from the disease. Fertilize early in the season, not late in the summer or fall.
• Monitor next spring for plant damage.
• Fungicide sprays can be applied before the disease is noticed or when the first spots are seen.
• If a plant is already showing severe symptoms from a fungal leaf spot, spraying fungicides on it will have little benefit as the fungicides are protective rather than curative.
• Contact a certified arborist for assessment and treatment options. www.treesaregood.org

Lilac issues and diseases | UMN Extension

For a confirmed and precise identification of the problem, I recommend sending all your images by email to the MSU Plant & Pest Diagnostic Lab. The experts there can also provide you with the best treatment options. Their email is <personal data hidden> They may request that you send in a sample. If you chose to do that there may be a small fee. Their website is here: Plant & Pest Diagnostics

Hope this helps.