In an ideal world, our deciduous trees and shrubs have beautiful green leaves from the minute they emerge in the spring until it is time for fall colors to arrive. Of course, nature and the environment will influence the longevity of those leaves depending on the weather and site history. And it appears that this year, we’ll likely see some early defoliation on our lilacs again, just like we saw two years ago. 
The weather has been highly conducive to the foliar fungal disease of lilac called Pseudocercospora leaf spot. In Iowa in 2024, we experienced what I called the Pseudocercospor-apocolypse in lilacs- an overwhelming amount of this disease showing up on lilacs early in the fall, leading to premature defoliation. However, in 2024 I first diagnosed Pseudocercospora leaf spot in late July, and this year I’m seeing it more than two weeks earlier.
What should you do if you think you have Pseudocercospora leaf spot on your lilacs?
- Don’t panic! Lilacs are tough plants and should be able to tolerate some defoliation as long as it isn’t a consistent occurrence.
- Don’t reach for the fungicide! Given the “polycyclic” nature (ability to repeatedly infect) of this fungal pathogen, fungicide applications will not help. In fact, fungicide applications are seldom warranted for foliar fungal diseases on trees and shrubs in the landscape, with rare exceptions.
- Do reach for the rake! The fungus causing this disease will remain in the environment on fallen and decaying leaves. Raking and removing fallen leaves is a critical tactic for limiting the potential for this pathogen to cause disease in the future.
- Do note that not every lilac shrub is susceptible to this disease. There are hundreds of cultivars of common lilac (Syringa vulgaris). With that kind of variability, naturally, the susceptibility to plant disease will also vary across cultivars. While we don’t have any data on which cultivars are more susceptible to Pseudocercospora leaf spot, I have observed that some lilac shrubs are virtually unaffected by this fungal disease, but may be affected by powdery mildew, another fungal foliar disease. So, knowing whether your lilac has a history of this disease is informative- applying a prophylactic fungicide for Pseudocercospora on a lilac shrub that has no history of the disease does not adhere to good integrated pest management practices.
You can read more about managing Pseudocercospora leaf spot in the Encyclopedia article on our website. An important first step to managing plant health problems is accurate identification of the cause of the plant health problem. If there is any uncertainty about what is afflicting your lilacs late this summer or fall, submit a sample to the Plant & Insect Diagnostic Clinic! The goal of the Plant & Insect Diagnostic Clinic is to provide accurate, reliable, and expedient plant health problem diagnoses so clients can make informed and sound management decisions. We are here to serve the state of Iowa.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 13, 2026. The information contained within may not be the most current and accurate depending on when it is accessed.