Symptoms and Spread
Lilac leaf spot typically appears in late summer, from July to September. Leaves start to yellow, then develop brown, blocky spots that grow larger over time, causing the leaves to twist, curl, and eventually fall off. The fungus thrives in warm, humid conditions, especially when temperatures range between 70- and 80-degrees Fahrenheit.
The disease likely begins in the spring or early summer when spores from overwintered leaves land on new foliage. Depending on the weather, the disease can spread quickly or take weeks to develop. This leaf drop can happen well before fall, which can weaken the overall health of the plant.
High-Risk Areas
While lilac leaf spot is usually not deadly, repeated infections over several years can weaken plants, potentially leading to branch dieback. Overgrown lilac bushes and dense windbreaks, especially in humid areas, are at higher risk. Even mature plants that have never had issues before are being affected, raising concerns about the disease's long-term impact.
Management Challenges
There are currently no fungicides specifically labeled for lilac leaf spot, and the timing for treatment is unclear. The University of Minnesota Extension recommends non-chemical management, such as proper pruning and removing leaf litter, to reduce the chance of reinfection.
Weather's Role
The severity of lilac leaf spot is closely linked to weather patterns. The disease was particularly bad in Minnesota during the warm, humid years of 2020 and 2021. However, the drought and extreme heat of 2022 and 2023 led to fewer reported cases, although some infections were noticed later in the season.
Identifying Lilac Leaf Spot
Gardeners should know that lilac leaf spot can be mistaken for other diseases like lilac bacterial blight or verticillium wilt. Correct identification is important since treatments differ. For example, bacterial blight often causes curled, drooping shoots and can produce a visible ooze, while verticillium wilt causes sudden branch die-off due to blocked water flow in the plant.
Looking Ahead
While lilac leaf spot is unlikely to kill healthy lilacs, repeated infections and other stress factors could lead to a decline in plant health over time. Researchers are studying this disease to better understand its long-term impact and potential management strategies. In the meantime, homeowners should monitor their lilacs, practice good sanitation, and consult with local extension services if they suspect an infection.
Author: Robin Trott, Extension educator, Douglas County