BRIAN PAIGE
DISEASE RECEIVES UPDATE BEACON AWARD
BRIAN PAIGE
DISEASE RECEIVES UPDATE BEACON AWARD
Pseudocercospora Leaf Blotch of Common Lilac
By Nicholas J. Brazee, UMass Extension Plant Pathologist
For the past three years( 2023 – 2025), common lilac( Syringa vulgaris) has suffered from a serious late season foliar disease caused by the fungal pathogen Pseudocercospora. Relatively little is known about the disease, but it has been widely reported on lilacs throughout the region. Overall, there are numerous Pseudocercospora species that cause disease on a variety of woody and non-woody plants. Symptoms of Pseudocercospora leaf blotch first appear in middle to late July but can be easily missed without careful scouting. In August and early September, symptoms intensify and appear as:( 1) light to dark brown spots and blotches scattered across interior portions of the leaf, and;( 2) a progressively worsening marginal leaf blight. As individual leaf spots coalesce, large necrotic blotches can form, leading to a complete foliar blight. Pseudocercospora sporulates from the dead tissue on the underside of the leaf, producing large numbers of cream-colored spore tendrils. These spores are splashed and blown onto adjacent leaves and plants, initiating new infection centers. The recordsetting rainfall in July of 2023 is likely to blame for the initial outbreak that year. This created a large amount of overwintering inoculum that helped to fuel additional outbreaks in 2024 and 2025. While the region has experienced dry conditions in late summer over the past two years, the mild and wet conditions in the spring combined with abundant inoculum likely allowed for widespread disease establishment.
While Pseudocercospora is primarily responsible for the leaf blotch and premature leaf shedding, another foliar disease pathogen( Septoria) may be present in some cases. Septoria is similar in that there are many species in the genus that attack a wide array of deciduous trees and shrubs. Septoria leaf spot diseases often appear later in the growing season on dogwood( Cornus) and birch( Betula), but cause little, if any, lasting injury. Lilac powdery mildew( Microsphaera syringae) is a perennial disease of lilacs in the region and may contribute to early leaf shedding as well. For some plants, the premature loss of foliage causes a sporadic flush of new growth and flowers in September and early October. Subsequently, this tender new growth does not have time to harden off and dies over the winter.
2. Symptoms of Pseudocercospora leaf blotch on lilac( Syringa vulgaris). Photo by N. Brazee
1. Symptoms of Pseudocercospora leaf blotch on lilac( Syringa vulgaris). Photo by N. Brazee
Common lilacs are extremely hardy ornamentals, but three successive years of early defoliation combined with drought stress may result in canopy dieback. Stressed lilacs are also more susceptible to opportunistic cankering pathogens( e. g. Phomopsis) and insect pests. Therefore, thoroughly collect and remove all diseased
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