BRIAN PAIGE
RECEIVES
AlertBEACON AWARD
BRIAN PAIGE
Disease
RECEIVES
AlertBEACON AWARD
Pseudocercospora leaf blotch of lilac ( Syringa vulgaris )
Nicholas J . Brazee , UMass Extension Plant Pathologist
In 2023 and 2024 , lilacs ( Syringa vulgaris ) in southern New England have suffered from a serious late season foliar disease caused by the fungal pathogen Pseudocercospora . Overall , relatively little is known about the disease , but it has been widely reported from lilacs in the Midwest prior to the outbreak here . While poorly studied on lilac , Pseudocercospora species are well known pathogens of many other woody and nonwoody plants . Symptoms of Pseudocercospora leaf blotch typically develop sometime in mid-summer but can be easily missed without careful scouting . Humid conditions with plentiful rainfall spur asexual spore production , dispersal , and germination . 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 . The record-setting rainfall in July of 2023 is likely to blame for the severe outbreak last year . This created a large amount of overwintering inoculum that helped to fuel another outbreak in 2024 . While the summer 2024 was very dry across much of the Commonwealth , the mild and wet conditions in the spring combined with abundant inoculum likely allowed for widespread disease establishment .
A large number of lilacs in the region were defoliated by Pseudocercospora leaf blotch by early autumn in 2023 and again in 2024 . In some cases , another foliar disease pathogen ( Septoria ) was also present . 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 often appears later in the growing season on hosts such as dogwood and birch . Lilac powdery mildew ( Microsphaera syringae ) has also been abundant the past two years and was a contributed to the early leaf shedding . In 2023 , many defoliated lilacs had a sporadic flush of new growth and flowers in September and early October . Subsequently , this tender new growth did not have time to harden off and died over the winter . In 2024 , this late season flush was relatively uncommon .
While lilacs are extremely hardy , two successive years of early defoliation in combination with drought stress from this past summer could 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 foliage from sites with Pseudocercospora . This will help to reduce overwintering inoculum used by the pathogen to initiate new outbreaks in 2025 . However , keep in mind that the pathogen is widespread in the environment and even with good cultural control , the disease can reappear . Prune and discard blighted twigs and branches throughout the canopy during this upcoming winter . For mature and dense lilacs , consider thinning out the canopy to allow for better air flow and sunlight penetration . Foliar disease fungi thrive in shaded and moist areas of the inner canopy . There are no chemicals labeled specifically for use against Pseudocercospora on lilac . However , both propiconazole and copper hydroxide are labeled for use on lilac in Massachusetts . These broad-spectrum , contact fungicides should have utility against the pathogen . While the exact timing is unknown , fungicide applications should be made early in the growing season and on regular intervals until the foliage is fully developed . However , avoid fungicide application while lilacs are in bloom to reduce the impact on pollinators . Mild ( i . e ., temperatures > 75 ° F ) and humid conditions are conducive to spore production and disease development . If we experience a dry spring and early summer , the fungicide application interval can likely be extended . Scout plants carefully during the month of July for any symptoms and make spot treatments as necessary to suppress further disease development .
Reprinted from UMass Extension Hort Notes
MALP Newsline | Winter 2025 17