Pest Update
Didymascella leaf blight of Green Giant arborvitae
By Nicholas J . Brazee , UMass Extension Plant Pathologist
Didymascella leaf blight ( Didymascella thujina ) can be a serious disease of western red-cedar ( Thuja plicata ) and Oriental arborvitae ( Platycladus orientalis ) ( Sinclair and Lyon 2005 ). Other common names for the disease include Keithia leaf blight and Cedar leaf blight . In recent years , Green Giant arborvitae ( T . plicata × standishii ‘ Green Giant ’) has become very popular in New England due to their rapid growth rates and dense canopies for screening between neighboring properties . While Japanese arborvitae ( T . standishii ) is considered resistant to infection , the hybridization with western red-cedar makes Green Giant susceptible . Northern white-cedar ( T . occidentalis ) is resistant to the disease along with false-cypress ( Chamaecyparis species ). Therefore , the most common arborvitae cultivars in this region , such as Emerald Green ( T . occidentalis ‘ Smargd ’) and Dark American ( T . occidentalis ‘ Dark American ’), are not considered at risk for the disease .
Didymascella leaf blight is widespread in the Pacific Northwest , where western red-cedar is native . It has been reported in eastern North America but is considered a relatively uncommon disease in New England . However , in 2024 , the UMass Plant Diagnostic Lab received several Green Giant samples with the disease . While trees of any age are susceptible , young trees suffer higher rates of disease incidence and severity ( Kope 2018 ). Symptoms of infection appear in the spring as brown lesions on the upper surface of one year old needles . Within these lesions , dark-colored fruiting bodies develop and rupture to release large volumes of spores . The spore dispersal period occurs in late spring and early summer ( late April into late June ) and late summer into autumn ( late September into
early November ). A larger volume of spores are released in the spring and the spores are known to travel considerable distances on local air currents ( Kope 2018 ). If infected shoots are not killed , they can continue to elongate and produce new needles the following year . In these cases , the old lesions may appear as deep , brown-colored pits . Variation in susceptibility among species and cultivars often determines the severity of the disease .
Due to the popularity of the Green Giant arborvitae , it ' s conceivable that Didymascella leaf blight may also become more common in future years . However , it ’ s important to note that this is not a lethal disease of mature trees and is most severe on young seedlings and saplings . Abundant rainfall or overhead irrigation during fungal sporulation can help to initiate disease outbreaks . Needle blight diseases of conifers often develop in the lower canopy where shade and moisture are more abundant . Over time , they can spread upward in the canopy . Regular scouting during the late spring and early summer for the foliar lesions can help to manage this disease . When the immature lesions are detected , these shoots can be pruned and discarded from the canopy before the fungus can sporulate . In certain cases , chemical management may provide some level of additional control . Because the disease is considered relatively rare in the region , regular sanitation pruning may effectively control the pathogen when it ' s detected .
Citations Kope HH . 2018 . Cedar Leaf Blight . Pp . 123 – 125 , In Compendium of Conifer Diseases , 2nd edn . APS Press , St . Paul , MN . Sinclair WA and Lyon HH . 2005 . Diseases of Trees and Shrubs , 2nd edn . Cornell University Press , Ithaca , NY .
UMass Hort Notes 2024 Vol . 35:10
MAA NEWS WINTER 2025 17