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 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 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
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.
Reprinted from UMass Hort Notes Vol. 35:10
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