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Cedar Leaf Blight (Didymascella thujina [= Keithia thujina])
Cedar leaf blight is a well-known disease of western red-cedar (Thuja plicata) in the western United States and Canada. It has also been reported previously from a few eastern U.S. states, including Maine, on northern white-cedar (Thuja occidentalis). Although the disease can occur on trees of any age or size, most damage has been reported on young seedlings and saplings. Damage is most severe where dense shading and high foliage moisture levels occur. Symptoms first appear as light-brown areas on the individual scale-like leaves. The fruiting bodies (apothecia) of the fungus appear in June on the upper surface of infected needles. The apothecia rupture through the needle epidermis, which often stays attached to the needle (see photos). Heavily-infected needles and twigs are shed later in the fall. Application of fungicides to manage the disease is rarely necessary. A copper fungicide (Bordeaux Mix) or mancozeb (Dithane) should be effective if applied early in the season, when susceptible foliage is developing.
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