Symptoms and Control Methods of Spruce Felt Blight
[Distribution and Damage] This disease occurs in the Songhua River area and Yichun forest region of Heilongjiang Province, damaging 16-21 year old plantations of Picea koraiensis (Korean spruce). In affected stands, naturally regenerated young trees of Abies nephrolepis (Khingan fir) and Pinus koraiensis (Korean pine) are also susceptible. A felt-like mycelial layer grows on branches and needles, causing wilting and felt blight. It spreads from the lower crown upward, expanding in a cone shape along the trunk. At the lower quarter of the crown, the damage rate of branches and needles can reach over 50%; at the lower third of the crown, the damage rate of branches and needles can reach 30%-50%; at the middle half of the crown, the damage rate of branches and needles is below 30%. In severe cases, almost the entire crown's branches and needles become damaged, yellowed, wilted and dead.
[Symptoms] Initially, a spiderweb-like mycelium grows on the branches and needles in the dense lower part of the crown, entwining the needles and twigs, gradually forming mycelium. A felt-like mycelial layer forms beneath the bases of branches and needles, about 1-2mm thick, gradually spreading to the green branches and needles. Initially grayish-white, gradually turning gray; on the upper surface of affected branches, a black mold layer forms. When the mycelial layer is peeled back, the cortical tissue remains green and the vascular tissue is intact, so the top branches and needles remain normal. In severe cases, twigs and needles are completely covered by the felt-like mycelial layer, forming a mycelial sheath; mycelial membrane can even form beneath the bark, clogging lenticels and stomata and causing suffocation, leading to yellowing, wilting and death of branches and needles. Subsequently, within or on top of the mycelial layer, black spherical bodies form in groups or side by side - these are the fungal perithecia.
[Pathogen] Rosellinia herpotrichioides Hepting & Davidson. Perithecia are grouped or side by side, densely clustered on the felt-like mycelial layer or embedded within it. Nearly spherical, dark brown, carbonaceous, 501-855μm in diameter, with a distinct black nipple-shaped ostiole. Asci are cylindrical, with a short stalk, thickened at the apex, with a small pore channel and an amyloid apical ring (plug) in the center. Staining with Melzer's solution turns blue, 180-207μm x 10.5-13.6μm. Ascospores are arranged in a single oblique row, unicellular, inequilateral elliptical to square, with a germ slit on one side, sometimes pointed at both ends, containing 1-2 oil droplets, 20-24μm x 8.5-10μm.
Ascospores do not germinate in water droplets, but can germinate in 2% glucose, sucrose, maltose and other nutrient solutions. When 2% maltose agar film was used for spore germination tests, the germination rate increased significantly.
[Disease Pattern] The fungus overwinters as perithecia within the host mycelial layer. Ascospores are the primary infection source, usually epiphytic on the host surface. When seedlings or stands are too dense, with continuous summer rain and high humidity, the fungus can grow, reproduce and spread rapidly, causing damage to the host. According to surveys conducted by the Suiling Forestry Bureau in Heilongjiang Province under the same site conditions, in Picea koraiensis plantations, the greater the canopy density, the more severe the disease on shaded slopes, lower slopes, inside the forest and in unthinned stands.
[Control Measures] Timely silvicultural measures such as forest clearing, pruning and thinning (removal of overstory) in overly dense stands can reduce the disease, with thinning showing significant preventive effect. Both the incidence rate and disease index decreased markedly, with a control effect reaching 91.2%. Where conditions permit, using 75% chlorothalonil smoke agent at 15kg/ha has a certain control effect.