Symptom Identification and Control Methods for Spruce Snow Blight
[Distribution and Damage] Abroad, it is distributed in North America, Northern Europe and Central Asia, damaging Norway spruce (Picea abies), white spruce (P. glauca), black spruce (P. mariana), etc. In China, it is distributed in the Tianshan Mountains, the Altai Mountains, the western mountains of the Junggar Basin and the Kunlun Mountains. It damages Schrenk spruce (P. schrenkiana), Siberian spruce (P. obovata), Qinghai spruce (P. crassifolia) and Sichuan spruce (P. balfouriana). In the late 1950s, when silvicultural activities had just begun, this disease occurred only sporadically in natural forests. Now the disease has become widespread in continuous-cropping nurseries, and in some areas it has broken out into disaster; it has also spread widely in regenerated young forests and shows a tendency to develop and spread further. The average incidence in artificially regenerated young forests in various places is 10%-43%, reaching as high as 88% in severe cases.
[Symptoms] In the early stage of the disease, the needles under the snow appear dark grayish green and scalded by water; a dirty white to light brown mycelium or mycelial film forms on the surface, and many dark brown to reddish brown small sclerotia are visible. After more than a month, raised swellings nearly 1 mm in size with a dark brown center can be seen on both sides of the needles, sometimes exuding dirty white drops or tendrils. In the later stage, dark brown line dots appear along the stomatal lines of the diseased leaves, later joining into line segments, changing from sunken to flat, then becoming raised, appearing blister-like and straw-colored; old diseased leaves from the previous year are grayish brown, with the cortex broken intermittently in many places.
[Pathogen] The pathogen is Lophophacidium hyperboreum Lagerb. The stroma is formed on the underside of leaves or all around, mostly on the underside, initially embedded, scattered to clustered, 500-1000 μm × 200-500 μm, often with several stromata fusing, reaching several millimeters in length, some almost as long as the needles. To the naked eye, the stomatal lines appear as interrupted black dots or line segments. The stroma grows within the cortex and becomes raised after maturity. The apothecia are longitudinally elongated, blister-like, straw-colored, opening by a longitudinal split at the top or side. The pseudothecium covering is a covering layer composed jointly of host and fungal tissues. The outer layer consists of brown, spherical pseudoparenchymatous cells, and the inner layer is nearly colorless. On the inner upper margin grow irregularly branched pseudoparaphyses, 40 μm × 2.5-4 μm. The wall is smooth, septate, and at the center of the top has a nucleus-like structure composed of dark brown, tightly packed spherical cells, 40-150 μm × 20-50 μm. The hymenium is straw-yellow and contains gelatinous material. The asci are club-shaped, 89-22 μm × 14-19 μm, containing 8 spores in two rows, with the apical pore turning blue in iodine solution. The ascospores are long ovoid to fusiform, straight or slightly curved, often swollen on one side, containing 2-3 oil droplets, 14-21 μm × 5-7 μm, nearly colorless. The paraphyses are numerous, filiform, 90-130 μm, with 2-4 septa, sometimes branched. The asexual stage belongs to the genus Apostrasseria. The stroma of the conidiomata is formed on the underside of leaves or all around, mostly on the underside, scattered, sometimes clustered. It is often connected with the sexual stromata and embedded in the needle cortex. At maturity the top is slightly raised, 200-500 μm × 130-220 μm, nearly spherical to flattened spherical, or with a sunken base, irregular in shape. It is unilocular, or forms labyrinth-like false multilocular chambers; the wall is composed of brown pseudoparenchymatous cells, 15-20 μm, rich in gelatinous material, sometimes exuding white gelatinous drops or tendrils outward. The conidiogenous cells are flask-shaped to nearly cylindrical, colorless, with smooth walls, 9-15 μm × 2-3 μm, producing conidia by phialides. The conidia are immersed in gelatinous material and not easily dispersed, colorless, single-celled, fusiform to elliptical, 2.5-40 μm × 1-1.5 μm, with a length-to-width ratio of 2.5-2.7:1, and with irregular gelatinous appendages at the apex. In the superficial floccose hyphae or mycelial film, dark brown small sclerotia are contained, mostly on the upper side of the leaves. They stand erect singly, sometimes clustered, 10-25 per needle, and fall off easily after snowmelt. The small sclerotia are elongated, granular, slightly curved at the top or base, with a rough surface, formed by brown, longitudinally elongated, septate, thick-walled hyphae gathered into bundles, easily breaking apart under light pressure, 80-320 μm × 40-120 μm.
[Disease Development Pattern] The pathogen damages plants of the genus Picea. Long-term stable snow cover in winter (over 40-50 cm) is the dominant ecological condition for disease development. Artificial snow-piling inoculation of Tianshan spruce introduced and cultivated in Urumqi, China, can also cause the disease. Because the susceptible hosts Schrenk spruce and Siberian spruce are widespread species in ** mountain forests, the pathogen has both ecological and host suitability and adaptability, and thus spruce snow blight has become a widely distributed and frequently occurring disease in **. Disease is severe in areas with many obstacles, low-lying and wind-sheltered valleys, deep snow cover, late snowmelt and a long snowmelt period. Spruce snow blight and snow mold often occur simultaneously in the same area and on the same plant, with snow mold mostly occurring on the lower part of the plant.
[Control Measures]
1. It is strictly forbidden to use diseased seedlings for bed transfer and transplanting, for out-planting and afforestation, or to transfer them elsewhere.
2. Eradicate the centers of infection and promptly remove diseased plants in June-July. Pull out all severely diseased plants and prune the diseased branches of lightly diseased plants. For other silvicultural and chemical control techniques, see "Spruce Snow Mold."