Symptom Identification and Control Methods of Spruce Snow Mold

[Distribution and Damage] This disease occurs in North America, Northern Europe, Japan and other countries, damaging tree species such as fir (Abies), spruce (Picea), and Japanese cedar (Cryptomeria fortunei). In China**, it is distributed in the western Tianshan Mountains, the western Junggar Mountains, and the Altai Mountains and other forest areas. The main affected areas include Gongliu, Yining, Tacheng and other places. In severely affected nurseries, the annual loss rate is 33-62%. In artificially regenerated young forests, this disease is also very serious, especially when using small seedlings, weak seedlings, and diseased seedlings for regeneration and afforestation. In recent years, due to the damage of snow mold, seedling shortages have occurred, quality has declined, and survival and preservation rates are low, affecting the restoration and development of forest resources.

[Symptoms] The needles and the whole aboveground part of the plant are infected under the snow, covered by gray-brown to dark brown mycelium, or invaded internally, causing seedling mold rot and suffocation. The texture and thickness of the mycelial layer vary with disease severity; when mildly affected it is flocculent, and when severe it is felt-like. The color differs depending on the duration of damage; early-infected parts are brown, while those infected just before snowmelt are white or gray-brown. When damage is light, it appears in clumps; after snowmelt, diseased leaves fall off, the cortex of the stem rots, and the aboveground part gradually dies.

[Pathogens] The main pathogens include Botrytis cinerea Pers., Sclerotinia sclerotiorum (Lib.) De Bary, Truncatella, and others such as Cladosporium sp., Rhizopus sp., Alternaria sp., Epicoccum sp., etc. The morphology of the main pathogens is described as follows:

1. The sclerotia of Botrytis cinerea are nearly black, with a rough surface, nearly round to irregular, 0.1-1 mm on leaves and 2-3 mm on stems; when mature they are semi-exposed and not easily detached. Vegetative hyphae are colorless and transparent, containing many granular bodies, 4-11 μm wide. Conidiophores are produced from sclerotia and hyphae; the base of the conidiophore is light brown, gradually tapering upward and nearly colorless, 0.5-2 mm long and 8-16 μm wide, branching repeatedly at the tip; the branchlets swell to form sterigmata, and budding conidia aggregate into clusters. Conidia are colorless, single-celled, oval, elliptical to oblong, 8-18 μm x 7-12 μm. Old conidiophores are flat and twisted, brown. Conidial clusters are not only terminal but also intercalary on extending conidiophores, with spacing of 26-59 μm. Appressoria initially branch dichotomously, later aggregate, dark green to black. Microspores are colorless, nearly round, about 3 μm.

2. The sclerotia of Sclerotinia sclerotiorum are black, smooth on the surface, broad-bean-shaped, dumbbell-shaped or nearly round, 4-8 mm. They are borne on young stems, twigs, cotyledon nodes and new bud tips of spruce, occasionally on leaves, and are easily detached. Generally, hyphae are produced from sclerotia; the hyphae are colorless and transparent, and the appressoria are dark green to black. One sclerotium can also produce 1-15 apothecia, usually 4-6. Apothecia are pale flesh-colored to dark flesh-colored, initially cup-shaped, and dish-shaped after expansion, 1-11 mm in diameter; the stipe is 1-2 cm long, often varying with the environment, and can reach more than 4 cm. The hymenium is nearly colorless; asci are club-shaped, 108-145 μm x 6-8 μm, and the apical ring turns blue with iodine staining. Ascospores are 8, uniseriate, single-celled, mostly elliptical, 8-12 μm x 5-8 μm, containing 2-3 oil droplets. Paraphyses are slightly longer than the asci, with slightly swollen tips.

3. The conidiomata of Truncatella are stem-borne, flask-shaped to dish-shaped, immersed, scattered or merging, 250-1100 μm x 100-450 μm; the wall is composed of brown, polygonal cells and dehisces irregularly. The hymenium is peripheral; conidiophores are colorless, linear, 25-46 μm x 2 μm, sparsely septate, with 1-3 annellations. Conidia are fusiform, 16-21 μm x 6-9 μm, with 3 true septa; the two middle cells are brown, 11-14 μm long; the basal cell is truncate and the apical cell is conical, both colorless; the apical cell has extended appendages, variable in shape, dichotomously branched or irregularly branched, sometimes single.

[Disease Cycle] In China**, snow mold not only damages Schrenk spruce (P. schrenkiana) and Siberian spruce (P. obovata), but also damages Norway spruce (P. abies), white spruce (P. glauca), Qinghai spruce (P. crassifolia), Sichuan spruce (P. balfouriana), etc. After spruce snow mold becomes established during the growing season, it continues to develop and spread under the snow and causes damage. Early winter and early spring are the main occurrence periods, with rapid spread in early spring. The common characteristics of the three pathogens such as Botrytis cinerea are: from 0-22°C, growth increases rapidly with rising temperature; above 28°C, growth is extremely slow and gradually stagnates; below 22°C, vegetative growth is vigorous. Mature sclerotia generally have no dormancy period. They can germinate when temperature and humidity are suitable. In the western Tianshan Mountains, the western Junggar Mountains, and the Altai Mountains forest areas, snow cover is deep (taking Xinyuan and Burqin as examples); the average maximum snow depth over the years is 100.4 cm. Under snow cover, the soil does not freeze in winter, or only in individual years does the shallow soil layer freeze slightly for a short time. The seedling canopy layer and soil layer remain at low temperature and high humidity around 0°C for a long time. Because the surface temperature under snow cover remains around 0°C for nearly 5-6 months, and during the snowmelt period in March-April, temperatures above 0°C last for nearly 2 months. Such ecological conditions are suitable for the occurrence of snow mold. In particular, unstable temperatures in early winter and early spring, delayed snowmelt, frequent freezing rain, sleet, and ice crust formation under the snow layer flatten and lodge seedlings, resulting in poor aeration, further favoring the development and spread of the disease. The pathogens are weak parasites; frost damage, sunscald and various other natural and human-caused injuries to seedlings, as well as extensive management, many years of continuous cropping, high-density planting, insufficient light, and microhabitats unfavorable for drainage, ventilation and cooling, all lead to more severe damage.

[Control Measures] In lightly affected areas, integrated control should be emphasized. In severely affected areas, chemical control should be widely adopted to control damage and epidemics.

Seedling-raising technical control. Choose lower-elevation sites in the mid-mountain belt to establish standard nurseries. Strictly control planting density, thin and finalize seedlings at the proper time, and remove weak and diseased seedlings. Change seedbeds in time, transplant by grade, and increase light. During leaf expansion and new shoot growth, take measures such as covering with screens to prevent late frost. Apply chemical fertilizers, growth promoters, and herbicides in appropriate amounts and at the proper time, but prevent burning. In autumn, clear channels and drainage outlets and remove obstacles such as weeds; in early spring, remove ice and silt. For small nurseries lacking chemicals and equipment, young seedlings can be protected by building snow-barrier sheds over the seedbeds.

Silvicultural technical control. In conjunction with site regeneration, carry out forest clearing and site preparation before afforestation. Use qualified, vigorous seedlings for afforestation and replanting. During lifting, heeling-in, and planting, take care to protect seedlings and maintain their vitality. Strengthen tending of young forests, promptly straighten seedlings, earth up, weed, cut brush, and clear collapsed stones and other obstacles around the plants to prevent them from being pressed down or lodging.

Chemical control. Before snow cover in early winter (mid-to-late October in nurseries, late September to early October in forest land), use broad-spectrum fungicides with long efficacy and strong systemic and adsorptive properties. Such as 20% triadimefon EC (1.5-4.5 kg/ha), 70% thiophanate-methyl (3.0-9.0 kg/ha), 25% carbendazim or 40% lime sulfur mixture, carbendazim-thiram mixture (9-12 kg/ha), etc. Use an electric handheld ultra-low-volume sprayer, dilute the chemicals 5-10 times, and spray once; the control effect is best, and it is especially suitable for mountainous areas and early winter work with changeable climate. For control in young forests, choose carbendazim suspension concentrate, triadimefon EC, etc., and spray the undiluted liquid.