Overview of Grey Snow Mold

Disease lesson (Snow Mold)

DISEASE: Grey Snow Mold

PATHOGEN:  Typhula blight (Grey snow mold)

HOSTS: All types of common turfgrasses- Bluegrass (Poa. spp.), fescues (Festuca spp.), ryegrasses (Lolium spp.), bentgrasses (Agrostis spp.) (7).

Symptoms and Signs

As shown in Figure 1, white to greyish, straw-colored circular patches appeared on the turf in the late winter or early spring. Each patch will not be very big, but they can connect to infect really large areas. The disease causes roots, leaves and stems to rot (4). There is another similar disease called pink snow mold. The grey snow mold and the pink snow mold will have the similar symptoms (2). The pink snow mold will have a pale pink color at the margins of the circle. The difference between the two pathogens is whether there is snow covering on the turf. The pink snow mold does not require snow cover to cause disease (2). Symptoms will become more severe when the time of snow covering gets longer (2). Generally, grey snow mold only kills the leaves and new leaves will form from the plant crowns again (4). If infected areas are left untreated, patches will stay for the whole growing season. With the areas of lighter damage, plants will resume growth after winter and soon look same as healthy plants (2). Masses of sclerotia, the survival structures can be seen as black spots on leaves and they will become light brown color when soaked with water (2). 

Pathogen Biology

     Snow mold is a facultative psychrophilic (winter-loving) fungus (14). Snow mold diseases are caused by fungi that attack dormant plants during winter at low temperature under the cover of snow (1).  Snow molds are favored by excessive applications of nitrogen fertilizers in fall season, excessive shade, thick thatch, or mulches of moisture- holding components (4).  The snow mold requires extended time periods of snow cover, at least 60 days to develop (13). The disease may be caused by either of the two species: Typhula ishikariensis and Typhula incarnata (12). T. incarnata is less severe and causes damages tend to be recovered quickly in the spring (12). T. incarnata has been reported from northern Europe, Canada, the northern United States and Japan (16). T. incarnata is generally found in the area with more than 65 days of snow cover, but it is not common for extremely cold areas (16). During the summer, the viability of T. incarnata can be reduced greatly (22). T. incarnata is less virulent than T. ishikariensis but has higher ability to survive as saprophytes (17). T. ishikariensis has been reported from northern Japan, northern Scandinavia, Canada, the northern United States, and Russian as well (16). T. ishikariensis is usually virulent and theinfection of T. ishikariensis may damage the crowns of grasses, which cause plant death or severe and long-lasting damages (12). T. ishikariensis is usually soilborne and spreads mainly by vegetative growth under snow (2). Sclerotia of T. ishikariensis is smaller than those of T. incarnata, but sclerotia of T. ishikariensis can survive better through summer and are often produced on roots and leaves as well (2). T. ishikariensis is usually virulent and cause severe damages on hosts.

Disease Cycle

Figure 2 is the general disease cycle of the grey snow mold. The disease cycle is the opposite way of most other diseases (7).

The fungi produce sclerotia as the resting structures in the summer for survival (7). Usually the sclerotia stay in or attached to the leaves or the crowns of the infected plants (4). In late fall, cool and wet weather triggers sclerotia to germinate (8). If the environment is not suitable for fungi growth, the sclerotia will not germinate. Sclerotia can survive for a long period of time as the surviving spores. The sclerotia germinate under snow cover and produce mycelia (9). Mycelia spread, produce infection and then penetrate into plant tissues directly or through stomata (10). Mycelia cause plant tissues to decompose and fall on the ground, which produce new sclerotia for survival in the thatch or soil for the whole summer (2). Once gray snow mold infected in a specific area, it is likely to reoccur if not managed (7).

Management

Winter injury can be caused by biotic sources but also can come from abiotic sources as desiccation, frost injury and freezing damages (2).

–          Use resistant varieties.  L. perenne is considered the most susceptible to Typhula blight (2). Bentgrasses (Agrostis spp.) is considered the most susceptible to Typhula blight (7). The physiological condition of the turfgrass has the strongest influence on its ability to resist the attack of Typhula blight (20). If the plant is not winter-hardened, they will suffer more damages during snow time (21). So, choosing the resistant varieties will be the most important step to prevent snow mold diseases.

–          Follow a recommended fertilizer program for a specific area and the grasses you use. Snow mold damage can be controlled when using balanced fertilization (4). Maintain the high value of potassium and phosphorus in the soil (4). Avoid late fall nitrogen application before turf dormancy (5). Nitrogen should be avoided at least 6 weeks before the first heavy snow (4).

–          Avoid excessive mulches and thatch accumulations that hold moisture (4). If the thatch or mulches are more than ½ inch thick, a specific machine has to be used to remove it to reduce the snow mold disease. Machines can be found in stores (4).

–          Reduce the cutting height of the turf based on the types of grass to reduce the possibility of infections. Keep mowing in the autumn until the grasses go dormancy (4).

–          Before the first heavy snow of each year, apply suggested fungicide to reduce the damage or infections of the snow molds (4). Generally, application of fungicides should be made before snowfall when the temperature drops lower than 45 degrees Fahrenheit (5). Use lower fungicide rates in preventive programs, use higher rates for curative programs (4). Mixtures of multiple fungicides are best for management of the disease (13).

-          Improve soil aeration by pruning shrubs and trees (5) and improve light circulation (4).

-          Remove snow covering to avoid severe symptoms. Snow fences and similar barriers should be placed on turf (4).

-          Compost-amended topdressings are found to reduce snow molds significantly. The compost improves growth and recovery in the early spring because of its nitrogen content (23).

Significance

The basidiomycetous genus Typhula has been assigned to different families over time (2). According to the ‘Dictionary of the fungi’, the genus Typhula was assigned to Cantharellales before (18). Previously than Cantharellales, it was placed into Clavarianceae of the Aphyllophorales (19). Variation of the T. ishikariensis makes it hard for assigning. The nomenclature of different species, varieties, biotypes or groups differs among the different researchers according to Table 1 (2).

    According to golf course superintendents, snow molds are the fifth damaging disease in the US (2). Approximately 40 fungal species have been described as pathogens of seedlings, but only five of them have been found to be pathogenic under snow covering (11). The severity of the snow mold disease varies depending on the environment and the time length of snow covering. Although the disease is unsightly, it does not usually kill the grass (7). The infected parts are normally the blades (15). The snow mold disease does not occur every year so using appropriate fungicide and fertilizer before grass dormancy will be a good way to prevent fungi infection. Information about specific fungicides can be found online easily and it is not costly to use for management. 

References

1. Matsumoto, N. 1994. Ecological adaptations of low temperature plant pathogenic fungi to diverse winter climates. Can. J. Plant. Pathol. 16:237-240. 43.
2. Matsumoto, Tom Hsiang, Naoyuki Matsumoto, Steve M. Millet. Biology and Management of Typhula Snow Molds of Turfgrass. Plant Disease /Vol 83 No.9. From https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS.1999.83.9.788
3. Figure 1: https://www.forestryimages.org/browse/detail.cfm?imgnum=5357034
4. University of Illinois Extension, Reports on Plant Diseases No. 404- Snow Molds of Turfgrasses, July 1997. From http://ipm.illinois.edu/diseases/series400/rpd404/index.html
5. Franc, Gary D. (April 1998). “Snow molds of turfgrass B-868” (PDF). Univ. of Wyoming Extension. Retrieved November 11, 2011. http://www.wyomingextension.org/agpubs/pubs/B868.PDF
6. Figure 2: Bruehl, G. W., and Cunfer, B. M. 1971. Physiologic and environmental factors that affect the severity of snow mold of wheat. Phytopathology 61:792-799.
7. Cornell University. ‘Grey Snow Mold on Turfgrass: Typhula spp. Last updated, SLJ 2/15. From http://plantclinic.cornell.edu/factsheets/graysnowmold.pdf
8. Smith, J. D., Jackson, N., and Woolhouse, A.R. 1989. Fungal diseases of amenity turfgrasses. E. & F. N. Spon, New York.
9. Vargas, J. M., Jr. 1994. Management of Turfgrass Diseases. CRC Press, Boca Raton, FL.
10. Oshiman, K., Kobayashi, I., Shigemitsu, H., and Kunoh, H. 1995. Studies on turfgrass snow mold caused by Typhula ishikariensis. II. Microscopical observation of infected bentgrass leaves. Mycoscience 36:179-185.
11. Wiese, M. V. 1977. Compendium of Wheat Diseases. American Phytopathological Society, St. Paul, MN
12. Richard Latin, Professor of Plant Pathology, Purdue Extension. ‘Turfgrass Disease profiles.’  From https://www.extension.purdue.edu/extmedia/bp/bp-101-w.pdf
13. NC State Extension TurfFiles. ‘Gray Snow Mold in Turf’. From https://www.turffiles.ncsu.edu/diseases-in-turf/gray-snow-mold-in-turf/
14. McBeath, J.H. 2002. Snow mold-plant-antagonist interactions: survival of the fittest under the snow. The Plant Health Instructor. DOI: 10.1094/PHI-I-2002-1010-01. From http://www.apsnet.org/publications/apsnetfeatures/pages/snowmold.aspx
15. “Snow Mold Fact Sheet”. University of Rhode Island Landscape Horticulture Program. Retrieved 2012-10-07.
16. Smith, J. D., Jackson, N., and Woolhouse, A.R. 1989. Fungal diseases of amenity turf grasses. E. & F. N. Spon, New York.
17. Matsumoto, N. 1994. Ecological adaptations of low temperature plant pathogenic fungi to diverse winter climates. Can. J. Plant. Pathol. 16:237-240.
18. Hawksworth, D. L., Kirk, P. M., Sutton, B. C., and Pegler, D. N. 1995. Dictionary of the Fungi. 8th ed. CAB International, Oxon, UK.
19. Hawksworth, D. L., Sutton, B. C., and Ainsworth, G. C. 1983. Dictionary of the Fungi. 7th ed. Commonwealth Mycological Institute, Kew, UK.
20. Bruehl, G. W., and Cunfer, B. M. 1971. Physiologic and environmental factors that affect the severity of snow mold of wheat. Phytopathology 61:792-799.
21. Burke, J. J., Gusta, L., Quamme, H. A., Weiser, C. J., and Li, P. H. 1976. Freezing and injury in plants. Annu. Rev. Plant Physiol. 27:507-528
22. Matsumoto, N., and Tajimi, A. 1985. Field survival of sclerotia of Typhula incarnata and of T. ishikariensis biotype A. Can. J. Bot. 63:1126-1128.
23. Nelson, E. B. 1992. Biological Control of Turfgrass Diseases. Cornell Coop. Ext. Publ. Inf. Bull. 220. Cornell University, Ithaca, NY