Welcome to the module on Snow Scald, a disease that affects cool-season turfgrass. Although not commonly observed, snow scald is believed to occur on various cool-season turfgrass species, with annual bluegrass and creeping bentgrass being the most frequently affected. This disease remains somewhat mysterious, and much is still unknown about its causal agent and the factors that contribute to its development. In this module, we will explore the characteristics of Snow Scald, including its host range, favored conditions, symptoms, signs, and available control measures. While our understanding of this disease may be limited, we will delve into the existing knowledge to shed light on this intriguing turfgrass issue.
Causal Agent
Sclerotinia borealis
Primary Hosts
All cool season grass species are susceptible to snow scald, but creeping bentgrass and annual bluegrass appear to be more susceptible.
Disease Cycle
The disease cycle of snow scald begins with the production of sclerotia by the pathogen Sclerotinia borealis. These sclerotia serve as long-term survival structures that allow the fungus to persist during the summer. In the fall, fungal hyphae germinate from the sclerotia, and with the onset of winter, they initiate new infections. The mycelium of S. borealis grows at temperatures as low as -5°C (23°F) and continues to develop during the winter months. As the snow recedes in spring, the disease symptoms become visible, and new sclerotia are formed on the affected turf.
Epidemiology
Snow scald is favored by the presence of deep snow covering frozen soil, particularly in regions with cold, wintry winters such as western Minnesota and the northern Great Plains of the United States and Canada. The disease occurs on various cool-season turfgrass species but is most frequently observed on annual bluegrass and creeping bentgrass. The fungus S. borealis can grow at temperatures ranging from 5-10°C (41-50°F), allowing it to thrive during the cool spring conditions. The development and spread of snow scald are influenced by the duration and depth of snow cover, as well as the availability of suitable hosts for the pathogen.
Symptoms
Visual symptoms of snow scald resemble those of gray snow mold (Typhula incarnata) or speckled snow mold (T. ishikariensis). They appear as roughly circular patches measuring 15-30 cm in diameter as the snow recedes. Affected leaves initially exhibit water-soaked characteristics and are covered with sparse, gray mycelium and tan sclerotia. Over time, the leaves become bleached and turn tan or white.
Credit: Paul Koch/University of Wisconsin
Signs
The sclerotia produced by the causal agent Sclerotinia borealis are a distinguishing sign of snow scald. These sclerotia are considerably larger than those produced by Typhula species. They turn black as they mature and resemble mouse droppings embedded in the affected turf. The mycelium of S. borealis is gray in color and does not possess clamp connections. The sclerotia are typically irregular, oval, or flake-like in shape and range in size from 0.5 to 7 mm in length.
Credit: Paul Koch/University of Wisconsin
Management
Cultural
Cultural management practices play a crucial role in mitigating the development of snow scald in turfgrass. One important aspect is to improve soil moisture by using wetting agents, particularly before the winter season. Adequate soil moisture levels help suppress the disease and create less favorable conditions for its development.
In the spring, promoting the recovery of damaged areas is essential. This can be achieved by lightly raking the affected patches, which facilitates sunlight and oxygen penetration to the turfgrass. Additionally, applying a light fertilization with quick-release nitrogen sources can aid in the recovery process. These cultural practices contribute to creating a healthier turfgrass environment and reducing the impact of snow scald.
Chemical
Chemical control options can be utilized to manage snow scald in turfgrass, particularly in areas where favorable conditions for the disease are prevalent. While research on the most effective fungicides against snow scald is limited, preventive fungicide applications have shown promise in suppressing the disease.
Fungicide mixtures that provide excellent control of speckled snow mold have been reported to have varying levels of efficacy against snow scald. It is important to note that the effectiveness of specific fungicides against snow scald may differ. Therefore, it is recommended to consult with local turfgrass specialists or extension services for fungicide recommendations specific to the region and prevailing conditions.