First Report of Dollar Spot, Caused by Sclerotinia homoeocarpa, of Creeping Bentgrass in Norway

Abstract

In September 2013, symptoms similar to dollar spot caused by Sclerotinia homoeocarpa F.T. Benn., were observed on creeping bentgrass (Agrostis stolonifera L.) fairways at Losby Golf Course, Lørenskog, Akershus County, in Norway (59.8864′ N, 10.9862′ E). There were small, circular spots and larger irregular patches of sunken, bleached, straw-colored turf. Affected leaves had light-tan lesions with light reddish-brown margins (2). Abundant aerial mycelium was observed in the diseased turf after incubation for 24 h at room temperature in a moist chamber. The mycelium was septate with y-shaped branches. No spores were observed. Diseased leaf segments were washed 30 min in cold running water, surface-sterilized for 60 s using 70% ethanol, placed on water agar, and incubated at room temperature. After 4 days, water agar plugs containing the fungus were transferred to 50% potato dextrose agar (PDA; 19.5 g PDA and 7.5 g agar per 1 liter of media). The fungus colonized the entire 9-cm PDA plates in 6 days. The diameter of the hyphae varied from 2.5 to 12.5 μm. The white, floccose mycelium turned olive green after 7 days and cinnamon brown after 21 days. The cultures became brown from the bottom, forming flat, dark-brown stroma of 0.5 to 5.0 mm in diameter. DNA was extracted from three isolates (from different plants) using a DNeasy Plant Mini Kit (Qiagen). The ribosomal internal transcribed spacer (ITS) region was PCR-amplified using primers ITS1 and ITS4 (3). All three isolates were identical in sequence (GenBank Accession No. KJ775860) and showed up to 97.6% similarity with isolates of S. homoeocarpa of the common type (C-type; e.g., GenBank Accession No. HQ449691) (1). This similarity is considered quite low within a species and indicates that the Norwegian isolates are distinct from other S. homoeocarpa. For Koch's postulates, the fungus was scraped off 21-day-old PDA cultures and chopped, using a sterile scalpel. All three sequenced isolates were pooled and mixed with 200 ml of autoclaved water. Four mature, healthy sod plugs of creeping bentgrass cv. Independence (10-cm-diameter and 10-cm-depth) were taken from an experimental golf green at Landvik, inserted into pots, and inoculated by even distribution of 50 ml of the fungal suspension. Two control pots with creeping bentgrass received 50 ml of sterile water only. All six pots were incubated individually in plastic bags at room temperature and 16-h daylight. After 14 days, 30 to 90% of the inoculated pots of turfgrass exhibited dollar spot symptoms and controls remained healthy. The fungus was recovered from inoculated turf and identified morphologically. This is the first report of dollar spot on any grass species in Norway. For climatic reasons, dollar spot has been considered to be nonexistent in Scandinavia. However, during recent years, symptoms resembling dollar spot have been observed on more than 15 golf courses in Sweden, Denmark, and Norway, and the damage has varied from low to severe. The disease has been given the Norwegian name myntflekk (i.e., coin spot). References: (1) D. Liberti et al. Phytopathology 102:506, 2012. (2) J. D. Smith et al. Fungal diseases of amenity turf grasses. E. & F. E. Spon, London. 1989. (3) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.