ABSTRACT Dollar spot is a fungal disease that affects grass, and is caused by the fungus Sclerotinia homoeocarpa. It was discovered for the first time in Scandinavia in 2013. Dollar spot is a very serious disease of grass in the United States and a lot of money is spent to control the fungus. In the present study, dollar spot development in potted turfgrass, caused by four geographically distinct, Scandinavian isolates of S. homoeocarpa, was investigated at three different temperatures; 6 °C, 15 °C and 21 °C. The objective was to identify the lowest temperature of development of disease, and to establish possible differences among the isolates with regard to disease development. The isolates used originated from Denmark, Norway and Sweden. The grass samples that were inoculated with S. homoeocarpa came from two golf courses in Sweden, located in Uppsala and in Gävle, respectively. Turf grass was dug out and placed in pots, and subsequently inoculated with fungal hyphae from cultures of S. homoeocarpa. These pots were individually covered with plastic bags and placed in different climate chambers at set temperatures. At three time points (day 7, 14 and 21), the ratio of area infected grass to total grass-covered area was estimated. After all the results had been analysed, it was concluded that dollar spot had the most severe disease development at 6 °C. Differences in disease development of dollar spot were also observed among the four isolates at different temperatures. There was a link between geographical origin of an isolate and its optimum temperature for disease development, which may suggest that the isolates can have adapted to the environment in which they have established themselves. S. homoeocarpa is apparently able to proliferate at low temperatures. The present study demonstrates it should be able to infect grass and establish in turf grass even in a colder climate, and thus it is very likely that dollar spot has gained a foothold in Scandinavia, and that it is here to stay. With climate change taking place, i.e. increasing temperatures and precipitation, which in turn results in elevated humidity, the disease will be able to establish and spread yet more, because the fungus thrives in humid conditions. If the climate zones will continue to move northward in the Nordic countries, it will increase the potential for continued establishment and spread of dollar spot. / SAMMANFATTNING Dollar spot är en svampsjukdom som drabbar gräs och som orsakas av svampen Sclerotinia homoeocarpa. Under 2013 upptäcktes dollar spot för första gången i Norden. Dollar spot är en mycket allvarlig grässjukdom i USA och man spenderar massor med pengar för att bekämpa svampen. I denna uppsats har sjukdomsutvecklingen av dollar spot, från fyra geografiskt skilda isolat, undersökts vid olika temperaturer; 6 °C, 15 °C och 21°C. Detta för att hitta den lägsta tillväxttemperaturen hos svampen som orsakar sjukdomen och även för att upptäcka eventuell skillnad mellan de olika isolatens sjukdomsutveckling i gräs. Isolaten som använts i experimenten kom ifrån Danmark, Norge och Sverige. Gräsproverna som svampen S. homoeocarpa odlades på kom ifrån två golfbanor, från Uppsala respektive Gävle. Krukor med gräs inokulerades med S. homoeocarpa och dessa krukor placerades i klimatskåp som höll olika temperatur. Vid tre tidpunkter (dag 7, 14 och 21) lästes den infekterade gräsarean av. Efter att alla resultat hade sammanställts visade det sig att dollar spot hade den snabbaste och mest aggressiva sjukdomsutvecklingen vid 6 °C. Det fanns även skillnader i sjukdomsutveckling av dollar spot mellan de fyra isolaten vid olika temperaturer. Det fanns en viss koppling mellan geografisk härkomst och optimumtemperatur för sjukdomsutveckling hos isolaten, vilket kan tyda på att de kan ha anpassat sig till den omgivning som de etablerat sig i. Med de låga tillväxttemperaturer som svampen S. homoeocarpa klarar för infektion och etablering i grönytor är det mycket troligt att sjukdomen dollar spot fått fäste i Norden, och kommit för att stanna. Med de klimatförändringar som sker, dvs. ökade temperaturer och ökad nederbörd, som i sin tur medför förhöjd luftfuktighet, kommer sjukdomen att kunna breda ut sig ännu mer, eftersom svampen trivs vid hög luftfuktighet. Flyttas klimatzonerna högre upp i Norden kommer det att bidra till att utbredningen av dollar spot ökar.

Sclerotinia homoeocarpa F.T. Bennett, the causal agent associated with dollar spot, is a common and economically challenging turfgrass disease in North America. Acceptable turfgrass quality requires the optimization of cultural practices and the judicious use of fungicides. Sclerotinia homoeocarpa causes significant damage to turfgrass swards from May to October annually, therefore, requiring multiple fungicide applications to maintain satisfactory turfgrass quality throughout the growing season. Sterol demethylation inhibitor (DMI) fungicides are among the most widely used in the United States and frequent use has led to the development of fungicide resistance to the DMI fungicide class. The precise mechanism of DMI fungicide resistance in S. homoeocarpa is not completely understood, however over expression of the CPY51A gene or efflux transporter genes have been reported as the molecular mechanism for other fungal systems. Fungicide resistance to the DMI class exhibits a gradual population shift towards insensitivity that can be monitored using In vitro fungicide sensitivity assays. In vitro fungicide sensitivity assays have been used to detect fungicide sensitivity differences in different S. homoeocarpa isolates and currently serve as the most accurate methodology to detect DMI insensitivity in S. homoeocarpa. Documentation of DMI field efficacy on native S. homoeocarpa populations with differing in vitro sensitivities has not been examined thoroughly and would provide critical information regarding the association between in vitro fungicide sensitivity and DMI field efficacy. The objectives of this research are (i) to determine the association between in vitro propiconazole sensitivity and reduced field efficacy for five native S. homoeocarpa populations and (ii) to develop a qualitative in vitro sensitivity assay for detection of S. homoeocarpa isolates responsible for practical field resistance.

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