Overwintering of the Dollar Spot Pathogen Occurs in Shoots and May Be Influenced by Temperature.

Dollar spot, caused by the fungus currently classified as Sclerotinia homoeocarpa, is one of the most common diseases of turfgrass worldwide. In spite of this, research-based evidence concerning sources of initial inoculum, such as pathogen overwintering, is limited. In the current study, in planta overwintering of S. homoeocarpa was investigated. Individual dollar spot foci were marked on putting green height creeping bentgrass at the OJ Noer Turfgrass Research Center in Verona, WI in the fall of 2010 and 2011. In December of both years, a standard soil core was collected from symptomatic turf and asymptomatic turf 3-5 in. from foci. Roots and shoots were surface disinfested and plated on antibiotic amended potato dextrose agar (PDA+++) using sterile forceps. After 48-72 hours, fungal colonies resembling S. homoeocarpa were transferred to fresh PDA+++ to obtain a pure culture. Identification of S. homoeocarpa was confirmed by colony morphology and sequencing of the ITS region. The process was repeated in the spring of 2011 and 2012. Rates of S. homoeocarpa isolation from roots were very low at all sampling dates and the pathogen was not isolated from roots in either spring. Shoot isolations were most common in both seasons and S. homoeocarpa was isolated once from symptomatic shoots in spring 2011 and multiple times from both symptomatic and asymptomatic shoots in spring 2012. The rate of fall shoot isolations in 2010 and 2011 was not different but spring isolations from symptomatic shoots were significantly different between 2011 and 2012 (P=0.02). The winter of 2011-2012 was extremely more mild than that of 2010-2011 and normal historical highs,with average monthly air temperatures approximately 7°F higher. This research indicates that overwintering in shoots provides initial inoculum for S. homoeocarpa and that winter temperatures may influence winter survival of the pathogen.