Seasonal availability of inoculum of the Heterobasidion root disease pathogen in central Wisconsin

After deposition of airborne basidiospores, the root disease pathogen Heterobasidion irregulare Garbelotto and Otrosina infects fresh conifer stumps and spreads through root grafts or by root contact to adjacent trees. Infection can be prevented, however, by borate application. Because the need for stump protection depends on inoculum availability, spore trapping was conducted (usually biweekly) from September 2009 through December 2011 in three infested plantations of predominantly red pine (Pinus resinosa Aiton) in central Wisconsin. A semiselective medium in Petri plates was exposed for 1 h in daylight at each of four locations in each plantation. After 7-10 days incubation at 20 °C, plates were examined and presence and abundance of colonies of the Spiniger asexual stage were recorded. Heterobasidion irregulare was detected on most dates during the two growing seasons, but colonies were most abundant during late summer, fall, and early winter. Relatively fewer colonies developed on medium exposed during periods of coldest winter temperatures, but colonies of the pathogen did develop frequently on medium exposed at ≤ 5 °C and occasionally on medium exposed at ≤ 0 °C. Biologically based guidelines for stump treatment require additional studies of seasonal factors influencing inoculum availability, in situ spore germination, infection, and establishment of the pathogen.