The epidemiology of dothistroma needle blight in Britain

Abstract

The epidemiology of D. septosporum in Britain, from its behaviour and effects in a forest stand to its spread and population biology was investigated in this thesis. The peak infection period for the pathogen was determined to be spring and summer (June to September) albeit with low levels of infection occurring throughout the year. Contrary to previous belief the pathogen was found to have more than one life cycle per year in England. This is undoubtedly a factor contributing to the severity of the disease seen in this country. Such high disease levels across multiple years reduce the length of needles on affected trees. This reduced needle length compounds the effects necrosis and premature defoliation have on photosynthetic capacity and contributes to the reduced main stem increment seen in DNB affected trees. Once needles are shed from the tree the fungus does not survive for extended periods of time. In needles that reach the forest floor 90% of infective propagules die within three months whereas this figure rises to six months in needles that remain lodged in the canopy. Nonetheless, the pathogen is successfully dispersed far greater distances than previously believed. Infection was detected over 1,400 m from an inoculum source, which is over five times the greatest distance previously reported. Microsatellite analysis of isolates from across Britain revealed six distinct population clusters: two restricted to Scotland, two predominantly in England and Wales and two occurring throughout Britain. The two England and Wales populations had considerable overlap with two populations from Brittany, France, suggesting that some degree of pathogen exchange had occurred between the two countries. Furthermore, one of the Scottish population clusters was found to have links with western North America, the origin of its dominant host, lodgepole pine, indicating its introduction from this area into Scotland. Together the findings presented in this thesis can be used to guide improved management practices of dothistroma needle blight in Britain.