Disease development of Dothistroma needle blight in seedlings of Pinus sylvestris and Pinus contorta under Nordic conditions By H. Millberg 1,3 , A. J. M. Hopkins 1, *, J. Boberg 1 , K. Davydenko 2 and J. Stenlid 1 1 Department of Forest Mycology and Plant Pathology, Swedish University of Agricultural Sciences, Box 7026, Uppsala 750 07, Sweden; 2 Ukrainian Research Institute of Forestry and Forest Melioration, Kharkiv, Ukraine; *Present address:School of Veterinary and Life Sciences, Murdoch University, Murdoch, WA, Australia; 3 E-mail: hanna.millberg@slu.se (for correspondence) Summary Dothistroma needle blight (DNB), caused by Dothistroma septosporum, was observed for the first time in the Nordic countries during the 21st century, and the dynamics of the disease under Nordic conditions are still poorly explored. In this study, we followed the develop- ment of DNB on seedlings of Pinus sylvestris and Pinus contorta, planted at two forest sites in central Sweden. PCR with species-specific primers was used to detect infections of D. septosporum in needle samples collected over a two-year period. The seedlings were also exam- ined for typical red bands and fruit bodies (conidiomata). One-year-old needles that were present on the seedlings at the time of planting became infected during the first summer. The first conidiomata appeared on P. sylvestris in autumn the same year and on P. contorta in spring the following year. The first infections of the current-year needles of both host species occurred in summer, as they were starting to elongate, and the following spring the first conidiomata appeared. On one of the sites, many seedlings carried latent infections without any symptom development. At some time points, infections of D. septosporum were detected in more than 50% of the seedlings, but red bands and conidiomata were only observed on a small number of the seedlings throughout the study period. No clear difference was observed in the susceptibility of infection between the two host species; nonetheless, at the same site, the mortality of P. sylvestris was higher than that of P. contorta, and this is likely primarily due to other stresses and the presence of another needle pathogen, Lophoder- mium seditiosum. 1. Introduction Dothistroma needle blight (DNB) is a serious disease of pine needles with a worldwide distribution and over 80 reported pine host species (Watt et al. 2009). DNB is caused by the two fungal species Dothistroma septosporum (Dorog.) M. Morelet and Dothistroma pini Hulbary (Barnes et al. 2004). Dothistroma septosporum has a worldwide distribution, while D. pini has been found in the USA, eastern parts of Europe, in France and Switzerland (Barnes et al. 2004, 2008, 2011; Ioos et al. 2010; Queloz et al. 2014). The disease causes premature needle fall and reductions in photosynthetic capacity, followed by growth loss and, in severe cases, tree mortality (Bradshaw 2004; Woods et al. 2005). In the 1960s, the disease first emerged as a problem in exotic pine plantations in the Southern Hemisphere (Bradshaw 2004). DNB was also present in Europe and North America, and since the 1990s, there has been an increase in disease severity and frequency in the North- ern Hemisphere. During the last 15 years D. septosporum has caused severe outbreaks in countries such as the UK and France and in British Columbia in Canada (Woods et al. 2005; Brown and Webber 2008; Fabre et al. 2012). Recently, D. septosporum has been reported in the Nordic countries (Drenkhan and Hanso 2009; Muller et al. 2009; Solheim and Vuorinen 2011). In Sweden, D. septosporum was discovered for the first time in 2007, on needles of P. sylvestris (Stenlid, unpublished). In this geographic area, the characteristic red bands and fruit bodies are predominantly visible in spring and early summer on two-year-old needles, a large number of which are shed in early summer (Stenlid, unpublished). However, the disease prevalence and dynamics have so far not been extensively studied under Nordic conditions. Which needle cohorts are affected by the disease will have an effect on the impact on the tree. In P. sylvestris, the primary pine species grown in Nordic countries, net photosynthetic capacity is highest in one-year-old needles and, in late summer, also in cur- rent-year needles (Aagren et al. 1980). These cohorts have been shown to be the most important contributors to tree growth (Drenkhan et al. 2006). Consequently, disease in younger needle cohorts will result in more severe consequences for the tree. Both environmental factors and host species will affect the impact of DNB (Bradshaw 2004; Bulman et al. 2013). The cli- matic requirements of DNB have been subject to thorough investigations where temperature, rainfall, humidity and needle area wetness have been identified as factors influencing spore production, the ability to infect and the time it takes for D. septosporum to complete its lifecycle (Gadgil 1977; Gilmour 1981; Karadzic 1989; Dvorak et al. 2012; Boateng and Lewis 2015). The cold climate in the Nordic countries could be limiting for the disease. Modelling of the possible geographic range of DNB based on its climatic requirements suggests that the climate in the Nordic countries, except for the northern- most part, is suitable for the disease, however not optimal (Watt et al. 2009). It has been projected that under anticipated climate change, with increased mean temperatures and precipitation in the Nordic countries, the climate will become more favourable for the disease and the area suitable for the disease will expand northwards (Watt et al. 2011; Swedish Meteo- rological and Hydrological Institute 2014). Host species differ in their susceptibility to DNB, and there can also be variation within a single host species (Fraser et al. 2015a,b). Pinus sylvestris is the only native pine species in the Nordic countries, and on the 23.3 million hectares of Received: 15.5.2015; accepted: 15.9.2015; editor: S. Woodward For. Path. doi: 10.1111/efp.12242 © 2015 Blackwell Verlag GmbH http://wileyonlinelibrary.com/