Forest Pathology. 2018;48:e12435. wileyonlinelibrary.com/journal/efp | 1 of 9 https://doi.org/10.1111/efp.12435 © 2018 Blackwell Verlag GmbH 1 | INTRODUCTION Mistletoes are highly specialized perennial, evergreen, flower- ing semiparasitic angiosperms living on aerial parts of their hosts. They use root-like structures called haustorium into host tree branches and trunk (Mathiasen, Nickrent, Shaw, & Watson, 2008) to take up water and minerals (Glatzel, 1983; Popp & Richter, 1998; Tennakoon, Chak, Lim, & Bolin, 2014; Watson, 2001; Zuber, 2004), leading to several physiological and morphological changes in the tree (Stanton, 2007). The transpiration rate of mistletoe is higher than that of the host (Mathiasen et al., 2008; Ullmann et al., 1985). This exacerbates drought stress of trees especially in water-deficit areas (Glatzel & Geils, 2009), leading to growth loses and acceler- ated mortality by such secondary factors as fungi and bark beetles (Hawksworth & Wiens, 1996). Mistletoe can also be responsible for reduced seed production and low wood quality in the host plants (Hawksworth & Wiens, 1996; Muir & Hennon, 2007). In addition, mistletoe infection causes crown degradation by reducing the num- ber of needles, needle length and width and the longevity of needles on the infected branches (Rigling, Eilmann, Koechli, & Dobbertin, Received: 29 December 2017 | Accepted: 15 March 2018 DOI: 10.1111/efp.12435 ORIGINAL ARTICLE Quantifying the effect of pine mistletoe on the growth of Scots pine E. Bilgili 1 | M. Ozturk 1 | K. A. Coskuner 1 | I. Baysal 2 | B. Serdar 1 | H. Yavuz 1 | M. Eroglu 1 | Y. Usta 1 1 Faculty of Forestry, Karadeniz Technical University, Trabzon, Turkey 2 Faculty of Forestry, Duzce University, Duzce, Turkey Correspondence Kadir Alperen Coskuner, Faculty of Forestry, Karadeniz Technical University, Trabzon, Turkey. Email: kacoskuner@ktu.edu.tr Funding information The Scientific and Technological Research Council of Turkey (TUBITAK), Grant/Award Number: 112O258 Editor: H. T. Doğmuş-Lehtijärvi Summary Mistletoe infection results in substantial growth losses in mistletoe-infected forests. This study reports and evaluates the results of retrospective analyses of radial growth of Scots pine (Pinus sylvestris) in relation to the level of infection of pine mis- tletoe (Viscum album ssp. austriacum). A total of 43 Scots pine trees were destruc- tively sampled from different sites. Of these trees, 14 were uninfected and 29 were infected. Infection classes were determined using six-class dwarf mistletoe rating system (DMRS). All needle and mistletoe biomass were removed completely and weighed for each sampled tree. Subsamples from needles and all mistletoe biomass were taken to the laboratory for oven-dried weight determinations. Five-cm-thick wood discs were cut from the stem at the breast height (1.3 m) to determine annual basal area increment for the last 25 years. In addition to DMRS, new infection classes were created using mistletoe-to-needle biomass (MB/NB) ratio. The results showed that the radial growth losses could be as much as 41% to 64% at different infection levels. The rate of growth loss in relation to DMRS and MB/NB ratio was similar, but with a larger variability in DMRS values. The results showed that both DMRS rating and MB/NB ratio seem to be important for quantifying growth loss on Scots pine trees infected with mistletoe. The results of this study can also be invaluable in mod- elling the effects of mistletoe on the growth of Scots pine trees. KEYWORDS Viscum album, Pinus sylvestris, basal area increment, growth loss