Ash dieback is an emerging disease caused by the fungus Chalara fraxinea that severely affects Fraxinus excelsior and F. angustifolia stands in Europe. Previous studies have shown that this pathogen prefers temperatures around 20°C, while its growth in pure cultures at 30°C proved to be very limited. The purpose of this study was to determine the effects of temperature on the development and growth of C. fraxinea in pure cultures and in plant tissues, as well as to test the heat tolerance of F. excelsior saplings. The sensitivity of fungus to heat in ash tissues was higher than in pure cultures. Low isolation success rate from diseased ash tissue after a five-hour hot water treatment at 36°C and the relatively high survival rate of ash saplings after hot water treatments at 36°C and 40°C indicate possibilities for the development of a C. fraxinea eradication method in ash saplings. Field monitoring showed that in hot weather periods, thermal conditions inside the ash tissues can be extreme enough to markedly decrease the viability of C. fraxinea in infected plant tissues.

Brown spot needle blight (BSNB), a disease of pine trees caused by the fungus Lecanosticta acicola, has been known in Slovenia since 2008 and in Croatia since 1975. Recent outbreaks in Slovenia prompted this study to compare L. acicola populations in these two neighbouring European countries. Sixty-nine isolates collected from three pine species (Pinus mugo, P. halepensis and P. nigra) were used to determine the phylogenetic relationships, genetic structure, and reproductive strategy of the pathogen. EF1-a sequences showed that Slovenian and Croatian isolates share a common ancestry with individuals from central and northern Europe. Population structure analysis revealed four distinct population clusters of L. acicola in these two countries, generally corresponding to their respective geographic location and host. An unequal ratio of mating types and a low overall genetic diversity in the population indicated a strong influence of asexual reproduction. Although some of the oldest recorded European occurrences of BSNB are from Croatia, this study provided no evidence that the population studied in Croatia was the source of the sampled outbreaks in Slovenia. Recent outbreaks of L. acicola in Slovenia are most likely due to introductions from other, yet to be identified, sources.

Summary The presumed resistance of individual ash trees to ash dieback caused by invasive pathogen Hymenoscyphus fraxineus is an important issue for the maintenance of ash in European forests. All known studies regarding the resistance of ash trees to ash dieback were conducted in plantations and stands of F. excelsior; however, no such data exist for F. angustifolia. Crown damage assessments were performed over four consecutive years between 2009 and 2012 at a F. angustifolia clonal plantation in Hraščica, Slovenia. Inoculation of H. fraxineus into the branches of the most and least damaged clones of F. angustifolia and leaf phenology assessments was performed to verify the presence of defence mechanisms that limit fungal growth or promote disease escape. Additionally, root collars of selected clones were inspected for fungal infections. The crown damage assessments showed considerable differences among F. angustifolia clones, indicating genetic variability in susceptibility to ash dieback. Crown dieback progressed significantly over the 4‐year time period; the mean crown damage of individual clones in 2012 varied between 16.7% and 83.8%. Significant differences among F. angustifolia clones were found in the inoculation trials and leaf phenology assessments. However, defence mechanisms such as early leaf flushing, early leaf shedding and the ability to inhibit pathogen growth in host tissues were not confirmed. High frequency of Armillaria spp. and H. fraxineus root collar infection demonstrated the need for whole tree inspection to determine causal agent of damages on individual ash trees. Armillaria spp. may be highly associated with ash decline epidemiology.

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© iForest -Biogeosciences and Forestry IntroductionAsh dieback is an emerging fungal disease caused by the ascomycete Hymenoscyphus pseudoalbidus Queloz et al. (2011), anamorph Chalara fraxinea Kowalski (2006). This highly pathogenic fungus was most likely introduced from the Far East (Zhao et al. 2012). The first symptoms of the disease were observed in Poland in 1992, and since the pathogen has spread throughout Europe (Timmermann et al. 2011, Pautasso et al. 2013. Common ash (Fraxinus excelsior L.) and narrow-leaved ash (F. angustifolia Vahl.) are European ash species the most heavily affected by the pathogen (Kirisits et al. 2009, Gross et al. 2014, although some introduced ash species could also be attacked (Drenkhan & Hanso 2010).Control of the ash dieback is hardly possible in areas where the disease is already established (Gross et al. 2014). However, proper control measures should still be taken to prevent or at least to slow down further spread of the pathogen (Pautasso et al. 2013), or to eradicate the pathogen at locations where this is still possible (relatively small, spatially limited and recently infected areas). Furthermore, the protection of individual high-value ash trees should also be often provided. Chemical treatment may be one of the options in some cases, but data on effective chemical agents for control of the ash dieback pathogen are lacking.Ascospores that develop in apothecia on infected ash leaf debris from the previous year are the major source of new infections (Gross et al. 2012(Gross et al. , 2014. Leaf debris from the previous year represents an inoculum source for some other well-known tree diseases. Good examples are cherry leaf spot caused by Blumeriella jaapii (Rehm) Arx and apple scab caused by Venturia inaequalis (Cook) Wint. For control of both mentioned diseases, urea treatments of the infected leaves are used to reduce pathogen sporulation (Sutton et al. 2000. Thus, removal of the infected ash leaf debris (if possible) or treatment of the debris to prevent sporulation of H. pseudoalbidus could therefore be regarded as potential control measures (Cooke et al. 2013).The main aim of this study was to examine the effect of eight different fungicides that are effective against different pathogens of the Helotiales order as well as the effect of urea on mycelial growth in Petri plates and development of H. pseudoalbidus apothecia on fallen ash leaf petioles. Materials and Methods H. pseudoalbidus isolates used in testingFour Slovenian isolates of Hymenoscyphus pseudoalbidus were used in this study (Tab. 1). Isolates were previously obtained from necrotic F. excelsior wood samples and stored in the culture collection of the Laboratory for Forest Protection at the Slovenian Forestry Institute (ZLVG). The selected isolates were of different morphology in culture and represented different geographical regions of the country. Three weeks before treatment experiments (preliminary testing was planned on July 13 th , 2010, and the main testing on August 26 th , 2010), all four is...