Fusarium circinatum, the causal agent of pine pitch canker (PPC), is currently one of the most important threats of Pinus spp. globally. This pathogen is known in many pine-growing regions, including natural and planted forests, and can affect all life stages of trees, from emerging seedlings to mature trees. Despite the importance of PPC, the global distribution of F. circinatum is poorly documented, and this problem is also true of the hosts within countries that are affected. The aim of this study was to review the global distribution of F. circinatum, with a particular focus on Europe. We considered (1) the current and historical pathogen records, both positive and negative, based on confirmed reports from Europe and globally; (2) the genetic diversity and population structure of the pathogen; (3) the current distribution of PPC in Europe, comparing published models of predicted disease distribution; and (4) host susceptibility by reviewing literature and generating a comprehensive list of known hosts for the fungus. These data were collated from 41 countries and used to compile a specially constructed geo-database. A review of 6297 observation records showed that F. circinatum and the symptoms it causes on conifers occurred in 14 countries, including four in Europe, and is absent in 28 countries. Field observations and experimental data from 138 host species revealed 106 susceptible host species including 85 Pinus species, 6 non-pine tree species and 15 grass and herb species. Our data confirm that susceptibility to F. circinatum varies between different host species, tree ages and environmental characteristics. Knowledge on the geographic distribution, host range and the relative susceptibility of different hosts is essential for disease management, mitigation and containment strategies. The findings reported in this review will support countries that are currently free of F. circinatum in implementing effective procedures and restrictions and prevent further spread of the pathogen.

The aim of this study was to investigate the composition of the fungal community associated with the red-haired bark beetle (Hylurgus ligniperda Fabricius) in two plantations of Pinus sylvestris L. located in the Kharkiv and Luhansk regions (ca. 250 km apart) in the forest-steppe zone in eastern Ukraine. In each plantation, 48 beetles were collected from butts of living trees and 48 beetles from stems of fallen trees, i.e., a total of 96. Half of the beetles from each site were used for culturing fungi and the other half for direct sequencing the internal transcribed spacer of fungal ribosomal RNA (ITS rRNA). Thirty distinct fungal taxa were identified by culturing and 31 by direct sequencing. When pooled, there were 40 fungal taxa among which Ophiostoma piceae (Münch) Sydow & P. Sydow (10.3%), Alternaria alternata (Fries) Keissler (9.7%), Ogataea neopini Nagatsuka, S. Saito & Sugiyama (8.0%), Botryotinia fuckeliana (de Bary) Whetzel (5.1%), Cladosporium sp. Link (5.1%) and Sydowia polyspora (Brefeld & Tavel) E. Müller (4.6%) were the most common. Species of the genus Ophiostoma were the most abundant and included five different taxa O. piceae, O. bicolor R.W. Davidson & D.E. Wells, O. ips (Rumbold) Nannfeldt, O. canum (Münch) Sydow & P. Sydow and O. rectangulosporium Ohtaka, Masuya & Yamaoka, all of which are known to be at most weak pathogens of trees. The plant pathogen Botryotinia fuckeliana and insect pathogens Isaria farinose (Holmskjold) Fries and Beauveria bassiana (Balsamo-Crivelli) Vuillemin were also detected. Basidiomycetes were rare, among which three wood-decaying fungi Bjerkandera adusta (Willdenow) P. Karsten, Fomitopsis pinicola (Swartz) P. Karsten and Heterobasidion annosum (Fries) Brefeld were detected. In conclusion, in the forest-steppe zone in eastern Ukraine H. ligniperda is a vector of diverse communities of fungi the majority of which, if at all, are only weak pathogens of trees.

Conifer bark beetles are well known to be associated with fungal complexes, which consist of pathogenic ophiostomatoid fungi as well as obligate saprotroph species. However, there is little information on fungi associated with Ips acuminatus in central and eastern Europe. The aim of the study was to investigate the composition of the fungal communities associated with the pine engraver beetle, I. acuminatus, in the forest-steppe zone in Ukraine and to evaluate the pathogenicity of six associated ophiostomatoid species by inoculating three-year-old Scots pine seedlings with these fungi. In total, 384 adult beetles were collected from under the bark of declining and dead Scots pine trees at two different sites. Fungal culturing from 192 beetles resulted in 447 cultures and direct sequencing of ITS rRNA from 192 beetles in 496 high-quality sequences. Identification of the above revealed that the overall fungal community was composed of 60 species. Among these, the most common were Entomocorticium sp. (24.5%), Diplodia pinea (24.0%), Ophiostoma ips (16.7%), Sydowia polyspora (15.1%), Graphilbum cf rectangulosporium (15.1%), Ophiostoma minus (13.8%) and Cladosporium pini-ponderosae (13.0%). Pathogenicity tests were done using six species of ophiostomatoid fungi, which were inoculated into Scots pine seedlings. All ophiostomatoid fungi tested successfully infected seedlings of Scots pine with varying degrees of virulence. Ophiostoma minus was the only fungus that caused dieback in inoculated seedlings. It is concluded that I. acuminatus vectors a species-rich fungal community including pathogens such as D. pinea and O. minus. The fungal community reported in the present study is different from that reported in other regions of Europe. Pathogenicity tests showed that O. minus was the most virulent causing dieback in seedlings of Scots pine, while other fungi tested appeared to be only slightly pathogenic or completely non-pathogenic.

Pines are major components of native forests and plantations in Europe, where they have both economic significance and an important ecological role. Diseases of pines are mainly caused by fungal and oomycete pathogens, and can significantly reduce the survival, vigor, and yield of both individual trees and entire stands or plantations. Pine pitch canker (PPC), caused by Fusarium circinatum (Nirenberg and O’Donnell), is among the most devastating pine diseases in the world, and is an example of an emergent invasive disease in Europe. The effects of microbial interactions on plant health, as well as the possible roles plant microbiomes may have in disease expression, have been the focus of several recent studies. Here, we describe the possible effects of co-infection with pathogenic fungi and oomycetes with F. circinatum on the health of pine seedlings and mature plants, in an attempt to expand our understanding of the role that biotic interactions may play in the future of PPC disease in European nurseries and forests. The available information on pine pathogens that are able to co-occur with F. circinatum in Europe is here reviewed and interpreted to theoretically predict the effects of such co-occurrences on pine survival, growth, and yield. Beside the awareness that F. circinatum may co-occurr on pines with other pathogens, an additional outcome from this review is an updating of the literature, including the so-called grey literature, to document the geographical distribution of the relevant pathogens and to facilitate differential diagnoses, particularly in nurseries, where some of them may cause symptoms similar to those induced by F. circinatum. An early and accurate diagnosis of F. circinatum, a pathogen that has been recently introduced and that is currently regulated in Europe, is essential to prevent its introduction and spread in plantings and forests.