Scots pine (Pinus sylvestris L) is the main forest-forming tree species in Ukraine. For many years, a gradual decline of Scots pine forest has been observed in Ukrainian regions caused by forest pests and diseases. Among forest infectious diseases, Sphaeropsis sapinea (Fr.) Dyko & B.Sutton causes shoot blight, canker, collar rot, root disease, and blue stain of many pine species and conifers of various ages in forest stands. The pathogen was found in Ukraine for the first time in the 1990s but no records of an outbreak or massive decline caused by S. sapinea had been observed until 2010. Moreover, S. sapinea was found to be in close association with the pine engraver beetle, Ips acuminatus in Ukraine as well as in other countries.
The aim of the study was to determine whether the pine engraver beetle Ips acuminatus is a vector for the S. sapinea pathogen and to confirm Leach’s postulates for this: (1) a close association between I. acuminatus and trees affected by Diplodia tip blight; (2) regular visit by I. acuminatus to healthy Scots pine forest; (3) the presence of the pathogen on the insect in nature; and (4) whether I. acuminatus can successfully vector the pathogen to disease-free host material under controlled conditions.
Materials and Methods
The field study was carried out in 2016 in Sumy Region in Ukraine. To confirm a connection between I. acuminatus and diseased trees (postulate 1), an inspection of cut Scots pine trees attacked by I. acuminatus was carried out to find trees infested by bark beetles and S. sapinea. Simultaneously, examination and sampling of Scots pine trees attacked by I. acuminatus were carried out in disease-free stands (postulate 2). To determine whether the pathogen occurs on the insects in nature (postulate 3), samples of I. acuminatus were checked for the pathogen presence using morphological and molecular (PCR) identification.
To confirm the vector of the pathogen by I. acuminatus, the disease was produced experimentally under controlled conditions (postulate 4) on healthy shoots which were attacked by artificially inoculated specimens of I. acuminatus. Afterward, all branches were visually checked to find symptoms of Spaheropsis shoot blight.
A sampling of Scots pine needles and shoots resulted in 197 morphological groups of fungal cultures. Molecular analyses of fungal morphological groups using S. sapinea specific primers demonstrated the absence of S. sapinea at the disease-free area as well as S. sapinea presence at the infested site. Samples of specimens of I. acuminatus and shoots with signs of maturation feeding or breeding galleries were analysed aiming to identify fungal phytopathogens, in particular ophiostomatoid fungi and S. sapinea.
To confirm that the bark beetles of I. acuminatus can vector pathogen S. sapinea during maturation feeding or making breeding galleries into branches with I. acuminatus, the disease was produced experimentally under controlled conditions. The branch samples examination on presence/absence S. sapinea demonstrated a capacity of I. acuminatus to vector S. Sapinea. In general, 62.9% of all branches (44% of needle samples and 82% of wood samples) showed presence of S. sapinea, while no confirmation of presence S. sapinea in control samples was n found, and all groups showed significant difference comparing with control.
To conclude, I. acuminatusis is probably a vector of S. sapinea, according to Leach’s postulates. Our study demonstrated that specimens of I. acuminatus were associated with numerous of fungi species which were generally dominated by tree pathogens, namely Sphaeropsis sapinea and ophiostomatoid species. The connection between opportunistic pathogen S. sapinea and I. acuminatus is of considerable importance to forest health, particularly to drought-stressed Scots pines. The presence of S. sapinea in galleries and on the surface of the beetle indicates that I. acuminatus may transport the pathogen and later introduce it into healthy trees.
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Key words: Sphaeropsis tip blight, insect-fungus interaction, Scots pine.