First report of Diplodia sapinea causing Diplodia tip blight on Juniperus communis subsp. communis in Finland

Abstract

In August 2022, blighted shoots were observed on juniper (Juniperus communis subsp. communis) in coastal Finland. The sites were previously confirmed to contain Scots pine (Pinus sylvestris) trees with Diplodia tip blight disease, caused by Diplodia sapinea (Terhonen, 2022) (Fig. 1). Declining shoots from 32 juniper plants were collected in four different locations in the Turku archipelago. Sixty-nine isolates of D. sapinea were taken from the borderline of living and dead phloem and sapwood of surface-sterilised juniper shoots and cultured on malt yeast peptone agar (MYP). Diplodia sapinea was isolated from all the samples. The species identification was confirmed from representative isolates using morphological characters and sequencing of the ITS region of rDNA and with species-specific primers (Adamson et al., 2021). The ITS sequences of six isolates were deposited in GenBank (Accession Nos. OP847850 - OP847855). All the isolates had 99 - 100 % identity with the sequence of the type material of D. sapinea (NR152452.1). In September 2022, branches of J. communis subsp. communis were inoculated with four different isolates of D. sapinea. Bark and phloem were removed with a cork borer (3 mm diameter) and mycelial plugs (3 mm diameter) of seven-day-old D. sapinea MYP cultures were placed on the exposed sapwood surface. The mycelial plugs were wrapped with plastic film and the branches were kept outside for five weeks and then moved to a growth chamber at 20°C with a 16 h photoperiod for an additional four weeks. The juniper branches showed necrosis in the sapwood (Fig. 2a) and occasionally a blue stain (Fig. 2b) when inoculated with D. sapinea. All control inoculations remained healthy (Fig. 2c). Diplodia sapinea was reisolated only from the necrotic lesions and from the blue stain. In November 2022, the same isolates were inoculated on two-year-old Scots pine seedlings as described by Blumenstein et al. (2022) and kept in a growth chamber at 20°C with a 16 h photoperiod for 14 days. The D. sapinea isolates caused similar symptoms in Scots pine to those observed in the field (Fig. 3a). The fungus was reisolated from the inoculated parts. The control plants stayed healthy (Fig. 3b) and no D. sapinea was isolated. This is the first report of Diplodia tip blight on J. communis subsp. communis. The disease agent, D. sapinea, was previously found in Finland on Scots pine cones collected from the ground (Müller et al., 2019) and from healthy Scots pine twigs (Terhonen et al., 2021). Besides J. communis reported here, D. sapinea has been recently demonstrated to cause Diplodia tip blight in Finland on Scots pine (Terhonen, 2022). Diplodia sapinea can be endophytic and may turn pathogenic in drought-stressed trees (Blumenstein et al., 2022). Accumulating data suggest that D. sapinea is an opportunistic pathogen which also infects alternative host species Blumenstein et al. 2022 possibly related to the high load of spores from the canopy (Caballol et al., 2022). Diplodia sapinea is a new, broad-host opportunistic pathogen in Finland and the disease Diplodia tip blight can be expected to increase in alternative hosts, especially in areas that are vulnerable to abiotic changes (for example, drought). The authors wish to thank landowners and Fredrik Granberg for helping with sample collection and Linda Mutanen for her excellent laboratory work. We thank reviewer Béatrice Henricot for her valuable comments to improve this article. This study was funded by Alfred Kordelin Foundation. The authors declare that there is no conflict of interest.