First Report of Dodder (<i>Cuscuta japonica</i>) Parasitizing Japanese Red Pine (<i>Pinus densiflora</i>) in China

Dodders (Cuscuta spp.) are widespread parasitic weeds on many plants all over the world, which can lead to considerable damage to ecosystems (Costea et al. 2011). Many Cuscuta species are recognized as class 1 designated Prohibited Noxious Weeds (Government of Canada 2016), because they can reduce the biomass of hosts and modify the structure of plant communities (Pennings and Callaway 2002). Cuscuta japonica is a hemiparasitic plant with many hosts including Araliaceae, Asteraceae, Fabaceae, Polygonaceae, Rutaceae, and Salicaceae (Chen et al. 2012; Yuan et al. 2015). Until to now, C. japonica has not been reported parasitizing hosts from gymnosperm species. In September 2019, dodder was observed parasitizing Pinus densiflora in Lushan Mountain (36°18′17.99″N, 118°5′50.27″E, elevation 662 m), Zibo City, Shandong Province, China. P. densiflora, commonly known as Japanese red pine, is an evergreen pine with a range including Japan, the Korean Peninsula, and northeastern China. It is widely cultivated due to its timber for construction, trunk for resin, bark for tannin, and needles for medicine. One individual of P. densiflora, approximately 5 m tall, was infected, and the branches and needles of the host were wrapped by the leafless stems of this dodder. The haustoria penetrating branches and needles of the host were visible to the naked eye. Uneven holes emerged on Pinus needle after removal of the dodder stem, and the tissues of Pinus needle around the holes became black. Some needles were chlorotic, wilted, and even defoliated. This dodder can be morphologically identified as C. japonica according to previous investigation records and descriptions, including slightly yellow stems with branches 1.4 to 2.6 mm in diameter, capsules globose and smooth, one style, and stigma two-lobed with elongated lobes. An area of ∼5.8 ha around the infected P. densiflora was carefully examined for this dodder, and only three infections were discovered on one individual of P. densiflora. No dodders were found on the nearby deciduous trees within the above examined area. However, this dodder was found parasitizing deciduous trees such as Ulmus pumila outside of the examined area in the Lushan mountain, so P. densiflora might not be a primary host for this dodder from the point of view of the entire mountain. Therefore, we tentatively classify P. densiflora as a rare host of this dodder. Species identification was further conducted by characterizing the whole plastome. First, total DNA was extracted from silica-dried stems of this dodder. Then the genome-skimming method was applied to generate the whole plastome as described in Qu et al. (2019). One sequence of 121,005 bp was deposited in GenBank (accession no. MN585289). BLAST analysis reconfirmed the identity of this dodder as C. japonica (99.92%, target accession no. MH780080). Furthermore, phylogenetic analysis supported this dodder as sister to C. japonica with nearly the same branch length (0.0002 versus 0.0003) with 100% bootstrap support value. Thus, this dodder observed in Zibo City was identified as C. japonica based on evidence from morphology, sequence, and phylogeny. Specimens of C. japonica on P. densiflora were collected and deposited at the herbarium of the College of Life Sciences, Shandong Normal University (no. 092217A). C. japonica seems to be an extremely rare occurrence on P. densiflora. It will need more evidence to judge if it is a real infection on P. densiflora. It is probable that C. japonica is not a parasite that can threaten populations of P. densiflora at present. Therefore, C. japonica is not considered as a management concern on Japanese red pine. To the best of our knowledge, this is the first report of C. japonica parasitizing P. densiflora, and the only known example of C. japonica parasitizing a gymnosperm species in the world.