First Report of Perenniporia fraxinea Causing Heart Rot Disease of Styphnolobium japonicum in China
Authors: Su Dan , Wang Jinge , Yang Hong , Xyu Rongfan , Sun Xiaodong , Hsiang Tom , Li Linlin , Lyu Guozhong 
Published: December 2024

Styphnolobium japonicum (L.) Schott, is an ornamental species of Leguminosae, widely planted as a roadside tree in north regions of China (Kite et al. 2007). In mid-June of 2022, 45 ca. 30-year-old plants of S. japonicum growing along roadsides in Dalian, China were found to be infected by a polypore fungus that appeared to be killing many of these plants. The infected trees show leaf wilt early in the growing season, and in mid-summer, pale yellow fruiting bodies appeared at the base of the tree trunks. The fruiting bodies were effused-reflexed, with a pale yellow to yellow-brown pileus when dry, and gradually formed whitish pores on their undersurface. Over two years, approximately 15% of S. japonicum trees became infected in a ca. 10-ha area. Thirteen morphologically identical strains were generated from 20 samples by collecting 8 mm3 tissue pieces from 20 fruiting body stroma and growing these out on potato dextrose agar (PDA). The basidiospores from field collected fruiting bodies were abundantly produced in pores which were circular and 4-6 per mm, hemispherical and dropletshaped, hyaline, thick-walled and smooth, (5.8–6.9)×(4.8–5.7) μm. On PDA, the mycelia were initially white and sparse, later becoming fluffy in the center, white at the margins, and pale yellow on the underside. These morphological characteristics were consistent with Perenniporia spp., as described recently in Cui et al. (2019). To further confirm species identification, representative isolate DL was selected for molecular identification. The small subunit (SSU), internal transcribed spacer (ITS), and large subunit (LSU) of the ribosomal DNA (rDNA) were amplified with primers NS1/NS4, ITS1/ITS4 (White et al 1990), and LROR/LR7 (Aveskamp et al. 2010), respectively. The sequences were uploaded to GenBank as accession OQ592891 for ITS; OQ629793 for SSU; and OQ629858 for LSU. Sequence comparisons against the GenBank database showed 99% to 100% identity (GenBank accessions KX081110.1, MG847270.1, JF706344.1) with a strain of Perenniporia fraxinea (Bull.) Ryvarden [syn.: Vanderbylia fraxinea (Bull.) D.A. Reid]. Phylogenetic analysis also placed these isolates in a highly supported clade with the other isolates of Perenniporia fraxinea. To test pathogenicity, fresh hyphal plugs were inoculated onto sterile wounds artificially punctured on the stem surface of five healthy one-year-old S. japonicum seedlings, and plants were incubated at 28℃ with a 12-hour photoperiod in a humid chamber for 7 days. PDA plugs were used as controls and the pathogenicity test was repeated three times. After 7 days, lesions were observed on all inoculated stems, while the mock-inoculated stems were asymptomatic, and no fungal pathogen was isolated. The same fungus was re-isolated from the inoculated wounds to complete Koch's Postulates. The original pathogenic and the re-isolated inoculated fungus was identified as P. fraxinea. This fungus has also been found on Robinia pseudoacacia in Japan (Matsumoto et al. 2023) and in Germany (Kehr et al. 2020). However, to our knowledge, this is the first report of P. fraxinea causing heart rot of S. japonicum in China. The wide occurrence of this pathogenic fungus on stem bases of S. japonicum, and its lethal effect on host plants should be given more attention by landscape maintenance workers and extension agents.