Leaf spot disease on caused by in China.

L. is commercially important for its edible nuts, which is a major species of walnut trees in Sichuan Province (Luo et al. 2020). In September 2021, brown leaf spot symptoms were observed on roughly 75% of 60 J. regia trees surveyed in an orchard of Chongzhou city (30°40'6''N, 103°40'18''E). Initially, the lesions measuring 2-10 mm were reddish to brown with a yellowish halo, then increased in size and coalesced to cover the whole leaf, eventually resulting in severe defoliation. Six symptomatic leaves from different trees were collected, and a single fungal isolate was obtained from each of the sampled leaves using single-spore isolation (Chomnunti et al. 2014). The isolates were incubated on potato dextrose agar (PDA) with a 12h photoperiod at 25 ℃, and deposited at the Culture Collection of Sichuan Agricultural University. Colonies were identical with black center and reddish-brown periphery, and the diameter reached 2 cm after 7 days. On the host, conidiophores were mostly reduced to conidiogenous cells, with prominent and thickened conidiogenous loci. Conidia were light green to light brown, and curved with a thickened and darked hilum at the base, 0-17 septate, tapering toward the distal end, and measuring 20-120 × 3-5 μm ((x ) ̅= 56 × 4, n = 30). Morphological characteristics fit the description of (Heald & F.A. Wolf) Videira & Crous (Synonym: (Heald & F. A. Wolf) Deighton) (Poletto et al. 2017). The internal transcribed spacer (ITS) region, the large subunit of the nrDNA (LSU), and RNA polymerase II second largest subunit (2) were amplified by polymerase chain reaction and sequenced with primers ITS5/ITS4 (White et al. 1990), LR0R/LR5 (Vilgalys & Hester 1990), fRPB2-5F/Rpb2-R3 (Liu et al. 1999, Videira et al. 2017), respectively. The nucleotide blast of the two isolates (SICAUCC 22-0077, SICAUCC 22-0078) showed 99.7% and 99.5% (ITS, 472/473 bp, 471/473 bp), 100% (LSU, 725/725 bp, 725/725 bp), 99.8% (2, 866/867 bp, 866/867 bp) identities with the ex-type strain of (CBS 106.14). The phylogenetic tree combined with ITS, LSU, and rpb2 genes and morphological characteristics confirmed the identification as . These sequences of the three gene regions of two isolates were deposited in GenBank with accession numbers ON409525 and ON409526 (ITS), ON409559 and ON409560 (LSU), ON417473 and ON417474 (rpb2), respectively. The isolate SICAUCC 22-0077 was used for pathogenicity test to fulfill Koch's postulates. Three leaves of each walnut seedlings (2-year-old seedlings) were inoculated by placing a mycelium plug onto fresh wounds on the upper leaf surface punctured via a fine needle (0.7 mm in diameter), and three replicate seedlings were inoculated. For the control, a sterile PDA plug was placed on the same number of replicate leaves on the plants. The inoculated and control plants were placed in a growth chamber at 25°C with relative humidity >80% and a 12-h photoperiod. Irregular light to dark brown spots developed on inoculated leaves after twenty days, and no symptoms were observed on controls. The re-isolation and examination of the fungus showed it to be morphologically and phylogenetically identical to the originally isolated pathogen. has been described on in Mexico (Farr & Rossman 2022). To our knowledge, this is the first report of causing brown leaf spot on in China. The identification of the pathogen will provide a basis for disease management in walnut planting areas.