Trunk canker of Juglans sigillata caused by Lasiodiplodia pseudotheobromae in China.

Abstract

Iron walnut (Juglans sigillata Dode) is a temperate deciduous tree indigenous to China. It is mainly distributed in southwestern China, and valued for its wood and nuts (Feng et al. 2018). In September 2020, symptoms of canker on J. sigillata were observed in an orchard measuring 2 hectares located in Chongzhou City, Sichuan Province (31°5' 25″N, 105°27'36″E, 365 m altitude). Twenty percent of plants showed canker symptoms during the 50 surveyed plants. The infected trunk showed necrotic lesions with black pycnidia, that led to necrosis of branches and death of the whole plant in severe cases (Fig. 1). Six specimens from different diseased plants were collected for pathogen isolation and morphological observation. Pure cultures were obtained from single conidium on potato-dextrose agar (PDA) media according to the method described by Chomnunti (Chomnunti et al. 2014). Colonies grew fast and reached 3 cm after 5 days. The aerial mycelium was abundant, which was initially white and then grayish. Conidiomata on the host were measured 160-280 µm × 140-190 µm (average: 220 × 165 µm, n = 20), stromatic, uniloculate, dark brown to black, immersed, and erumpent when mature. Pycnidial walls 32-58 µm wide, were composed of 5-7 layers of brown to dark brown cells. Conidia were hyaline, and ellipsoidal with rounded apex and base, widest at the middle, thick-walled, and unicellular, with a size 21.5-31 µm × 11.5-15.7 µm (average: 27 × 13.5 µm, n = 50). Morphological characteristics fit the description of Lasiodiplodia pseudotheobromae A.J.L. Phillips, A. Alves & Crous (Aives et al. 2008). The internal transcribed spacers (ITS), 18S small subunit rRNA (SSU), 28S large subunit rDNA (LSU), translation elongation factor 1-alpha (tef1-α), and beta-tubulin (tub2) were amplified by polymerase chain reaction and sequenced with primers ITS1/ITS4, NS1/NS4, LR0R/LR5, EF1-728F/EF1-986R and Bt2a/Bt2b, respectively (Li et al. 2018). The sequences of the representative isolate (SICAUCC 22-0079) were deposited in NCBI with accession numbers ON090365 (ITS), ON090406 (SSU), ON090418 (LSU), ON112377 (tef1-α), and ON112378 (tub2), respectively. Nucleotide blast showed 100% similarity of all the analyzed and NCBI submitted isolates with L. pseudotheobromae (CBS116459; holotype) (accession numbers EF622077, EU673199, EU673256, EF622057, EU673111). Phylogenetic analyses based on a combined dataset showed 100% bootstrap support values in a clade with L. pseudotheobromae complexes (Fig. 2). Based on morphological and molecular analyses, the fungal pathogen was identified as L. pseudotheobromae. To conduct Koch's postulates, four 2-year-old healthy plants of J. sigillata were inoculated with 10 μL spore suspension (105 conidia/mL) onto the wounded sites via sterile pin. As control, four healthy plants were treated with sterile distilled water. The inoculated and untreated plants were placed in a growth chamber at 25°C with relative humidity >90% and 12-h photoperiod. Trunk canker symptoms appeared on inoculated plants after 15-20 days, and the pathogen was re-isolated and the controls were symptomless, confirming Koch's postulates. L. pseudotheobromae is widely distributed in various plants all over the world, usually as a pathogen associated with damping-off, wilt, die-back, root rot, collar rot, witches' brooms, or fruit rots (Zhao et al. 2010). To our knowledge, this is the first report of trunk canker on J. sigillata caused by L. pseudotheobromae in China. Trunk canker caused by L. pseudotheobromae is becoming a potential threat to walnut production, and some necessary measures for integrated management should be made.