First report of Alternaria crassa causing yellow spots on Daturastramonium in China.

Abstract

DaturastramoniumL.(jimson weed)is an invasiveweedin agricultural fieldsand a medicinalplant. In April 2022, a leaf disease on D. stramonium was observed in Zhanjiang (21.17 N, 110.18 E), Guangdong province, China. Early symptoms were small yellow spots on leaves. Later, the spots gradually expanded and turned becoming necrotic with a clear yellow halo and a white center. The disease incidence in the field was 85% (n= 50, about 1 ha). Twenty diseased leaves were collected from the field. The margin of the diseased tissues was cut into 2 mm × 2 mm pieces, surface disinfected with 75% ethanol and 2% sodium hypochlorite for 30 and 60 s, respectively, and rinsed twice with sterile water before isolation. The tissues were plated onto potato dextrose agar (PDA) medium and incubated at 28 ℃. After 2-day incubation, grayish fungal colonies appeared on the PDA, then pure cultures were produced by transferring hyphal tips to new PDA plates. Single-spore isolation method was used to recover pure cultures for three isolates (DSAC-1, DSAC-2, and DSAC-3). The isolates were morphologically identical.They colonies were gray to brownish black. Conidiophores were branched, brown. Conidia were brown, long ellipsoid, had 4-12 transverse and 0-3 longitudinal septa; measured within 67.5-127.8 (average =105.6) × 12.5-27.8 (average=20.4) µm (n=30). Apical beak was longer than conidia body. measured within 40.5-423.5 (average =365.2) × 2.5-5.8 (average=3.2) µm (n=30). Based on morphological characteristics, the three isolates were identified asAlternariacrassa(Sacc.) Rands (Simmons 2007). Molecular identification was performed using the colony polymerase chain reaction method with MightyAmp DNA Polymerase (Takara-Bio, Dalian, China) (Lu et al. 2012) to amplify internal transcribed spacer (ITS) region, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), RNA polymerase second largest subunit (RPB2) and translation elongation factor (TEF1) with primers of ITS1/ITS4, GDF1/GDR1, RPB2-5F2/fRPB2-7cR, and EF-1α-F/EF-1α-R, respectively (Walther et al. 2013; Woudenberg et al. 2015; Nishikawa and Nakashima. 2020). Amplicons of the isolates were sequenced and submitted to GenBank (ITS, ON430524-ON430526; GAPDH, ON500656-ON500658; RPB2, ON500659-ON500661; TEF1, ON500662-ON500664). The sequences were 100% identical with those of Alternaria crassa strain CBS 116647 upon BLAST analysis. The sequences were also concatenated for phylogenetic analysis by maximum likelihood. The isolates clustered withA. crassa (CBS 116647, CBS 116648, CBS CBS-110.38, and CBS_103.18 ).Thus, the fungus associated with leafyellow spoton D. stramonium was identified asA. crassa.Pathogenicity tests were conducted in a greenhouse at 24℃-30℃ with 80% relative humidityusing 3 isolates. Individual plants were grown in pots (n = 5, 1 month old). The unwounded leaflets were inoculated using three isolates (DSAC-1, DSAC-2, and DSAC-3). The fungal mycelium on 5 mm-diameter PDA plugs were placed faced down to the leaves. Sterile PDA was used for mock inoculated comtrols.. The test was performed three times. Disease symptoms were observed on the leaves after 7 days, whereas the controls remained healthy. The pathogen was re-isolated from infected leaves and was morphologically identical to the original isolates, fulfilling Koch's postulates.A. crassa was reported causing leaf spot on D. stramonium in Algeria (Nabahat et al. 2020). To our knowledge, this report is the first report of A. crassa causing leaf yellow spot on D. stramonium in China. This pathogen possessespotential biocontrol properties on the invasiveweed, while this study also provides an important reference for the control of the diseaseof the medicinalplant.