Leaf Spot of Sanhua Plum caused by Botryosphaeria wangensis in China.

Abstract

Sanhua plum (Prunus salicina L.) is planted widely in Babu district of Hezhou, Guangxi with a planting history of more than 70 years (Zhou et al., 2021). In August 2021, leaf spot disease was observed with approximately 50% incidence on Sanhua plum leaves in Babu district in Hezhou, Guangxi (N23°49'-24°48', E111°12'-112°03'). The symptoms initially appeared as small, round, and chlorotic spots. As the disease progressed, the lesions enlarged and margins became dark brown. To isolate the pathogen, small pieces (5 × 5 mm) of the infected tissue margins were sterilized by exposure to 75% ethanol for 10 sec, 2% sodium hypochlorite for 1 min and rinsed three times in sterile water. Pieces were incubated on potato dextrose agar (PDA) at 28℃. In total, 75 isolates were obtained from leaves which were collected from three trees. Fifty of them were morphologically identical with a 67% average isolation frequency. Three representative isolates (HZ13-1, HZ26-3 and HZ47-1) were selected for further study. The cultures on PDA were initially white, fluffy with uneven margins and turned smoky gray to olivaceous at the surface. The reverse sides were olivaceous gray to iron gray after seven days. The growth rate of mycelium was 2.5 cm/day. Conidia were produced after two weeks by exposure to near-fluorescent light for 10 hours per day. Conidia were fusiform, hyaline, thin-walled, smooth with granular contents unicellular, and 19.7 ± 0.13 × 5.8 ± 0.06 μm (n=90), 19.8 ± 0.09 × 6.5 ± 0.23 μm (n=90), and 20.6 ± 0.20 × 6.7 ± 0.12 μm (n=90) for HZ13-1, HZ26-3 and HZ47-1, respectively. These characteristics were consistent with the descriptions of the Botryosphaeria wangensis (Hattori et al. 2021). The DNA was extracted from mycelia, and the internal transcribed spacer (ITS), elongation factor 1-alpha gene (EF1-α) and β-tubulin (TUB2) were amplified using the primer pairs ITS1/ITS4, EF1-728F/EF1-986R and T1/BT2b (White et al. 1990, Carbone et al. 1999, Yu et al. 2021), respectively. The sequences were compared with GenBank and they all showed over 99% identity to the type strain of B. wangensis CERC 2298 (Li et al. 2020). Sequences of the three isolates were deposited in GenBank (Accession Nos.: ITS, OP804110-OP804112; EF1-α, OP821748-OP821750; TUB2, OP821745-OP821747). The three isolates were identified as B. wangensis based on the maximum likelihood phylogenetic tree of concatenated sequences of ITS, EF1-α, and TUB2 with RAxML version 2.0. Pathogenicity tests were performed on healthy leaves of 2-year-old Sanhua plum, which were wounded by a sterilized needle in a greenhouse. A 5-mm-diam hyphal plug was placed on the wound. Each isolate was used to inoculate three plants, with 20 leaves per plant. Control plants were inoculated with sterile PDA plugs. All the plants were sprayed with distilled water and covered with plastic bags. After four days of incubation at 28℃ with constant light, lesion began to develop in the inoculated leaves. After ten days, the average diameter of lesions was up to 1.5 cm but controls remained symptom-free. The fungi were reisolated from inoculated symptomatic leaves and were identical to the inoculated isolates, thus completing Koch's postulates. To our knowledge, this is the first report of B. wangensis associated with leaf spot of Sanhua plum in China. The results will contribute to accelerating the development of future epidemiological studies of B. wangensis on Sanhua plum.