First Report of Leaf Blight Caused by Botryosphaeria dothidea on Aucuba japonica in Zhejiang Province, China.

Abstract

Aucuba japonica var. variegata Dombrain is a common evergreen cultivated ornamental in China (Li et al. 2016). In December 2022, severe leaf blight on A. japonica was observed next to the Meishiyuan of Zhejiang Normal University (29°8'4″N, 119°37'54″E) in Jinhua City, Zhejiang Province, China. There were seven plants in the surveyed area, and over 50% of leaves were affected. The early symptoms were small gray spot parts with brown borders on the tip of the leaves. Then the grey parts gradually expanded and became brownish black. In severe cases, the whole leaves became black and blighted. To identify the pathogen, 5 symptomatic leaves were randomly collected from 5 plants and cut into small pieces (5 mm × 5 mm), surface disinfected in 1% sodium hypochlorite solution for 3 min, followed by 75% alcohol for 30 s, then rinsed in sterile distilled water thrice. Tissues were cultured on potato dextrose agar (PDA) and incubated at 28°C for 7 days. Pure cultures were obtained by the single-spore method. Thirteen strains were isolates from the tissues, and nine of them showed similar morphological characteristics. Colonies were white initially, then became gray. The undersides of the colonies became black gradually. Hyaline, fusiform conidia (n = 30) were 17.1 to 24.76 µm (average 20.39 ± 1.906 µm) in length and 5.4 to 6.61 µm (average 6.19 ± 0.434 µm) in width. The DNA of nine isolates were extracted by Ezup Column Bacteria Genomic DNA Purification Kit, and their sequences were identical, so they were named QM1. The internal transcribed spacer (ITS) region, translation elongation factor 1-α (TEF1), and β-tubulin (TUB2) genes were amplified with primer pairs ITS1/ITS4, TEF1-728F/TEF1-986R and βt2a/βt2b (Slippers et al. 2004), respectively. The BLAST analysis indicated that ITS (OR215464), TEF1 (OR243689), and TUB2 (OR243688) of the isolate QM1 were 99 to 100% identical to those of Botryosphaeria dothidea (GenBank accession nos. MH329646 for ITS sequences; OL891702 for TEF1 sequences; MK511445 for TUB2 sequences). In addition, the phylogenetic tree based on sequences from ITS, TEF1 and TUB2 was constructed with MEGA 11 by use of the maximum likelihood method with 1,000 bootstrapping iterations. Based on the multi-locus phylogeny and morphological features, the isolate QM1 was identified as B. dothidea. To test the Koch's postulates, ten leaves from three healthy two- to three-year-old A. japonica plants were surface disinfested with 75% ethanol for 30 s, rinsed with ddH2O three times. The leaves were wounded with a sterile needle and inoculated with 2ml drop of the isolate QM1 conidial suspension (106 spores/mL), with sterile distilled water as a control. All plants were placed in a greenhouse at 28°C, >70% relative humidity and 12 h light/day. The experiment was repeated three times. After 7 days, leaves of the inoculated group showed symptoms similar to those observed on the naturally infected leaves, while leaves of the control group remained asymptomatic. The pathogen was reisolated from inoculated leaves and was confirmed as B. dothidea based on morphological and molecular analyses. It has been reported B. dothidea cause leaf disease in a wide range of hosts in China, such as Camellia oleifera (Hao et al. 2023), Kadsura coccinea (Su et al. 2021). To our knowledge, this is the first report of Botryosphaeria dothidea causing leaf blight on Aucuba japonica in Zhejiang Province of China. B. dothidea are usually secondary invaders and are known to cause diseases in stressed plants. The results further expand the host-range of B. dothidea, and would help to establish control strategy against the disease.