First Report of Powdery Mildew Caused by Golovinomyces tabaci on Hibiscus trionum L. in Xinjiang, China.

Abstract

Hibiscus trionum L., a member of the Malvaceae family and Hibiscus genus, is a prevalent weed in agricultural fields across Xinjiang, China. In September 2023, powdery mildew was observed on H. trionum leaves in the cotton experimental field (0.133 ha) at Shihezi University, with the surfaces of the leaves covered with white powder. The mildly infected leaves displayed chlorosis, while those with severe infections exhibited senescence. In the field, we conducted a five-point sampling method across five 1 m² plots, examining a total of 100 weeds, of which 96% displayed these symptoms. Morphological analysis revealed conidiophores measuring 40.2-160.8 × 3.3-10 µm (without conidia), with a mean dimension of 80 × 9.2 µm, arising from the upper surface of hyphal mother cells or laterally, typically towards one end of the cell. Foot-cells were straight or curved, measuring 28.8-92.4 × 7.8-10.9 µm (mean 66 × 9.6 µm), followed by 2-4(-5) shorter cells, with the basal septum usually located at the junction with the supporting hypha. Conidia were transparent, cylindrical to nearly rectangular, with dimensions of 18.2-36.2 × 10.8-15.6 µm, consistent with the characteristics of Golovinomyces tabaci (Qiu et al., 2020). To confirm the morphological identification, the total genomic DNA of the representative strain GT-SHZ was extracted using the CTAB method (Doyle and Doyle. 1990). The ITS, 28S rDNA, and TUB2 sequences were amplified with the specific primers ITS5/ITS4(White et al., 1990), LSU1/LSU2 (Scholin et al., 1994), and TubF1/TubR1(Qiu et al., 2020), respectively. After TA cloning and sequencing, vector sequences were removed, and the sequences of the ITS, 28S rDNA, and TUB2 were deposited in GenBank under accession numbers PQ571799, PQ572139, and PQ586974, respectively. BLAST analysis indicated that the ITS, 28S rDNA, and TUB2 sequences of GT-SHZ were 100%, 98.58%, and 99.5% identical to ITS (MK937796.1), 28S rDNA (AB430816.1), and TUB2 (ON645928.1) of G. tabaci, respectively. A combined phylogenetic analysis of ITS, 28S rDNA, and TUB2 confirmed that the GT-SHZ clustered with G. tabaci. To assess the pathogenicity of the GT-SHZ, conidia collected from H. trionum leaves infected with GT-SHZ were formulated into a spore suspension (1 × 107 spores/mL) and sprayed onto the leaf surfaces of six healthy plants, with six non-inoculated plants as controls. Both groups were placed in two growth chambers with consistent environmental conditions (25°C temperature and 60% humidity). Twelve days post-inoculation, a large amount of white powder appeared on the leaves of the inoculated plants, while the controls remained healthy. The morphological characteristics and the nucleotide sequences of the ITS, 28S rDNA, and TUB2 from the fungus collected on the inoculated leaves are consistent with the GT-SHZ. Two pathogenicity tests obtained similar results, identifying G. tabaci as the pathogen causing powdery mildew on H. trionum. This pathogen has been reported on Cucumis sativus, Trigonotis peduncularis, and Rubia cordifolia in Henan, China (Qiu et al., 2020). However, this is the first report of G. tabaci causing powdery mildew on H. trionum in Xinjiang, China. The dry climatic conditions in Xinjiang facilitate the occurrence of powdery mildew, which is a common disease in agricultural production and seriously threatens crop health. The identification of this pathogen and the confirmation of new hosts provide valuable support for future powdery mildew control strategies.