First Report of Powdery Mildew of Excoecaria cochinchinensis Caused by Pseudoidium neolycopersici in China

Abstract

Excoecaria cochinchinensis Lour., a small evergreen shrub in the family Euphorbiaceae, is usually grown as a native ornamental plant in South China. In April 2019, severe powdery mildew infection was observed on E. cochinchinensis in a garden (23.9°N, 113.23°E) in Guangzhou, China. Nearly 60% of the Excoecaria plants were infected, and the foliage of infected plants turned yellow and necrotic, followed by rapid defoliation. Voucher specimens were deposited in the Herbarium of Mycology of Guangdong Institute of Traditional Chinese Medicine under accession number HMGDTCM18. White to gray rounded irregular patches, consisting of abundant mycelia and conidia of the fungus, were found on both leaf surfaces and green stems. Hyphae were septate, branched, and with moderately lobed appressoria. Conidia were solitary, borne on top of conidiophores, and were ellipsoidal, hyaline, 38 to 48 × 13 to 17 μm (mean 42 ± 3 × 15 ± 1 μm, n = 50), containing no distinct fibrosin bodies. Conidiophores were erect, unbranched, consisted of three cells, and 84 to 117 μm long (mean 106 ± 7 μm, n = 50). Chasmothecia were not observed on sample plants. The pathogenicity of this fungus was confirmed by gently pressing infected leaves onto healthy leaves and stems of three healthy seedlings in separate pots, and two noninoculated seedlings were used as a control. After maintaining at 25°C in a greenhouse for 10 days, similar disease symptoms were observed in the inoculated plants, whereas no symptoms developed on control plants. Molecular identification was conducted by extracting genomic DNA from conidia washed from infected leaves and using a Fungal DNA Kit (Omega Bio-tek, Guangzhou, China). The rDNA internal transcribed spacer region was amplified using primers ITS1/ITS4 (Glass and Donaldson 1995). The obtained sequence (GenBank accession no. MK811439) analysis by BLASTN search showed>99% similarity with Pseudoidium neolycopersici (GenBank accession nos. LC009912.1 and AB032484.1). On the basis of the morphological and molecular analyses, the fungus was identified as P. neolycopersici (Kiss et al. 2001). This pathogen has only been reported on Codiaeum variegatum, a euphorbiaceous host (Liu et al. 2015). To our knowledge, this is the first report of this fungus infecting Excoecaria. In addition, P. neolycopersici has been reported on tomato, papaya, and Physalis pubescens in China. Therefore, powdery mildew infection on E. cochinchinensis not only poses a potential threat to health and beauty of this ornamental plant but may also serve as a reservoir of the pathogen that could allow spread to other economically important hosts.