Occurrence of Powdery Mildew Caused by Pseudoidium neolycopersici on Papaya (Carica papaya) in China

Abstract

Papaya (Carica papaya L.) is an economically important fruit tree in tropical and subtropical countries. In late March 2017, symptoms of powdery mildew were observed on 80% of papaya plants that were grown at the campus of Fujian Agriculture and Forestry University, Fuzhou, China. The initial symptoms of powdery mildew on papaya appeared as circular to irregular white colonies on both sides of leaves of papaya. As the disease progressed, the leaves became covered with white mass of fungal colonies; eventually leaves showed chlorosis and necrosis followed by desiccation and rapid senescence. Fungal mycelium was amphigenous, white, and effuse. Hyphae were hyaline, septate, branched, and 5 to 8 μm wide with lobed appressoria. Conidiophores were approximately 90 to 150 × 8 to 10 μm with cylindrical foot cells of 30 to 60 × 8 to 10 μm followed by one to three short cells of 14 to 25 μm. Conidia were hyaline, ellipsoidal-ovoid, or doliform, 30 to 60 × 14 to 20 μm in size, and were produced singly on the tip of conidiophores. Germ tubes were terminal or subterminal, with lobed ends. The sexual morph was not found. Based on these characteristics, the pathogen was identified as a Pseudoidium sp. (Kiss et al. 2001). A voucher specimen (HMAS 247192) was deposited at the Institute of Microbiology, Chinese Academy of Sciences Herbarium Mycologium, Beijing, China. DNA was extracted and the nrDNA internal transcribed spacer (ITS) region was sequenced. The ITS sequence (deposited in GenBank as KY887788) was identical to that of Pseudoidium neolycopersici isolates AB032484, KY776199, and JQ972700. Pathogenicity was confirmed through gently pressing infected leaves onto the leaves of three asymptomatic papaya plants (approximately 90 cm high). The inoculated papaya plants were covered with plastic bags placed outdoors for 7 days at 25°C under natural daylight conditions in April. Typical symptoms of powdery mildew were observed after 7 days, whereas noninoculated plants remained asymptomatic. To confirm pathogen identity, an eggplant cultivar (unknown) and two tomato cultivars (Zaofen and Fenguo) were also cross inoculated (five plants of each cultivar) with inoculums from infected papaya plant. Only tomato cultivars showed powdery mildew symptoms. The ITS sequence of sample MG846017 from the tomato host plant was identical to that of the Pseudoidium sp. on papaya plants (KY887788). The inoculated eggplant cultivar remained asymptomatic. To our knowledge, this is the first report of papaya powdery mildew caused by P. neolycopersici in mainland China. The only report of this disease caused by P. neolycopersici along with Erysiphe diffusa and Podosphaera xanthii was in Taiwan (Tsay et al. 2011). Other powdery mildew species have been reported on papaya in many tropical and subtropical areas (Liberato et al. 2004). Papaya is a popular and widely grown crop in China, and the presence of a new pathogen could have an impact on its production. P. neolycopersici has been reported on tomato in China (Li et al. 2008). Powdery mildews with similar ITS sequences have recently been found on other plant species as well (Liu et al. 2015; Southwell et al. 2018).