First Report of Powdery Mildew Caused by Podosphaera xanthii on Hulless Cucurbita pepo of Western District in China (2020)

Hulless Cucurbita pepo is an annual herb in the Cucurbitaceae family and is one of the main economic vegetable crops in China, and is a raw material for cosmetics and health care products. It is also called hulless pumpkin because its seeds have no seed coat, which is a rare variation of the Cucurbita. In July 2010, powdery mildew was observed on hulless Cucurbita pepo 'Tianran' in fields of Wuwei District, China(Liang et al.2010). Disease incidence when first observed was 65.67%, but increased to 100% in July 2019. Early disease symptoms appeared as circular or irregular white powdery areas on both leaf surfaces. At later infection stages, entire leaves，petioles, and stems were covered with white fungal mycelia that resulted in leaf yellowing and senescence，but not defoliation . Fungal hyphae were septate, branched and flexuous to straigh. Conidiophores were unbranched, straight and grew vertically to the mycelium. Conidiophore foot cells of the were cylindrical with slight constriction at basal septa and followed by one to four short cells that eventually became conidia. Conidia were barrel-shaped with ends darker than the middle and measured 20 to 32 × 12 to 19μm.T Cleistothecium formed at 20 ℃, 70% relative humidity and light intensity of 4,400 lx. They were scattered, spherical, dark brown, with parietal cells irregularly rectangular or polygonal, and 70 to 75×90 to 95μm in diameter. Cleistothecia had four to eight appendages that were colorless to partially brown, each with three to five septa and lengths 0.5 to 3.0 times the diameter of cleistothecia. The powdery mildew fungus was tentatively identified as Podosphaera xanthii and showed characteristics similar to those reported by others (Cui et al. 2018; Choi et al. 2020). rDNA was extracted from pools of fungus conidia and the ITS region amplified using primers ITS 1:5'-TCCGTAGGT GAACCTGCGG-3'/ITS 4:5'-TCCTCCGCTTATTGATATGC-3' and then sequenced. BLASTn analysis of the 540-bp (MT250855) amplicon revealed 100% sequence identity with respective rDNA sequences of Podosphaera xanthii isolates from Momordica charantia(AB774158.1). Based on the morphological characteristics and ITS sequences, the fungal species was identified as P. xanthii .Pathogenicity of the powdery mildew fungus was tested by dusting conidia from infected hulless pumpkin leaves onto three asymptomatic plants. Three noninoculated plants was used as a controls. The Infection process of P.xanthii on pumpkins observed that the conidia began to germinate at 12h after inoculation.Twenty four to 72 h post inoculation, powdery mildew mycelia appeared on inoculated leaves. After 73 to 96 h, chains of conidia formed and these germinated to form secondary infection sites. In later stages of the disease cycle, dark brown cleistothecia formed on the yellowing plant foliage. Fungus morphology from inoculated leaves was identical to that observed on original naturally infected plants. Uninoculated plants remained healthy. Powdery mildew caused by P. xanthii is a major foliage disease that affects members of the Cucurbitaceae family worldwide. The fungus has previously been reported from China on Cucurbita moschata (DQ490752), Cucurbita maxima (DQ490759), Cucurbita pepo(DQ490750), Cucumis sativus(DQ490755)(Park et al.2010; Liang et al.2007) and Cucurbita maxima, Abelmoschus esculentus (okra), Sechium edule (mirliton, vegetable pear), and Lagenaria siceraria (bottle gourd) (Choi et al. 2020; Fan et al. 2019; Xu et al. 2020; Cui et al. 2018).To our knowledge, this is the first report on the occurrence of P. xanthii on hulless Cucurbita pepo in China.