First Report of Powdery Mildew Caused by Podosphaera xanthii on Cucumis dipsaceus in China

Abstract

Cucumis dipsaceus (hedgehog cucumber) belongs to the family of Cucurbitaceae and is widely spread in east tropical Africa and Asia (Geethakumary et al. 2015). The fruits serve as a source of food and have the potential to reduce the levels of human oxidative stresses (Urs et al. 2013). In May and June 2018, we observed that C. dipsaceus line PI504553 was infected with powdery mildew in a greenhouse located in Yangzhou University, Yangzhou, China (32.3988° N, 119.4294° E). Diffusely white powdery spots covering 5 to 40% of the surface of true leaves were observed. In later stages, the infected leaves became chlorotic and senesced. Two other lines of C. dipsaceus, PI441993 and PI390450, remained symptomless in the same greenhouse. Microscopic observations showed that conidia on infected leaves were ellipsoidal to cylindrical, contained fibrosin bodies, and were 24 to 38.4 × 16.2 to 24.5 μm (average 32.5 × 20.9 μm, n = 50). Conidiophores were unbranched, slightly roughened, 55 to 213 × 6.2 to 14.5 μm (average 132.5 × 10.4 μm, n = 50), with foot cells 40 to 90 μm long. Chasmothecia were not found in the collected infected tissues. Mycelium branched and grew superficially along plant surfaces. These characteristics fit the description of Podosphaera xanthii (Braun and Cook 2012). A pathogenicity test was carried out at the three-leaf stage by spraying a suspension of conidia (1 × 10⁶ conidia/ml) that had been collected from naturally infected leaves onto the healthy leaves of PI504553. The inoculated plants (50) and noninoculated plants (50) were grown in a growth chamber with 27/21°C (day/night) temperature, 85% relative humidity, and a 16-h/8-h (light/dark) photoperiod. Fifteen days after inoculation, powdery mildew symptoms were observed on all inoculated plants but not detectable on the noninoculated control. Further microscopic observations showed that the conidia were consistent with those on naturally infected plants, thus completing Koch’s postulates. The test was repeated once with a similar finding. To further confirm the identification, the internal transcribed spacer sequence was amplified from conidia on infected PI504553 leaves with the primer pair PxT/ITS4 (Rennberger et al. 2018). Sanger sequencing of the amplicon was performed by a commercial service (Sangon, Shanghai, China). BLASTn analysis of the resulting 459-bp sequence (GenBank accession no. MN227347) showed 100% identity with those of P. xanthii on melon from southeast China (KP980563), watermelon from Taiwan (KX369541), and Momordica charantia from Japan (AB774158). To our knowledge, this is the first report of P. xanthii on C. dipsaceus in China. This could cause significant yield losses and become a limiting factor for C. dipsaceus production with the spread of the powdery mildew. The present record also supports a wider host range of P. xanthii among Cucurbitaceae.