Cucurbita ficifolia Bouché, called fig-leaf gourd, is a cucurbitaceous climbing plant native to the Americas. During summer and autumn of 2020, almost all of fig-leaf gourds planted in an experimental plot in Jeonju (35°50'54″N, 127°07'46″E), Korea, were found to be infected by a powdery mildew with a 100% disease severity. Symptoms first appeared as white, small, irregular colonies, later coalesced into abundant hyphal growth on both sides of the leaves and young stems. Subsequently premature senescence and poor growth of affected leaves was observed. A voucher specimen was housed in the Korea University herbarium (KUS-F32173). For morphological characterization of the fungus, fresh materials were used for microscopy. Conidiophores (n = 30) were straight, 90 to 190 × 10 to 12 μm and produced three to six immature conidia in chains with a crenate outline. Conidia (n = 30) were ellipsoid-ovoid to barrel-shaped, measured 28 to 38 × 18 to 24 μm with a length/width ratio of 1.3 to 2.0, and contained distinct fibrosin bodies. No chasmothecia were found over the course of the season. The morphological feature of the fungus and the host genus were compatible with those of Podosphaera xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012), a well-known cucurbitaceous powdery mildew. For further confirmation, DNA was extracted from mycelium taken from above mentioned specimen. Internal transcribed spacers (ITS1 and ITS2) and large subunit (LSU) gene of the rDNA were amplified using primer pairs ITS1F/PM6 and PM3/TW14, respectively (Takamatsu and Kano 2001). Newly obtained sequences were registered to the GenBank under the accession numbers OL677355 for ITS and OL677356 for LSU. Our sequences shared 99.54~99.77% identity for ITS (MW559231 and MT250855) and 99.58% for LSU with sequences of P. xanthii (MK357445 and MK357438) in BLAST'n search. Pathogenicity tests were performed twice by pressing method of mycelial patches of the infected leaf onto the young leaves of five healthy fig-leaf gourd plants. Five non-inoculated plants were used as controls. Inoculated leaves started to develop powdery mildew signs after 5 days treatment, whereas the control plants remained symptomless. The fungus on the inoculated plants was morphologically identical to that originally observed on diseased plants, fulfilling Koch's postulates. To date three powdery mildew species have been reported on C. ficifolia (Farr and Rossman 2021). Among these reports, Golovinomyces orontii sensu lato was known from Chile, Mexico, United Kingdom, and Germany, Leveillula taurica from Korea and Podosphaera fuliginea (currently P. xanthii) was recorded Australia and India. To our knowledge, this is the first report of P. xanthii on C. ficifolia in Korea. Since this plant is used as grafting rootstock for preventing soil-borne gummy stem blight of cucumber caused by Didymella bryoniae in Korea (Choi et al. 2009), economic importance of the powdery mildew occurring on leaves of this plant is currently limited. Nevertheless, presence of a powdery mildew could be informative to breeding program of Cucurbita spp. and safe production of C. ficifolia fruits globally.
Keywords: ITS and LSU; climbing plant; cucurbitaceous; fig-leaf gourd.