First Report of Powdery Mildew Caused by Podosphaera xanthii on Lagenaria siceraria in Brazil

The bottle gourd (Lagenaria siceraria Mol.) is a plant in the Cucurbitaceae that is grown for its fruit, which can either be harvested immature for consuming as a vegetable, or mature, dried, and used for handcrafts as well as pipes and bottles. Powdery mildew on bottle gourd was observed on plants under field conditions in three farms in the city of Capão do Leão (Rio Grande do Sul State, Brazil) during the spring and summer of 2016–17. In the three areas, all bottle gourd plants were affected by powdery mildew, and foliar disease severity reached up to 50%. Powdery mildew symptoms characterized by circular white colonies on both leaf surfaces and on petioles were easily observed by the abundant production of mycelia, conidiophores, and conidia. On the oldest leaves, colonies coalesced, often covering the entire leaf surface, causing chlorosis and eventual death. Mycelia was superficial with nipple-shaped to almost absent appressoria and producing erect conidiophores with three to six immature conidia produced in chains. Conidiophores were unbranched, cylindric, 120 to 230 μm long (mean 200 μm), composed of a cylindrical foot cell 43 to 70 μm long (mean 54 μm) and 7 to 12 μm wide (mean 10 μm), and following one to four shorter cells. Conidia were hyaline, ellipsoid-ovoid to subcylindric, 25 to 35 μm long (mean 30 μm) and 12 to 19 μm wide (mean 16 μm), with fibrosin bodies. Chasmothecia were not observed on sampled plants. The morphological traits suggested that the fungus belonged to the Podosphaera genus (Braun and Cook 2012). DNA was extracted from conidia, conidiophores, and mycelium and used to amplify the ITS (ITS1-5.8s-ITS2) region using the ITS1-KYO2 and ITS4 primers (Braun et al. 2001), and ITS sequence (513 nt) was deposited under GenBank accession number MF508534. Searches using the BLASTn algorithm revealed 99% similarity with Podosphaera xanthii from Cucumis melo var. conomon (KP120972), Cucumis sativus (AB040330), and Trichosanthes kirilowii var. japonica (AB040316). Based on scanning electron microscopy, light microscopy, and sequence analysis of the ITS region, the fungus causing powdery mildew on bottle gourd was identified as P. xanthii. To fulfill Koch’s postulates, three bottle gourd plants were inoculated with 10 to 20 conidia (obtained from symptomatic plants) per leaf on their adaxial surface using an eyelash brush. Three noninoculated bottle gourd plants served as controls. Inoculated and noninoculated plants were kept in a greenhouse, but in a separate compartment, with relative humidity around 80% and temperature ranging from 20 to 30°C. Powdery mildew symptoms, similar to those on symptomatic plants in the field, were observed 6 days after inoculation. Morphological characteristics were used for identity confirmation of the pathogen from inoculated plants. The powdery mildew, caused by P. xanthii, is one of the major diseases of the Cucurbitaceae affecting a wide range of species worldwide, including in Brazil (Far and Rossman 2017). This disease on bottle gourd was previously reported in India (Nayak and Babu 2017), but to the best of our knowledge, this is the first report of powdery mildew caused by P. xanthii on bottle gourd in Brazil. Although in this study was not measured the yield damage caused by the disease, plants showing high powdery mildew severity (∼50% of foliar area affected) showed lower fruit size, which also were sunburned.