First Report of Downy Mildew on Buffalo Gourd (Cucurbita foetidissima) Caused by Pseudoperonospora cubensis in North Carolina

Cucurbita foetidissima (buffalo gourd) is a viny perennial crop native to the United States. This plant, characterized by long, triangular, gray-green leaves and fruits that are 5 to 10 cm in diameter and yellow at maturity, is feral and considered a weed to many. However, C. foetidissima serves several purposes in Native American cultures and its oil-rich seeds have potential economic profitability (Bemis et al. 1978; Clowney et al. 2013). C. foetidissima grows well in dry arid regions, but is widely distributed throughout the United States, growing in 22 states and Mexico (Bemis et al. 1978; USDA-NRCS PLANTS). In August through October of 2014, Pseudoperonospora cubensis was observed on C. foetidissima plants in Lenoir, Rowan, and Haywood counties in North Carolina. These plants were grown in sentinel plots as part of the CDM-IPM PIPE, the cucurbit downy mildew disease-forecasting system. The disease was characterized by irregular brown lesions with chlorotic halos and sporulation on the abaxial leaf surface. The oomycete was collected from infected leaves and observed with a microscope, revealing characteristic P. cubensis structures. An average sporangiophore trunk length of 307.6 µm and pigmented, papillated, lemon-shaped sporangia of 23.9 × 17.9 µm were observed. Sporangia collected from Lenoir County field samples were used to inoculate detached lab-grown C. foetidissima leaves. A 10⁴/ml suspension of sporangia was applied to the abaxial surface of the leaves using a Preval sprayer. Inoculated leaves were kept in clear, acrylic boxes and placed in incubators set for a cycle of 12 h of light at 21°C then 12 h of dark at 18°C. Humidity was maintained by the addition of a damp paper towel in the box. Six days post inoculation, sporulation of the pathogen was observed on the abaxial side of the leaf. Species confirmation was also carried out molecularly. DNA was extracted from sporulating lesions and PCR was used to amplify nuclear and mitochondrial regions. PCR products were sequenced and BLAST searches showed Beta Tubulin (Btub), NADH dehydrogenase subunit 1 (Nad1), and NADH dehydrogenase subunit 5 (Nad5), had 100% identity to P. cubensis sequences in NCBI (GenBank Accession Nos. JF304706.1, KJ141003.1, and HQ636556.1, respectively). Sequences of P. cubensis isolates from C. foetidissima were added to GenBank (Accession Nos. KP970684, KP970682, and KP970683). Little is known about the impact of wild cucurbits in the yearly cucurbit downy mildew epidemic, which warrants further research. The perennial nature of C. foetidissima makes this host of particular importance, as it is believed P. cubensis may overwinter on wild cucurbits (Ojiambo et al. 2011). This is the first report of P. cubensis infecting C. foetidissima in field settings in the United States. Identifying noncommercial and wild cucurbits that host the downy mildew pathogen is an important factor to identify as we learn more about the pathogen and details of the disease cycle.