First Report of Sclerotium rolfsii Causing Stem Rot of Luffa cylindrica in Brazil

Luffa cylindrica (L.) Roem. (Cucurbitaceae) is an Asian vine widely known as the source of loofah (Purseglove 1968). In Brazil (local name bucha), it is cultivated by small-scale producers as a cash crop. In February 2017, L. cylindrica plants with rotted and girdled stem bases were found in a commercial field (27°22′S; 53°25′W) in Frederico Westphalen, Rio Grande do Sul, Brazil. Disease symptoms later led to a collapse of stems. Necrotic areas were covered with white cotton mycelium as well as abundant spherical sclerotia. The fungus was isolated in pure culture by direct transfer of sclerotia in aseptic plates with potato dextrose agar (PDA). After 7 days, cottony white colonies started to appear, often forming fans, primary hyphae 2.0 to 5.0 μm in diameter, and bearing clamp connections; sclerotia formed after 10 days, initially white and becoming dark brown with age, and 0.65 to 2.15 mm in diameter. A specimen was deposited in the herbarium (Federal University of Pelotas Herbarium, Rio Grande do Sul State, Brazil) under accession no. LPS0025. Molecular characterization was performed by amplifying and sequencing the internal transcribed spacer (ITS) region using ITS1 and ITS4 primers (White et al. 1990) and the translation elongation factor (TEF-1α) gene region using primers EF595F and EF1160R (Wendland and Kothe 1997). Sequences of the studied DNA regions were submitted to GenBank (ITS: MF425542; and TEF-1α: MF434828). BLAST searches in GenBank showed 99 to 100% identity with the existing sequences of Athelia rolfsii (Curzi) C. C. Tu & Kimbr (anamorph: Sclerotium rolfsii Sacc.) (ITS: KU128903, KX139196, DQ059578, and GU080230; and TEF-1α: KM521767, JF267808, JF267798, and JF267798). To fulfill Koch’s postulates, 10 healthy, 20-day-old L. cylindrica seedlings were inoculated with sclerotia obtained from a 15-day-old culture. Dry sclerotia were placed directly at the base of the plant (four sclerotia per plant). Ten uninoculated L. cylindrica seedlings were used as the control treatment. The inoculated and uninoculated plants were incubated in a humid growth at 26°C for 24 h and then maintained in a greenhouse. Seven days later, pathogen-inoculated plants showed typical symptoms similar to those observed under natural conditions, but no symptoms were seen on the control plants. The pathogenicity tests were carried out three times. The fungus was reisolated from the artificially inoculated plants. According to morphological and molecular characteristic combinations, the isolate was confirmed as S. rolfsii. S. rolfsii has been reported on Citrullus lanatus, Cucumis sativus, Cucurbita pepo, and other species in the Cucurbitaceae family in Brazil (Queiroz et al. 2002), but not on Luffa cylindrica. To our knowledge, this is the first report of S. rolfsii causing disease on L. cylindrica in Brazil.