First Report of False Rust caused by Synchytrium psophocarpi on Winged Bean (Psophocarpus tetragonolobus) in Taiwan

From September 2020 to January 2021, an unknown disease of winged bean (Psophocarpus tetragonolobus) was reported by local growers in the Toucheng Town, Yilan County (N24.91, E121.85). The disease occurs in all age of winged bean, and the occurrence tended to be higher in humid environment, such as branches in lower canopy or branches in high density. The disease symptoms, which also appeared to be the sign of the pathogen, were spherical pustules in yellow to orange color on the stems, leaves, and pods of winged bean. Severely infected plants also exhibited growth reduction, malformation, and curling of the leaves and pods. According to the disease literature of winged bean, this unknown disease was likely to be the false rust caused by a chytrid pathogen, Synchytrium psophocarpi (UK, CAB International. 1993); and the uredinia-liked pustules could be the sori, which contain numerous ovoid to globose sporangia inside. In order to characterize the pathogen identity, the sori were manually ruptured to assess the size of individual sporangium, which had an average of 26.71 ± 4.25 μm x 26.61 ± 4.60 μm (n=42), similar to the size reported in literature (Drinkall and Price. 1979). To confirm the molecular identity, the full genomic sequences from the small subunit (SSU) to the internal transcribed spacer-1 (ITS-1), 5.8S unit, and ITS-2 were amplified using the primer sets NS3 and ITS4. The 2,263 bp amplicon was cloned and sequenced to reveal the identity (Smith et al. 2014). The BLASTN results matched the SSU of our isolate (MW649126.1) to the Synchytrium minutum (HQ324138.1) with 96% similarity (1,075 out of 1,121 bp in length), Synchytrium decipiens isolate DAOM_87618 (KF160868.1) with 92% similarity (1,215 out of 1,326 bp in length) and S. decipiens isolate AFTOL-ID 634 (DQ536475.1) with 92% similarity (1210 out of 1316 bp in length). Phylogenetic analysis using the SSU sequence revealed this unknown pathogen was the grouped within the clade of Synchytrium genus with 100% bootstrapping confidence (Smith et al. 2014). Accordingly, the pathogen was confirmed to be a Synchytrium chytrid fungus. To complete the Koch's postulates, the sori were collected from infected tissue. After vortexing washing in 1% bleach for surface sterilization, the sori were gently crashed by a plastic tube pestle to harvest sporangia. The sporangia were sprayed onto healthy winged beans cultivated in pots, and the inoculated plants were kept in a moisture bag in 25 °C. While leaf curling and malformation could be observed about 14 days post inoculation, the yellow to orange sori could be observed around 30 to 40 days post inoculation on the whole plants cultivated in pots. The sori were collected to confirm the sporangia and the sequences were identical to the original pathogen. Collectively, this study not only presents the first report for the false rust of winged bean in Taiwan, but also documents the first reference sequence of S. psophocarpi that will be useful for future molecular diagnosis. Since S. psophocarpi has been only reported in tropic regions including Indonesia, Malay Peninsula, Malaysia, Papua New Guinea, and Philippines, this report provides the first observation of S. psophocarpi moving in the subtropic region.