Occurrence of Puccinia polysora Causing Southern Corn Rust in the Northeast Huanghuaihai Region of China

The Huanghuaihai (HHH) region is the major summer corn (Zea mays L.) producing area in China, whereas spring corn is grown in northeast China. Although Tianjin is located in the northeast HHH region, it serves as a bridge area crossing the summer and spring corn growing regions in China. In 2016, corn leaf rust symptoms were observed on the regional trial corn varieties in Wuqing county (39.40°N, 117.05°E), Tianjin. Of 106 trial varieties, 98 were infected with infection types from III to VI according to Fu et al. (2011). The pustules were randomly scattered on leaves with color being cinnamon to yellowish brown. The pustules were found predominantly on the upper leaves but infrequently on the husks of the ear. The urediniospores were yellowish, and the teliospores were not found. Urediniospores were subglobose or globose showing yellow or golden color. The size of urediniospores was 25.0 to 30.0 × 28.0 to 37.0 μm. Finally, echinulates were observed on the spore surface. The corn leaves with urediniospores were collected from the field. Fresh urediniospores from single uredinial sori were used to inoculate on the fourth leaf of the sweet corn hybrid Jinnuo628. Incubation was performed for 24 h in darkness with 100% relatively humidity, and the inoculated plants were placed in the growth chamber at 28°C with 12 h natural light for 16 days. Urediniospores were collected after appearance of leaf rust symptoms. The inoculation process was repeated to obtain enough spores for DNA analysis. Genomic DNA was extracted from urediniospores, and a 1,331-bp fragment was amplified with the ITS1F/RUST1 primer pair (Anikster et al. 2004). BLAST analysis showed the sequence (Wuqing; GenBank MF033480) was 99% identical to Puccinia polysora (GenBank HQ189432) from isolate HSZ1878 (Crouch et al. 2011). Compared with the sequence from HSZ1878, there was one transition between guanine and adenine and one adenine base insertion in the ITS2 region of the sequence from the Wuqing isolate. To confirm the difference, another primer pair, PpolyF/PpolyR (Crouch et al. 2011), was used to amplify the ITS2 region of the Wuqing isolate and an isolate collected from Hainan Province, which was identified as P. polysora. Two ITS2 fragments 139-bp long were sequenced with pTZ57R/T vector. We confirmed the presence of a base transition and an insertion based on sequence alignment of ITS2 from the Wuqing and HSZ1878 isolates. The sequences of ITS2 from Wuqing and Hainan isolates were also 100% identical. Accordingly, this fungus was identified as P. polysora. Previously, P. polysora was reported mainly in southern China, such as Hainan, Guangxi, and so on. To our knowledge, this is the first report of corn rust disease caused by P. polysora in the northeast HHH region of China. The suitable temperature for P. polysora is 26 to 30°C, and the temperature in Tianjin ranged from 21 to 32°C during the summer corn growth stage. A violent storm struck more than half of China from south to north on July 20, 2016. The emergence of P. polysora in Tianjin was likely caused by long-distance wind dispersal from southern China. The fact that most maize germplasm is susceptible makes P. polysora a potential threat to corn production. We have begun to investigate the migration routes of P. polysora in China, and such research should be useful in understanding of spore dispersal of P. polysora.