Puccinia suaveolens Causing Leaf Rust on Cirsium setosum in China

Thistle, Cirsium setosum (Willd.) M. Bieb., is widely distributed in China as a common weed in fields. It is also used as a traditional Chinese medicine for cooling blood, stopping bleeding, dispelling stasis, detoxifying, and resolving carbuncle. In 2023, we found a rust disease on plants of Cirsium setosum in the experimental field of Hebei Agricultural University, Baoding, Hebei Province, China, with incidence of 15% - 25% (Fig. S1 A, B). The diseased leaves turned yellow, and the leaf edges were slightly rolled. The yellow, oil-like pycnia and pycniospores covered the baxial surface of leaves, and brown pustules were produced after 2-3 weeks. On the adaxial surface of the leaves, the brown rust pustules were mainly along the leaf veins. Stems were also be infected later, and dark pustules were scattered. The diseased plants were relatively short and small, and produced relatively small or no flowers compared to healthy plants. A total of 100 plants with typical leaf rust symptoms and signs were collected. To confirm the pathogenicity, healthy plants of thistle were sprayed with 5 ml of urediospores suspension (2.6×105/ml), and plants sprayed with sterile distilled water were treated as control. The sprayed plants were incubated under high moist conditions at 18°C for 24 h, and the inoculated plants were grown at 20°C in a greenhouse. Ten days after inoculation, the plants inoculated with urediniospores showed rust symptoms with uredinia and urediniospores on the leaves (Fig. S1 C), while the control plants were healthy. For morphological characterization, urediospores were picked from the naturally infected plants and placed in a drop of sterile water on a glass slide using a sterile needle, and observed and measured under a microscope. Urediospores were nearly spherical, brown-yellow, and measured 15 - 25 μm in diameter (n=100) (Fig. S1 D). Telia were scattered on the baxial surface of the naturally infected leaves, and teliospores were oval, yellow-brown, double-celled, with very short hyaline pedicels, and measured 15-20 × 15-30 μm (n=100) (Fig. S1 E). For molecular characterization, about 200 μg of urediniospores was collected and placed in a 1.5 ml sterile centrifuge tube, and genomic DNA was extracted using the cetyl-trimethylammonium bromide method (Gawel et al. 1991). The internal transcribed spacer (ITS) region of the rDNA and the D1/D2 domain were amplified using primer pairs ITS1/ITS4 (White et al. 1990) and NL1/NL4 (Borhani et al. 2013) in polymerase chain reaction (PCR), respectively. The PCR products were sequenced, and their sequences were aligned and compared with those deposited in GenBank. The obtained sequences were deposited in GenBank (OR600240 for ITS and OR598614 for D1/D2), which were 100% identical with 100% coverage to the ITS sequence (ON063373.1) and the D1/D2 sequence (ON063379.1) of Puccinia suaveolens (Menzies 1953). Based on the morphological characteristics and DNA sequences, the isolates were identified as P. suaveolens (Fig. S1 and Fig. S2). Thistle rust caused by Puccinia obtegens has been reported in some other parts of China (Zhang 2012). To the best of our knowledge, this is the first report of P. suaveolens causing leaf rust on C. setosum in China. This discovery is helpful for control of leaf rust on thistle grown for Chines medicine and other purposes, and the rust species could be used for biological control of thistle as a weed in crop fields.