LAUSR

Xanthium strumarium L. var. canadense (Mill.) Torr. & A. Gray (Canada cocklebur; Asteraceae), an annual herb native to North America, has been widely naturalized in Eurasia (Yin et al. 2012). This introduced plant was first recorded in Beijing in 1991 and now has spread into many provinces in China such as Hebei, Shandong, Liaoning, Jilin, Heilongjiang, Guangdong, Guangxi, and Xinjiang. This invasive, noxious weed generally invades disturbed habitats such as wastelands, riverbanks, roadsides, meadows, and croplands, causing serious ecological and economic impacts in China (Zhao et al. 2014). To determine its distributions and natural enemies in Liaoning Province, we carried out field surveys in the summer of 2016. On August 7, dodder was first found on Canada cocklebur at three disturbed sites (N 41°49′15.41″ to 41°49′20.81″, E 123°34′56.37″ to 123°35′05.41″, 54 to 57 m above sea level) along the Xinkai River in Shenyang (with temperate continental monsoon climate), Liaoning Province, Northeast China. At each site we found a nearly circular patch of Canada cocklebur (3, 5, and 8 m in diameter, respectively) that was parasitized by dodder. Cocklebur parasitized by the dodder showed wilting and senescent leaves, stunted growth, and few or no fruit produced. The dodder wrapped around petioles and stems of the invasive weed, and it occasionally spread onto cooccurring native plants such as Setaria viridis (L.) Beauv. and Humulus scandens (Lour.) Merr. Where the dodder penetrated the cocklebur stem tissues, brownish black lesions developed with small cracks. Based on its inflorescence, bracts, calyces, pedicles, flowers, capsule, ovary, and seed, the dodder was identified as Cuscuta australis R. Br. (Wu 1979; Zhao et al. 2015). The morphological identification was confirmed by molecular evidence. We measured the base sequence of the internal transcribed spacer (ITS) region of ribosomal DNA of the dodder (amplified with S2F-S3R as primers) and found that it (deposited into GenBank; accession no. MH370812) was 99.75% similar to that of C. australis in GenBank (AY554398). Our dodder was 96.47% similar to the nearest relative, C. epithymum (AY554400.1) (Revill et al. 2005). Dodder is distributed worldwide from temperate to tropical regions. It has been found parasitizing other invasive plants such as Ambrosia artemisiifolia (Zhao et al. 2015). To our knowledge, this is the first report of C. australis parasitizing Canada cocklebur in China. The dodder produced 228 to 2,985 seeds (1.05 to 1.22 mm in length) on each infested host and reduced aboveground biomass of the cocklebur by 68.70 to 80.69% and fruit production by 87.59 to 97.21% in the field (data not shown). The dodder and other natural enemies such as Puccinia xanthii (Zhao et al. 2014) may mitigate the impacts of the invasive cocklebur in China.