So what's the problem?
Cirsium arvense, or Canada thistle, is among the most important invasive plants in the world. Six biological control agents were studied and released in North America and several other natural enemies were accidentally introduced. Thus far, however, none have been able to halt the spread or reduce the impact of this invasive weed. Additional surveys in the likely area of origin of Canada thistle did not reveal any additional insect species specific enough to be considered for release in North America.
What is this project doing?
Pathogens can be more specific than insects, and host-specificity is the main hurdle to implementing biological control of this weed in North America. So, a new initiative was started in 2009 to look for co-evolved fungal pathogens. As a survey area, we chose China because a large number of Cirsium species are recorded from China, and some areas show a good eco-climatic match with infested areas in North America.
Because CABI’s fungal expertise lies with our staff in the UK, Drs Harry Evans and Carol Ellison are leading this project in cooperation with Dr Li Hongmei, Dr Zhang Feng and Wan Huanhuan at the Ministry of Agriculture (MoA) – CABI Joint Laboratory of Bio-safety (hosted by the Institute of Plant Protection of the Chinese Academy of Agricultural Sciences [IIP-CAAS]) in Beijing.
Results so far
Surveys planned in Xinjiang Province and Inner Mongolia in August 2009 had to be restricted to the latter, as civil unrest in Xinjiang Province earlier in the year made it impossible for foreign nationals to enter the region. Contacts were established with the Grasslands Research Institute in Hohhot, Inner Mongolia. Staff from here had previously worked with entomologists from USDA in the 1990s to find insect agents for C. arvense (Canada thistle). During the survey, 22 field sites were visited, and plant samples with various fungal pathogen symptoms were collected. However, it became obvious that the plant in Inner Mongolia is morphologically distinct from C. arvense, and it was identified by a botanist of the Plant Protection Institute (PPI) as C. setosum, a species closely related to C .arvense and treated as a synonym of C. arvense in North America. This was confirmed by molecular analyses conducted by Dr John Gaskin (USDA, ARS, Sidney, USA).
In 2010, surveys were therefore concentrated in Xinjiang and Gansu provinces in north-western China. In total, more than 40 field sites were visited and samples with disease symptoms were collected from 21 sites. Despite reports that up to four rust species occur on C. arvense in Xinjiang Province, initial findings suggest that only one species, the systemic rust Puccinia cf. punctiformis, is actually present. Since this rust species is already present in North America, its potential as a classical biological agent is limited unless more aggressive strains can be identified. A white blister ‘rust’ in the genus Pustula was identified as the most promising pathogen. Recent molecular phylogenetic and morphological studies clearly showed that the Pustula lineage, usually referred to as P. tragopogonis s.l., can be distinguished into several distinct species, including P. spinulosa on Cirsium spp., which is distinct from the species attacking Helianthus annus (sunflower). It is suggested that different pathotypes of P. spinulosa have evolved that are specific at the species level. This would be a necessary requirement, if this pathogen is to be considered for introduction to North America.
Surveys in 2011 therefore concentrated to collect samples of the white blister ‘rust’. Unfortunately, this fungal pathogen proved difficult to work with. Harry Evans visited China in October 2011 to provide assistance, and Wan Huanhuan has spent time in the UK and Switzerland in April/May 2012 for additional training. In August 2012, two field surveys were conducted and fresh material infected by the white blister ‘rust’ was collected in the provinces of Xinjiang and Gansu. Zoosporangia germination tests in local labs of both provinces were successful and the release of zoospores was observed. The same tests were conducted successfully at the joint lab in Beijing after the samples were brought back. The critical factor was that the zoosporangia had to be fresh, and not dried in order to be viable. Inoculation of potted plants of Cirsium arvense of Chinese origin at the joint lab was undertaken. Initial infection symptoms (leaf chlorosis) appeared on leaves 9-14 days after inoculation, but under ambient glasshouse conditions in Beijing no further development was observed. A further inoculation test was conducted in mid-September using freshly collected samples from Gansu, but this time the inoculated plants were kept in controlled conditions, similar to field conditions where the blister ‘rust’ was collected; 22°C/16°C (day/night) and relative humidity of 85%. After 17 days, fully developed symptoms, i.e. mature white pustules, started to appear on leaves of C. arvense from Gansu, Xinjiang, and Inner Mongolia. The next step is to achieve consistent and reliable infection under controlled conditions, in order to establish a rust culture, and then inoculate C. arvense of US origins. If the plants from the US are susceptible, then host-specificity testing can be undertaken.
Hariet L. Hinz
Address: Rue des Grillons 1, CH-2800 Delemont, Switzerland
Tel: +41 (0)32 4214872
Address: Bakeham Lane, Egham, Surrey, TW209TY, United Kingdom
Tel: +44 (0)1491 829003
Tel: +44 (0)1491 829153
Address: C/o CAAS-CABI Project Office, C/o Internal Post Box 56, Chinese Academy of Agricultural Sciences, 12 Zhongguancun Nandajie, Beijing 100081, China
Tel: +86 (0)10 6289 8289
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