Submitted to: Pest Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 2, 2010
Publication Date: N/A
Interpretive Summary: Canada thistle is an invasive weed in agricultural, range, and natural lands. Bacterial pathogens isolated from Canada thistle have been evaluated as biological control agents for this weed. One of the pathogens isolated, Pseudomonas syringae pv. tagetis 1-502a, has been evaluated extensively. The pathogen produces a phytotoxin, tagetitoxin, that prevents normal chloroplasts from developing,. This study demonstrated that a similar bacterial pathogen, strain CT99, may produce the same toxin. Furthermore, genetic analysis of the CT99 strain with respect to the P. syringae taxa suggests that it is not a strain of P. syringae pv. tagetis. This is the first report of a strain other than P. syringae pv. tagetis that may produce tagetitoxin. Based on what is known about P. syringae pv. tagetis and its biological activity, this pathovar may be a better biological control agent for annual weeds rather than perennial weeds for which it has been tested. Also, because of its biological activity, i.e., blockage of chloroplast development, tagetitoxin may be useful as a natural herbicide.
Technical Abstract: Pseudomonas syringae pv. tagetis (Pst) is a disease of plants in the family Asteraceae. A distinctive characteristic of this bacterial pathogen is the symptom of apical chlorosis in infected plants, caused by the phytotoxin tagetitoxin. Strains of Pst have been isolated from several plant species from a number of countries. One strain isolated from Cirsium arvense (Canada thistle) has been evaluated as a biological control agent for this invasive weed and other weeds in the family Asteraceae. Genetic analysis of the strains in this pathovar indicate that it is highly clonal. There is another strain of P. syringae (CT99) that was also isolated from Canada thistle and causes apical chlorosis that may produce tagetitoxin as well. However, multilocus sequence typing analysis indicates that it is not a Pst strain. The major impact of Pst on infected plants is stunting and the reduction in sexual reproductive structures, symptoms attributed to tagetitoxin. While initially considered for the control of Canada thistle, the utility of this pathogen as a biological control agent may be limited to controlling annual weeds. Alternatively, tagetitoxin may be of value as a natural herbicide because of its impact on chloroplasts.