Yellow starthistle (YST, Centaurea solstitialis) is a major weed pest of the western United States. Synchytrium solstitiale, a pathogen of YST, caused significant damage to symptomatic (versus asymptomatic) plants in a field study in France. Before it was evaluated as a candidate for biological control of YST in the United States, protocols for pathogen maintenance under greenhouse conditions were developed. Maintenance, increase, and host range determination protocols involved incubation at 10/15°C (night/day) with an 8-h photoperiod either of potted or exhumed (i.e., roots of 4-week-old plants grown in flasks of water) plants inoculated with galled leaf tissue, or potted plants in which inoculum was wrapped within healthy leaves by a plastic wrap. The leaf-wrap protocol, used during the host range determination, always resulted in disease of YST. Several safflower (Carthamus tinctorius) cultivars and other plants related to YST became diseased following this protocol, thus raising concern about host specificity. Development of disease on nontarget species precludes proposal of S. solstitiale for biological control of YST at this time.
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