Differential susceptibility of Russian thistle accessions to Colletotrichum gloeosporioides

Molecular information suggests that Russian thistle (Salsola tragus L.) in the US may consist of more than one genetic entity. This genetic variation needs to be taken into account when developing agents for biological control of this important weed. Preliminary evidence suggests that there are differences in susceptibility of Salsola sp. to infection by fungal pathogens. In the present study, an isolate of Colletotrichum gloeosporioides, a pathogen of Russian thistle collected in Hungary was tested for its ability to infect and damage California accessions of S. tragus (referred to as Type A) and the related S. tragus, Type B. The minimum dew period and temperature required for infection of S. tragus with C. gloeosporioides was determined to be 12–16 h and 25 °C. Both Type A and Type B were susceptible, but C. gloeosporioides caused greater damage and reductions in biomass of Type A than of Type B. Fresh weights of Type A and Type B were reduced from controls by 60 and 9%, respectively, after inoculations with C. gloeosporioides. Results from this study illustrate the importance of understanding target plant taxonomy in biological control evaluations.