2006 Annual Report 4d.Progress report. This report serves to document research conducted under a Specific Cooperative Agreement between ARS and the Pennsylvania State University. Additional details of research can be found in the report for the parent project 1920-22000-031-00D "Evaluation and Characterization of Exotic Plant Pathogens for Biological Control of Introduced, Invasive Weeds". Mowing strategies evaluated in this study increased the frequency of systemically infected Canada thistle shoots in plots over experimental plots that were not mowed. For all mowing treatments in both locations, mowing elevated the proportion of systemically infected shoots, in comparison to the un-mowed treatment, in an additive manner consistent with the replacement of mowed systemically infected shoots with two or more new systemically infected shoots after mowing. It is likely that mowing during the summer months when teliospores are present will increase disease incidence within the season by stimulating production of new systemically infected shoots, and also, by improving the dispersal of the teliospores. Cutting primary shoots eliminates apical dominance and is followed by the enhanced production of secondary adventitious root buds from both healthy and systemically infected rootstock. Timing of mowing is a key aspect in the enhancement of an epidemic because teliospores are the only spore stage that can lead to the development of systemically infected plants. Urediniospores themselves cannot cause systemic infections on Canada thistle. It is important that mowing occur soon after the preponderance of systemically infected thistle shoots bear mature teliospores so that the early summer inoculum is redistributed. After mowing, newly emerged systemically infected shoots develop and eventually produce a second crop of teliospores that become capable for infection after winter stratification. Plots that are not mowed can be expected to contain teliospores produced from systemically infected shoots present in the spring plus those from any new systemically infected shoots that develop over the summer months. Another important factor is the physical movement by the mower of systemically infected plant material bearing teliospores. Frantzen (1994) demonstrated that P. punctiformis is characterized by a steep dispersal gradient where teliospores are deposited on the soil very close to the infected plant from which they were produced. This phenomenon indicates reduced disease spread is probably a major limiting factor in producing widespread epidemics of P. punctiformis on Canada thistle. Physical redistribution of inoculum by mowing may assist pathogen movement within a thistle patch by creating a more uniform (homogenous) teliospore distribution. As a result, with homogenous rather than patchy distribution, there will likely be greater opportunities for teliospore root bud contact the following spring leading to a greater incidence of basidiospore infections, and ultimately enhanced levels of systemically infected shoots in the thistle patch. The synergistic affect of strategically timed mowing and the presence of Puccinia punctiformis may lead to acceptable levels of control when combined with other management practices particularly in areas where the use of herbicides are restricted or are not cost effective.