Enhancing incidence of Puccinia punctiformis, through mowing, to improve management of Canada thistle ( Cirsium arvense)

Biological control of Canada thistle using Puccinia punctiformis has been largely unsuccessful in part due to a low incidence of systemically infected shoots and heterogeneous distribution of teliospores in the soil. The present study investigated the feasibility of strategic mowing to improve incidence of systemically infected shoots, and enhance intra- and/or inter-season disease development in two unused pastures. Mid-season mowing of plots in July lead to a greater proportion of systemically infected shoots in experimental plots observed at seasons’ end compared to unmowed plots. Late-season mowing in September resulted in the highest levels of systemically infected shoots early the next summer. Over time, September mowing treatments significantly increased the proportion of systemically infected shoots compared to no mowing. The number of healthy shoots declined over time in mowed plots, whereas the number of healthy shoots in unmowed plots either increased or remained constant. These effects were observed in experimental plots in both pastures. It is proposed that mowing followed by regrowth of systemically infected shoots may help overcome the monocyclic nature of the pathogen and enhance severity of this disease.