Integrating Puccinia punctiformis, a biological control agent, into Cirsium arvense management in semi-arid organic agriculture

• P. punctiformis aids management of C. arvense in organic agroecosystems. • Reduced tillage increased P. punctiformis occurrence and symptomatic stem density. • Reduced tillage − P. punctiformis combination reduced C. arvense stem density. • Reduced tillage and P. punctiformis decreased C. arvense relative growth rate. • Crop response was positive to P. punctiformis presence under reduced tillage. Puccinia punctiformis is an obligate fungal pathogen of Cirsium arvense , a perennial rhizomatous weed common in temperate ecosystems. Cirsium arvense is difficult to manage in organic agriculture and integrated management is necessary to reduce its abundance and impact on crops. Puccinia punctiformis limits C. arvense growth but its effect has not been quantified in cropland field studies. In a four-year organic annual crop sequence at a semi-arid site in Montana, USA, we evaluated (1) P. punctiformis (occurrence, symptomatic stem density) and C. arvense (stem density, relative growth rate [RGR]) in response to tillage (standard vs. reduced), and the relationship between these two variables, and (2) final year crop stem density in response to tillage and P. punctiformis patch infection. Reduced tillage increased P. punctiformis occurrence and symptomatic stem density through time. Cirsium arvense stem density decreased through time in reduced tillage patches that displayed symptomatic infection. Cirsium arvense RGR decreased with increasing years that a patch had P. punctiformis symptomatic infection, and in the reduced tillage treatment regardless of symptoms. Final year crop stem density was higher under standard tillage but increased under reduced tillage with increasing years that a patch had P. punctiformis symptomatic infection. Reduced tillage practices complemented the impact of existing P. punctiformis by increasing its spread and symptomatic infection, two challenges previously identified with using P. punctiformis as a biocontrol agent. When coupled with complementary tactics, producers can integrate P. punctiformis into semi-arid organic agriculture weed management, potentially helping reduce C. arvense ’s impact on crops.