Fusarium crown rot under continuous cropping of susceptible and partially resistant wheat in microcosms at elevated CO 2

This study examines the CO 2 ‐mediated influence of plant resistance on crown rot dynamics under continuous cropping of partially resistant wheat line 249 and the susceptible cultivar T amaroi. Disease incidence, severity, deoxynivalenol and F usarium biomass were assessed after each cycle in microcosms established at ambient and 700 mg kg −1 CO 2 using soil and stubble of these wheat lines from a field experiment with free to air CO 2 enrichment. Monoconidial isolates from wheat stubble were collected initially, and after five cropping cycles, to compare the frequency and aggressiveness of F usarium species in the two populations. Aggressiveness was measured using a high‐throughput seedling bioassay. At elevated CO 2 , the higher initial incidence in T amaroi increased with cropping cycles, but incidence in 249 remained unchanged. Incidence at ambient CO 2 did not change for either line. Elevated CO 2 induced partial resistance in T amaroi, but not in 249. Increased F usarium biomass in wheat tissue at elevated CO 2 matched raised deoxynivalenol of the stem base in both lines. After five cycles of continuous wheat cropping, aggressiveness increased in pathogenic F . culmorum and F . pseudograminearum by 110%, but decreased in weakly pathogenic F . equiseti and F . oxysporum by 50%. CO 2 and host resistance interactively influenced species frequency, and the highly aggressive F . pseudograminearum became dominant on T amaroi irrespective of CO 2 concentration, while its frequency declined on 249. This study shows that induced resistance at elevated CO 2 will not reduce crown rot severity, or impede the selection and enrichment of F usarium populations with increased aggressiveness.