Field-Scale Gene Flow of Fungicide Resistance in <i>Pyrenophora teres</i> f. <i>teres</i> and the Effect of Selection Pressure on the Population Structure

The effectiveness of fungicides to control foliar fungal crop diseases is being diminished by the increasing spread of resistance to fungicides. One approach that may help to maintain efficacy is remediation of resistant populations by sensitive ones. However, the success of such approaches can be compromised by re-incursion of resistance through aerial spore dispersal, although knowledge of localized gene flow is lacking. Here, we report on a replicated mark-release-recapture field experiment with several treatments set up to study spore-dispersal-mediated gene flow of a mutated allele that confers demethylase inhibitor resistance in <i>Pyrenophora teres</i> f. <i>teres</i> (<i>Ptt</i>). Artificial inoculation of the host, barley (<i>Hordeum vulgare</i>), was successful across the 12-ha trial, where the introduced sensitive and resistant populations were, respectively, 6- and 13-fold the DNA concentration of the native <i>Ptt</i> population. Subsequent disease pressure remained low, which hampered spread of the epidemic to such an extent that gene flow was not detected at, or beyond, 2.5 m from source points. In the absence of gene flow, plots were assessed for treatment effects; fungicide applied to populations that contained 14.3% of allele mutation increased in frequency to 24.5%, whereas sensitive populations had no change in structure. Untreated controls of the native <i>Ptt</i> population remained genetically stable, yet untreated controls that were inoculated with sensitive <i>Ptt</i> had half the resistance frequency of the native population structure. The trial demonstrates the potential for management to remediate fungicide-resistant pathogen populations, where localized gene flow is minimal, to safeguard chemical crop protection into the future.