Methods to assess competitive ability of Monilinia fructicola isolates with reduced sensitivity to azoxystrobin

Peach brown rot (caused by Monilinia fructicola) is managed in Brazil by preventive spraying of fungicides, mostly single-target site, such as quinone-outside inhibitors (QoIs). Repeated use of these single-target site fungicides can induce shifts in pathogen populations to reduced fungicide sensitivity, which may result in failure of disease control. The dynamics of isolates with reduced sensitivity to fungicides over time can be predicted with competitive ability assays between high (HSI) and low (LSI) sensitivity isolates. However, there are limited reports concerning the methodology used in competitive ability assays of M. fructicola with reduced sensitivity to azoxystrobin (QoI fungicide). This study was undertaken with different variables (colony-forming units and spore germination) and substrates (in natura and canned peaches) to evaluate the competitive ability of M. fructicola isolates with different levels of sensitivity to azoxystrobin. A conidial suspension of 1:1 HSI:LSI was inoculated on fresh and canned peaches and the percentage of LSI in the population was assessed for 5 generations. Linear or generalized linear models (GLM) were fit to the progression of LSI over time. For all variables used to measure competitive ability, no fitness cost was associated to M. fructicola isolates with reduced sensitivity to azoxystrobin. After 5 disease cycles, LSI corresponded to 66% of the population in the mixture, regardless of the inoculum substrate (canned or fresh peaches). Spore germination as an assessment variable and canned peaches as a substrate are viable options for studies aiming to determine fitness cost of LSI to azoxystrobin. Feasibility, low cost, time requirements and low variability of results were considered in choosing the best method to assess competitive ability.