Absence of Fitness Trade-Offs in Fungicide-Resistant Venturia inaequalis: Evidence Along an Altitudinal Gradient

Venturia inaequalis, a hemibiotrophic ascomycete, is the causal agent of apple scab, a major disease affecting apple production worldwide. The widespread use of fungicides in orchard management has led to the selection of resistant strains. To limit the spread of these resistant strains, it is essential to understand their competitive fitness within the population. In this study, we evaluated the resistance profiles and fitness components of 23 V. inaequalis isolates from both treated and untreated orchards in Northern Italy, focusing on five fungicides: dodine, cyprodinil, trifloxystrobin, boscalid, and myclobutanil. Fitness parameters, including mycelial growth, conidia number, and conidia size, were assessed in relation to fungicide resistance and environmental factors, such as altitude. The results revealed that, overall, resistant and sensitive strains showed no significant differences in fitness, except for cyprodinil-resistant strains, which exhibited enhanced mycelial growth and increased conidia size, and dodine-resistant strains, which produced smaller conidia. Additionally, altitude influenced conidial size, with higher elevation sites correlating with smaller conidia. These findings suggest that both genetic and environmental factors contribute to conidial variation, which may impact pathogen dispersal and infection dynamics. This study highlights the resilience and potential for spread of fungicide-resistant strains and underscores the need for integrated resistance management strategies to maintain sustainable and high quality apple production.