Durable resistance is a key objective in genetic improvement for disease resistance in grapevines, which must survive for years in the field in the presence of adaptable pathogen populations. In this study, the adaptation of 72 Northern Italian isolates of Plasmopara viticola, the downy mildew agent, has been investigated into Bianca, possessing Rpv3-1, the most frequently exploited resistance locus for genetic improvement, and Mgaloblishvili, a Vitis vinifera variety possessing the newly discovered Rpv29 locus. Infection parameters (latency period, infection frequency, and disease severity) and oospore production and viability were evaluated and compared to those of Pinot noir, the susceptible reference. The expected levels of disease control were achieved by both resistant cultivars (>90% on Bianca; >25% on Mgaloblishvili), despite the high frequency of isolates able to grow on one (28%) or both (46%) accessions. The disease incidence and severity were limited by both resistant cultivars and the strains able to grow on resistant accessions showed signatures of fitness penalties (reduced virulence, infection frequency, and oospore density). Together, these results indicate an adequate pathogen control but suitable practices must be adopted in the field to prevent the diffusion of the partially adapted P. viticola strains to protect resistance genes from erosion.

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.