Novel Plasmopara viticola isolate surpasses grapevine Rpv3.1 and Rpv3.2 resistance but not Rpv12

Grape downy mildew, caused by the oomycete Plasmopara viticola, is a major threat to Vitis vinifera grape production. Several breeding programs rely on resistance factors (Rpvs) from American and Asian Vitis species to confer resistance to P. viticola. Although the use of these crossing hybrids offers a promising strategy for disease control, variants capable of overcoming host resistance are emerging. A thorough characterization of these isolates is needed to define new strategies for breeding programs. The investigation of varying aggressiveness of two P. viticola pathotypes was investigated. One pathotype is unable to overcome Rpv3.1 based resistance and another pathotype was isolated from a grapevine carrying the Rpv3.1 locus. Their aggressiveness was evaluated on five grapevine genotypes harboring distinct combinations of Rpv loci. Macroscopic and microscopic development of the pathogen was characterised throughout its colonization of grapevine leaves. Our findings demonstrate that the P. viticola isolate NW-10/16 is not only capable of overcoming Rpv3-based resistance but also exhibited a faster growth, increased sporulation, and a denser mycelial colonization within the plant mesophyll when compared with the avRpv3 + isolate in susceptible plants. The identification and characterization of new, more aggressive, P. viticola pathotypes is crucial to promote durable management of the resistant grapevine hybrids and to mitigate the threat of downy mildew.