INTRODUCTION

Grapevine (Vitis vinifera) is a widely grown fruit crop with various uses, including consumption as fresh fruit, production of jams, traditional medicine, and as raw material for juice and wine production [1,2]. Grape cultivation is socially and economically important in South Korea [3]. However, fungal diseases, particularly downy mildew, pose significant threats to commercially important grape cultivars and hybrids, resulting in substantial economic losses [4]. Oomycetes, a group of plant pathogens related to diatoms and brown algae, encompass approximately 800 species of downy mildew and over 120 species of Phytophthora [4]. Grapevine downy mildew is caused by the oomycete Plasmopara viticola and is more prevalent in regions with frequent rainfall and moderate temperatures [5].

P. viticola was initially reported in Vitis ficifolia by Choi et al. in 2017 [6] and in Vitis cognetiae by Kim et al. in 2019 in Korea [7]. However, the occurrence of this pathogen in cultivated Vitis vinifera and the methodology for growing and maintaining the pathogen under laboratory conditions require detailed reporting. Therefore, a more comprehensive identification and analysis of the behavioral characteristics of this pathogen during infection are necessary to understand its biological interactions with the host. Such comprehensive characterization holds great implications for discovering resistance genes in locally grown genotypes that can be introduced into susceptible cultivated cultivars through breeding programs [8].

Previous studies have characterized P. viticola isolates from other countries; however, the genome organization of Korean isolates has not yet been reported [9-11]. Traditional characterization methods for fungal pathogens pose challenges for P. viticola because of its biotrophic nature and inability to be cultured in artificial media. Multiple genealogy analyses using four genomic regions (internal transcribed spacer 1 [ITS1], actin [ACT], large subunit [LSU], and beta tubulin [TUB]) have been recommended for the identification of filamentous pathogens such as grapevine downy mildew. This approach has been successfully applied to characterize P. viticola populations in China and France [12,13].

MATERIALS AND METHODS

DISCUSSION

Plasmopara viticola is an oomycete that causes severe damage to vineyards worldwide [19]. Grapevine downy mildew is primarily established through infection by activated oospores that germinate on the leaf surface, penetrate the stomata, and develop hyphae, forming mycelial structures within the mesophyll cells. Colonization involves the development of globose hostoria that absorb nutrients from the host and secrete biomolecules for colonization [20,21]. Visible symptoms of downy mildew include oily spots on the upper leaf surface, whitish mycelial structures protruding through the stomata, and necrotic lesions that can lead to defoliation [17,22]. Primary infection with resting oospores is followed by a cycle of asexual zoospore production. Sporangia isolated from leaves produce active zoospores within three hours. Under favorable conditions, zoospores encyst upon reaching the stomata and develop germ tubes for penetration. In vitro spore isolation was conducted using susceptible grape cultivars such as Vitis vinifera ʻItalia’ and the hybrid Hucbarad, which allowed abundant sporulation. Leaf discs excised from healthy vines facilitated the isolation and maintenance processes. However, laboratory-controlled conditions may not fully reflect field conditions, and further analyses should involve in vivo experiments.

In addition to resistance genes grape cultivars exhibiting extreme resistance often possess visible trichome structures on the leaf surface, providing physical resistance by preventing pathogen contact with stomata. The absence of downy mildew symptoms in resistant cultivars can be attributed to the physical barrier created by leaf hair. Physiological adaptations and anatomical structures of stomata in resistant cultivars also contribute to the early arrest of the infection process, even in the absence of leaf hair structures [30,31].