page_title: Plasmopara viticola
section_title: Pathogenesis
hierarchical_section_title: Plasmopara viticola / Pathogenesis
caption_attribution_description: English: Plasmopara viticola sporangium and spores
context_page_description: Plasmopara viticola, the causal agent of grapevine downy mildew, is a heterothallic oomycete that overwinters as oospores in leaf litter and soil. In the spring, oospores germinate to produce macrosporangia, which under wet condition release zoospores. Zoospores are splashed by rain into the canopy, where they swim to and infect through stomata. After 7–10 days, yellow lesions appear on foliage. During favorable weather, the lesions sporulate and new secondary infections occur.
context_section_description: The disease development of grape downy mildew is known to be heavily reliant on the efficiency of the asexual propagation cycles. Kiefer et al. (2002) demonstrated that the early development of Plasmopara viticola is regulated specifically and coordinately by unknown factors originating from the host grapevine plant Vitis vinifera. The host factors influence the pathogen development in three ways: (i) accelerating the release of zoospores from mature sporangia, (ii) coordinating the morphogenesis of the germ tube through the reorientation of the polarity of the zoospores during the attachment to the host cell, and (iii) targeting the zoospores to the stomata by active chemotaxis from the open substomatal cavity. The expression of pathogenesis-related (PR) genes by the host plant V.vinifera during the infection of P.viticola has been investigated over the years. The expressions of PR-2, PR-3 and PR-4 genes are induced in the grapevine host during pathogen infection, which encode for cell wall-degrading enzymes B-1,3-glucanase (PR-2) and chitinases (PR-3 and PR-4). It was previously understood that oomycetes differ from true fungi by the presence of cellulose in the oomycetes’ cell walls as opposed to chitin in true fungi. However, it was found that chitin synthesis is regulated during in planta growth and asexual propagation of P.viticola and this is further demonstrated by the presence of chitin on the cell walls of the hyphae, sporangiophores and sporangia of the grape downy mildew pathogen. Hence, both the cell wall-degrading enzymes are synthesized by the host grapevine plant specifically to target and degrade the cell walls of the oomycete pathogen. In addition, the upregulation of the PR-9 gene that encodes for peroxidase, which is a reactive oxygen species is associated with the systemic acquired defense of the grapevine host. The roles of other constitutively expressed PR genes during P.viticola infection such as PR-5, PR-1 and PR-10 genes remain ambiguous. PR-5 is involved in the synthesis of thaumatin-like proteins and osmotins, which are believed to inhibit the spore germination and germ tube growth of Plasmopara viticola by creating transmembrane pores.
caption: Sporangium and spores of Plasmopara viticola
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