Abstract
Plasmopara viticola, the downy mildew pathogen, is one of the most important pathogens in European viticulture. This oomycete infects grapevine leaves via zoospores that encyst at stomata. A primary germ tube then enters the substomatal cavity and develops a tubular network of hyphae that proliferate intercellularly and parasitize the leaf mesophyll cells by haustoria. Leaf infections have thus been the primary object of multiple studies concerning the physiology of the pathogen and defense reactions of grapevines. Besides leaves, this oomycete pathogen is able to spread throughout the plant tissue. As shown here by microscopy, it colonizes leaf petioles, shoots, berries and seeds. Evidence is provided showing that this process is facilitated by formation of special fan-shaped hyphae that seem to be necessary to overcome physical barriers in plant tissues. Physical obstacles are mainly constituted by vascular tissue in leaf veins, leaf petioles and shoots. In grapevine shoots, the mycelium seems to extend along the cambial layer between xylem and phloem tissue. Infected young berries are completely colonized on the inside. Older infected “leather berries” show glossy appositions of the fan-shaped hyphae at the inner side of the berry skin. The seeds from that stage of infestation are devoid of endosperm and embryo and biologically dysfunctional. Furthermore, a classification system for P. viticola infection based on the degree of infections in petioles and shoot tips is presented. This study contributes to a better understanding of downy mildew pathogenesis in grapevine, a prerequisite for efficient control measures.
Highlights
Plasmopara viticola [(Berk. & Curt.) Berl. & de Toni], belonging to the order of Peronosporales, is an obligate biotrophic oomycete pathogen of grapevine and causes downy mildew (DM)
It provides the basis to study plant reactions operating in resistant grapevine genotypes beyond the leaf tissue. This is important, since in most cases of DM resistance identified in North American or Asian Vitis species and applied in resistance breeding, the pathogen is still able to enter the mesophyll through stomata and to establish the first infection structures (Unger et al, 2007; Diez-Navajas et al, 2008; Jürges et al, 2009)
For the natural infections from the field plants (“Riesling,” “Müller-Thurgau,” “Pinot blanc,” and “Pinot noir”) microscopy was done by collection of the different parts of these plants, KOH treatment and staining as above
Summary
Plasmopara viticola [(Berk. & Curt.) Berl. & de Toni], belonging to the order of Peronosporales, is an obligate biotrophic oomycete pathogen of grapevine and causes downy mildew (DM) (for review see Gessler et al, 2011; Fawke et al, 2015; Kassemeyer et al, 2015). After ∼5 days under optimal conditions at around 25◦C, the mycelium produces sporangiophores that grow out of stomata and discharge new sporangia at their tips, ready to spread the pathogen This asexual propagation cycle is responsible for DM epidemics arising each season in warm and humid weather conditions. It provides the basis to study plant reactions operating in resistant grapevine genotypes beyond the leaf tissue This is important, since in most cases of DM resistance identified in North American or Asian Vitis species and applied in resistance breeding, the pathogen is still able to enter the mesophyll through stomata and to establish the first infection structures (Unger et al, 2007; Diez-Navajas et al, 2008; Jürges et al, 2009). Resistance relies on following diminished propagation mediated by some—not yet fully understood— “post-penetration” mechanisms
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
