Pathogen development and host responses to Plasmopara viticola in resistant and susceptible grapevines: an ultrastructural study

Abstract

The downy mildew disease in grapevines is caused by <i>Plasmopara viticola</i>. This disease poses a serious threat wherever viticulture is practiced. Wild <i>Vitis</i> species showing resistance to <i>P. viticola</i> offer a promising pathway to develop new grapevine cultivars resistant to <i>P. viticola</i> which will allow reduced use of environmentally unfriendly fungicides. Here, transmission and scanning microscopy was used to compare the resistance responses to downy mildew of three resistant genotypes of <i>V. davidii var. cyanocarpa</i>, <i>V. piasesezkii</i> and <i>V. pseudoreticulata</i> and the suceptible <i>V. vinifera</i> cultivar 'Pinot Noir'. Following inoculation with sporangia of <i>P. viticola</i> isolate 'YL' on <i>V. vinifera</i> cv. 'Pinot Noir', the infection was characterized by a rapid spread of intercellular hyphae, a high frequency of haustorium formation within the host's mesophyll cells, the production of sporangia and by the absence of host-cell necrosis. In contrast zoospores were collapsed in the resistant <i>V. pseudoreticulata</i> 'Baihe-35-1', or secretions appeared arround stomata at the beginning of the infection period in <i>V. davidii var. cyanocarpa</i> 'Langao-5' and <i>V. piasezkii</i> 'Liuba-8'. The main characteristics of the resistance responses were the rapid depositions of callose and the appearance of empty hyphae and the plasmolysis of penetrated tissue. Moreover, collapsed haustoria were observed in <i>V. davidii var. cyanocarpa</i> 'Langao-5' at 5 days post inoculation (dpi) and in <i>V. piasezkii</i> 'Liuba-8' at 7 dpi. Lastly, necrosis extended beyond the zone of restricted colonization in all three resistant genotypes. Sporangia were absent in <i>V. piasezkii</i> 'Liuba-8' and greatly decreased in <i>V. davidii var. cyanocarpa</i> 'Langao-5' and in <i>V. pseudoreticulata</i> 'Baihe-35-1' compared with in <i>V. vinifera</i> cv. 'Pinot Noir'. Overall, these results provide insights into the cellular biological basis of the incompatible interactions between the pathogen and the host. They indicate a number of several resistant Chinese wild species that could be used in developing new cultivars having good levels of downy mildew resistance.