Abstract
The responses of grapevine varieties and hybrids, with a range of field disease resistance, to infection by downy mildew (<i>Plasmopara viticola</i>) were studied <i>in vitro</i>. Dual cultures of host and pathogen were established for Chambourcin, JS 23-416, Verdelet, Muscat St. Vallier, Cabernet Sauvignon, and Sultana by inoculating sterile rooted shoot cultures with a contaminant-free fungal spore isolate. It was demonstrated that resistance to downy mildew was expressed <i>in vitro</i>, and a scale of infection, based on foliar symptom development 10 days after inoculation, was described to rank the varieties. It can be summarized as I - discrete necrotic spots; II - extensive necrotic lesions; or III - extensive sporulation. The order of ranking of the varieties correlated with field and glasshouse assessments, although due to the high efficiency of <i>in vitro</i> inoculation and infection, symptom expression was more severe than in the field. These results were verified using full siblings of JS 23-416 x Sultana and Chambourcin x (Merlot x Sultana) differing in resistance to downy mildew. This report demonstrates that the dual culture technique can be used as a reproducible screening procedure, independent of season, to select for host resistance to an obligate parasite.
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