Development of methods for screening grapevines for resistance to infection by downy mildew. I. Dual culture in vitro.

Abstract

The responses of grapevine varieties and hybrids, with a range of field disease resistance, to infection by downy mildew ( Plasmopara viticola ) were studied in vitro . 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 in vitro , 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 in vitro 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.