Vitis 45 (4), 191–196 (2006) Summary Grapevine breeding is the most effective way to create cultivars resistant to downy mildew (Plasmopara viticola), and to reduce the number of fungicide appli- cations. Four criteria, including histological and biochemi- cal analyses, based on the level of different mechanisms of resistance to grapevine downy mildew, were tested on 42 different cultivars. Plantlets were artificially in- oculated with downy mildew and the sporangia density was measured spectrophotometrically 6 d after infec- tion. Callose synthesis in stomata and δ- and ε-viniferin levels at the site of infection were recorded 48 h after in- oculation. These observations have allowed the 42 cul- tivars to be divided into 5 groups: very resistant (VR), resistant (R), less susceptible (LS), susceptible (S) and highly susceptible (HS). All 4 criteria have to be applied to assign the resistance level closer to field conditions. This method allows to rapidly evaluate the level of re- sistance of seedlings to downy mildew thereby leading to a reduction in duration of the breeding program by several years. K e y w o r d s : Plasmopara viticola, callose, stilbene, Vitis vinifera, breeding. Introduction Downy mildew (Plasmopara viticola (Berk. and M.A. Curtis) de Bary) is one of the most serious diseases in vine- yards worldwide. In Switzerland, field observations have shown that downy mildew infection was rated as severe for 25 out of the last 50 years. Depending on meteorologi- cal conditions, 8-10 fungicide applications are necessary to control this disease (VIRET et al. 2001). Chasselas, Gamay and Pinot noir, the major varieties in Swiss vineyards, are highly susceptible to P. viticola. The number of fungicide applications can be reduced only by an efficient forecast of downy mildew infection and special cultural practices such as integrated pest management (VIRET et al. 2001). Although a long-term endeavor, traditional breeding for resistance is an excellent way to create fungus resistant grape cultivars which allow to reduce the number of fun- gicide applications. Grape metabolic reactions induced in response to biotic stresses may be used as criteria to rapidly evaluate disease resistance in seedlings of progeny popula- tions. For example, callose synthesis in stomata (GINDRO Histological and biochemical criteria for objective and early selection of grapevine cultivars resistant to Plasmopara viticola K. GINDRO, J. L. SPRING, R. PEZET, H. RICHTER and O. VIRET Swiss Federal Research Station for Plant Production of Changins (RAC), Nyon, Switzerland Correspondence to: Dr. K. GINDRO, Swiss Federal Research Station for Plant Production of Changins, Secteur 1 - Mycologie, Postal box 254, 1260 Nyon 1, Switzerland. Fax: +41-22-362-1325. E-mail: katia.gindro@rac.admin.ch et al. 2003), stilbene production at the site of P. viticola infection, or induced peroxidases (KORTEKAMP and ZYPRIAN 2003) were thought to be of particular importance (DERCKS and CREASY 1989; PEZET et al. 2003; PEZET et al. 2004 b; HAMMERSCHMIDT 2004). In recent years, histological observations of callose and qualitative and quantitative micro-analysis of stilbenes in grape leaves some hours after artificial inoculation of P. viticola, as well as evaluation of sporangial density (PEZET et al. 2004 a) have been used to select resistant seedlings. In Switzerland, a V. vinifera breeding program has re- leased the red cultivars Gamaret and Garanoir (Gamay x Reichensteiner), which are highly resistant to Botrytis ci- nerea. However, to date, no red cultivars with resistance to downy mildew and with satisfactory wine quality have been bred. Therefore, a large number of crossings have been realized and selected the first year for their resistance potential. This paper describes the use of metabolic resist- ance criteria that have been used for the early selection of resistant seedlings from our cross-breeding program, to ob- tain new red and white cultivars with resistance to downy mildew. Material and Methods Cuttings were obtained from grapevines (Vitis vinif- era L. cvs and interspecific grape cvs) of the experimental vineyards of Agroscope-RAC. Rooted plants were grown in glasshouse as described before (PEZET et al. 2004 a). At the 10-developed-leaf-stage they were placed in a growth chamber and kept at a photoperiod of 16 h (22 °C) and a dark period at 18 °C. Relative humidity was 60 %. For in- oculum, leaves infected with P. viticola were harvested in a vineyard in Perroy (canton Vaud, Switzerland) and spo- rangia were collected by vacuum aspiration, as described by PEZET and PONT (1990). They were stored in cryotubes at -80 °C until use. Eight cultivars, Solaris, Bronner, Jo- hanniter, Seyval blanc, Pinot noir, Gamaret, Gamay and Chasselas, known to differ in their susceptibility to downy mildew (PEZET et al. 2004a) were chosen as references. The other 34 cultivars tested in this work represent new germ- plasms, whose level of resistance to downy mildew had not been determined under field conditions. Their origin and characteristics are described in Tab. 1. D e t e r m i n a t i o n o f c a l l o s e p r o d u c t i o n : Callose synthesis in stomata was observed and quantified as described by GINDRO et al. (2003). Thin sections of leaves,