Knowledge of the relations between virulence and aggressiveness would help to understand dynamics of parasitic populations that use their pathogenicity to improve adaptation to their environment (Leach et al. 2001). Virulence is a driving force in host-pathogen co-evolution since it enables pathogens to overcome qualitative resistance genes R. Aggressiveness enables the pathogen to develop within the host plant (Van der Plank, 1968). Van der Plank (1968) indicated that virulence (vertical pathogenicity) and aggressiveness (horizontal pathogenicity) are often negatively correlated in the case for Phytophthora parasitica var. nicotianae on tobacco (Sullivan et al. 2005) and Leptoshaeria maculans on oilseed rape (Huang et al. 2006). Damgaard et al. (1999) suggested the absence of difference of aggressiveness between a virulent strain and nonvirulent strain in a theoretical model. Moreover, the two components of pathogenicity are positively correlated in the case of obligate parasite Phakospora pachyrhizi on soybean (Bonde et al. 2006). Virulence (qualitative pathogenicity) has been defined as specific disease-causing abilities and aggressiveness (quantitative pathogenicity) as non-specific disease-causing abilities (Van der Plank 1968).

For Plasmopara halstedii, no studies on the relationship between virulence and aggressiveness have been reported. P. halstedii (sunflower downy mildew) is an obligate endoparasite that cannot be cultivated independently from its plant host. P. halstedii is a homothallic oomycete, whose cycle is made up of a single sexual generation permitting overwintering and one or perhaps two asexual generations which occur during the growing season (Spring and Zipper 2006). P. halstedii displays a gene-for-gene interaction with its host plant and shows physiological races (pathotypes) capable of infecting a variable range of sunflower genotypes. The nomenclature of these races is based on the reaction of a series of differential lines (Tourvieille de Labrouhe et al. 2000). To date, there are at least 35 races in different parts of the world according to Gulya (2007).

Disease resistance in sunflowers to P. halstedii can be placed in one of two categories, the first is qualitative resistance (Tourvieille de Labrouhe et al. 2000) and the second is quantitative resistance (Tourvieille de Labrouhe et al. 2008).

In this study, aggressiveness was analysed for seven P. halstedii isolates: race 100 present in France since 1966, race 710 introduced from the USA during the 1980s (Tourvieille de Labrouhe et al. 2000) and 5 progeny isolates of races 300, 304, 700, 704 and 714 which appeared recently in plots where the first two races were present (Tourvieille de Labrouhe et al. 2010). Four aggressiveness criteria were evaluated: percentage infection, latent period, sporulation density and reduction of hypocotyl length (Sakr 2009,Sakr et al. 2011). These criteria were analysed by using a sunflower inbred line not carrying any qualitative resistance Pl gene but showing a high level of quantitative resistance (Tourvieille de Labrouhe et al. 2008). In this paper, our target was (i) to study the relationship between the two components of pathogenicity of progeny isolates as compared with parental ones and (ii) to analyze the specificity of this relationship using 12 EST-derived markers. Hence an attempt was made to generate information about the possibility of using the molecular markers to determine the relationship between virulence and aggressiveness in P. halstedii.

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