Comprehension of the interaction between Plasmopara halstedii and its host plant Helianthus annuus requires knowledge of the variability of pathogenicity. With this in mind, the relationship between virulence and aggressiveness was studied by using 5 progeny isolates of races 300, 304, 314, 704 and 714 arising from two parental ones of races 100 and 710. Using molecular markers helps to underline the specificity of the relationship between the two components of pathogenicity in P. halstedii which is characterized by a high level of evolutionary potential (Sakr 2011).

High percentage infection, short latent period, high sporulation density and, significant reduction in the length of the hypocotyl represent high aggressiveness (Sakr 2009, Sakr et al. 2011). Several relationship cases may be obtained when pathogenicity of progeny isolates is compared with parental ones. The two progeny isolates DU 1842 and DU 1767 were more virulent and aggressive than parental isolate MIL 001, and the correlation was positive.

For other Oomycete, Phytophthora infestans on potato, Miller et al. (1998) found that new populations were more virulent and aggressive than the old populations. However, the three progeny isolates DU 1943, DU 1734 and DU 1915 were more virulent and less aggressive than MIL 001; the progeny isolate DU 1842 was less virulent and more aggressive than parental isolate MIL 002, and the relation was negative. In similar experimental conditions, Murakami et al. (2007) found that variant strains were more virulent and less aggressive than parental strains in pathosystem Magnaporthe oryza / wheat and rice. Moreover, the progeny isolate DU 1915 was more virulent and did not show differences of aggressiveness as compared with parental MIL 002, and the relation was uncorrelated.

There are three types of relationship between virulence and aggressiveness in P. halstedii and two hypotheses could explain them. First, it seems that virulence and aggressiveness are independent, and the coincidence makes this relation positive, negative or uncorrelated in P. halstedii. Second, the isolates used in this study belong to several races and may be found to be an effect of additional virulence genes in P. halstedii isolates for variation of aggressiveness as observed for other pathogens (Leach et al. 2001). This makes it possible to have the three types of relationship between virulence and aggressiveness.

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