Genetic Similarity of Flag Shoot and Ascospore Subpopulations of Erysiphe necator in Italy

Abstract

The overwintering mode of the grape powdery mildew fungus, Erysiphe necator (syn. Uncinula necator), as mycelium in dormant buds (resulting in symptoms known as flag shoots) or as ascospores in cleistothecia, affects the temporal dynamics of epidemics early in the growing season. We tested whether distinct genetic groups (I and III) identified previously in E. necator correlate to overwintering modes in two vineyards in Tuscany, Italy, to determine whether diagnostic genetic markers could be used to predict overwintering. Samples from one vineyard were collected from flag shoots; the other vineyard, 60 km away, had no flag shoots, and mildew colonies were assumed to be derived from ascospores. Genetic markers putatively diagnostic for groups I and III showed that both types were common in the flag shoot subpopulation. Both genetic types were found in the ascospore population, although group III was dominant. We did not find strong genetic differentiation between the two subpopulations based on inter-simple sequence repeat markers. Although there was significant (P < 0.001) genetic differentiation between these subpopulations in 1997 and when 1997 and 1998 subpopulations were pooled ( ‫؍‬ 0.214 and 0.150, respectively), no differentiation was evident between vineyards in 1998 ( ‫؍‬ 0.138, P ‫؍‬ 0.872). Moreover, we did not observe distinct lineages corresponding to overwintering modes, as observed in previous studies. We could not determine if differentiation resulted from biological differences or restricted gene flow between the two vineyards. Our samples were taken from both subpopulations early in the epidemic, while previous studies confounded overwintering mode and sampling time. These results do not support a strong correlation between overwintering and genetic groups, highlighting the need to base population biology studies on sound biological and epidemiological knowledge.

Genetic Structure and Aggressiveness of Erysiphe necator Populations during Grapevine Powdery Mildew Epidemics

“…The temporal isolation of genetic groups shown in our study thus prevents recombination between group A and B isolates, accounting for the maintenance of the highly differentiated genetic structures in E. necator populations, despite demonstrations of interfertility between these groups in controlled crossing experiments (26,37). Group B isolates may overwinter either within buds (8,29,32,41) or within cleistothecia formed by sexual reproduction, whereas group A isolates probably survive the winter exclusively within buds, which become infected at the beginning of the growing season (30). From an evolutionary point of view, this difference in survival strategy between the two genetic groups should confer an advantage on group B isolates, as these isolates benefit from both clonal and sexual reproduction.…”

“…An association between flag shoot symptoms and infection with group A isolates has also been reported in other studies (1,10,26). However, the association between the genetic group and the overwintering strategy has been challenged by the findings in recent studies reporting that flag shoot symptoms may be caused by both groups (8,28,29,32,41).…”

“…The frequencies of E. necator genetic groups vary considerably among vineyards, and the two groups may coexist in the same vineyard (1,4,7,8,28,41). This finding suggests that the maintenance of a highly differentiated genetic structure in E. necator populations may result from the temporal isolation of genetic groups.…”

Spatio-temporal distribution of Erysiphe necator genetic groups and their relationship with disease levels in vineyards

“…Due to this association, these authors proposed that group A isolates may be responsible for early infections in the season while group B isolates may be responsible for late infections (Délye and Corio-Costet 1998;Miazzi et al 2003). However, the association between genetic groups and over-wintering survival has been challenged by recent studies reporting that flag-shoot symptoms may harbour both group A and B isolates (Cortesi et al 2005;Nuñez et al 2006;Péros et al 2005;Willocquet et al 2007). Moreover, the hypothesis of a temporal succession of genetic groups was based on genetic studies that suffered from sampling strategies confounding time during the epidemic with over-wintering mode and source of inoculum.…”

“…Data available showed that the frequencies of the groups could vary greatly from one field to another, suggesting a high level of spatial heterogeneity at the vineyard scale (Cortesi et al 2005;Amrani and Corio-Costet 2006;Bouscaut and Corio-Costet 2007;Willocquet et al 2007).…”