A single nucleotide polymorphism in the β‐tubulin gene distinguishing two genotypes of Erysiphe necator expressing different symptoms on grapevine

Abstract

The biotrophic fungus Erysiphe necator (formerly Uncinula necator), for which two genetic groups have been described in European vineyards, is the causal agent of grapevine powdery mildew. By analysing the pathogen population with respect to polymorphism in the sequence of the β‐tubulin gene, which distinguishes two groups of isolates, a new tool was developed for epidemiological and population studies and tested in the vineyard. As in many ascomycetes, the β‐tubulin gene of E. necator (Entub) includes six introns and seven exons and encodes a 447‐amino‐acid protein. A single nucleotide polymorphism (SNP) in the intron‐3 region of the Entub gene distinguished two genetic groups (A and B). This method was used to examine differences in the ratio of the two groups from a total of 289 grape powdery mildew samples collected at the beginning of the growing season from either flag shoots or leaves with sparse‐spot symptoms in four different vineyards. The SNP in the intron‐3 region of the β‐tubulin gene, similar to SNPs in the CYP51 gene, was associated with genotypes A and B of E. necator and confirmed the existence of two sympatric populations of the pathogen in the French vineyards. Differences in the relative proportions of each group varied with the presence or absence of flag‐shoot symptoms and with the region in which isolates had been collected.

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

“…Délye et al (11) suggested that group A isolates survive the winter as a resting mycelium within dormant buds, from which growth is reinitiated after budbreak, leading to the colonization of young flag shoots (30,34); in contrast, group B isolates originate from ascospores released from overwintering cleistothecia (19). 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

“…Several studies have suggested that genetic E. necator groups (A and B) correlated with ecological features of the pathogen; Délye et al (1997) proposed that group A isolates over-winter as resting mycelium within dormant buds that reinitiate growth after budbreak and colonise young flag-shoots (Pearson and Gärtel 1985), while group B isolates would survive as ascospores released from overwintering cleistothecia (Gadoury and Pearson 1988). Indeed, an association between flag-shoot symptoms and infection by group A isolates has been found in earlier studies in France (Délye and Corio-Costet 1998;Amrani and Corio-Costet 2006) and Italy (Miazzi et al 2003). 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).…”

“…The molecular method used to differentiate genetic groups was the amplification of the β-tubulin gene of E. necator (tub2, accession number AY074934) exhibiting a T/C single nucleotide polymorphism (SNP) between group A and group B isolates (Amrani and Corio-Costet 2006). SNP creates a recognition site of restriction endonuclease AccI that allows the characterisation of A or B isolates by Cleaved Amplified Polymorphic Sequence (CAPS) analysis (e.g.…”

“…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).…”