Relationship Between Biological Control, Incidence of Hypovirulence, and Diversity of Vegetative Compatibility Types of Cryphonectria parasitica in France

Abstract

In France, chestnut blight, caused by Cryphonectria parasitica, has been controlled since 1974 in orchards, but never in coppice forests, by releasing hypovirulent strains infected with CHV1 hypovirus. We tested the hypothesis that this biological control (BC) has lead to a decrease in blight severity, spread of hypovirulence, and change in C. parasitica populations. The low severity of chestnut blight was confirmed in the six regions studied (subdivided into zones). The remission of cankers was associated with the presence of white isolates presumed to be hypovirulent. These two parameters were also correlated, at the zonal level, to the frequency of sites where BC was used. However, the estimates of the natural background level of hypovirulence, independent of BC, ranged from 4% in forests in Dordogne to 60% in orchards in Lozère. Differences in the rate of hypovirulent isolates among regions were consistent with the diversity of vegetative compatibility (VC) types in populations of C. parasitica. The highest VC-type diversity and mean allelic diversity for known vegetative incompatibility (vic) genes were observed in Dordogne. We showed that the current diversity of VC types in populations of C. parasitica was lower than in 1981. We found 30 VC types among 1,113 isolates of C. parasitica. Ten VC types were incompatible with known EU testers, suggesting that one additional vic gene or allele at one of the six vic loci known should be present in Europe.

Multiple introductions of divergent genetic lineages in an invasive fungal pathogen, Cryphonectria parasitica, in France

“…However, the large geographical distribution of RE019 and RE092 detected in most of South-Eastern stands, as well as RE053 and RE028 in most of Atlantic stands, suggests an ancient presence of these genotypes in these areas. This ancient presence is also in agreement with the official presence of C. parasitica reported in 1956 in the South-Eastern France (Grente, 1981) and assumed before 1940 in Pyrénées (Darpoux, 1949), and the presence of vc types EU-2 and EU-5 associated with RE019 and RE092, and detected at least since 1970 in South-Eastern France (Robin et al, 2000;Cécile Robin, unpublished data). Furthermore, sexual structures were observed in France for 430 years (Grente, 1981), regularly detected in neighbour countries (Switzerland and Italy; for example Milgroom and Cortesi, 1999), and observed on 10 to 20% of cankers sampled in two French chestnut stands (Guérin et al, 2001;Cécile Robin, unpublished data).…”

“…Isolates were obtained from small pieces of bark removed from cankers as described in Robin et al (2000). Collected material was isolated and cultivated on potato dextrose agar medium (Difco Laboratories, Detroit) plates in the laboratory.…”

“…However, in contrast to Eastern European populations in which only one mating was generally detected (Milgroom et al, 2008), in these Western populations both mating types were often found allowing sexual reproduction among isolates in this heterothallic haploid species (Braganca et al, 2007;Dutech et al, 2008;Robin et al, 2009). The studies in the Atlantic part of Europe were performed with phenotypic markers for which the genetic determinants have not been completely elucidated (that is vegetative compatible (vc) types; Robin et al, 2000;Braganca et al, 2007;Montenegro et al, 2008;Robin et al, 2009) or with molecular markers, but for a low number of sampled populations (Breuillin et al, 2006). Consequently, they have precluded any robust conclusions about the level of genetic relatedness among C. parasitica isolates, especially relative to Italian isolates, and on recombination processes in Western Europe.…”

Engineering super mycovirus donor strains of chestnut blight fungus by systematic disruption of multilocus vic genes

“…Although the currently defined six diallelic vic loci account for the vic diversity of C. parasitica populations in the forest setting in the eastern United States (36), it is clear that additional vic diversity is present in Europe (19,20,37) and Asia (38). It is envisioned that the comparative genomic approach (22,23) for identifying the currently defined vic loci could be used to identify additional C. parasitica vic alleles of existing vic loci or entirely new vic loci.…”

“…A negative correlation between vic diversity and virus transmission has been reported for several fungal hosts (1,16), but has most extensively been demonstrated for the chestnut blight fungus Cryphonectria parasitica infected with virulence-attenuating hypoviruses (7,(17)(18)(19)(20). Genetic analyses have defined six diallelic vic genetic loci for C. parasitica (21).…”