Castanea sativa: genotype-dependent recovery from chestnut blight

In Lovran (coastal Croatia), a unique forest/orchard of evenly mixed grafted marrons and naturally growing nongrafted sweet chestnut trees exists. This old chestnut population has been devastated by chestnut blight, caused by an aggressive introduced pathogenic fungus, Cryphonectria parasitica. However, initial observations indicated recovery of naturally growing chestnut trees in that area, mediated by Cryphonectria-associated hypovirus (Cryphonectria hypovirus 1 (CHV-1)). Such recovery was not observed on grafted trees. Genotyping both, we confirmed the clonal origin of the grafted ones—marrons. No significant difference was observed between fungal strains isolated from naturally growing trees and the ones from marrons regarding fungal vegetative compatibility types or the prevalence of CHV-1. A strong correlation was observed between the types of canker: active/deep-expanding versus healing callus or superficial necrosis and the absence or presence of CHV-1 in the fungal isolates, sampled from naturally growing trees (Spearman rho 0.686, p value 7.81 × 10⁻⁵, Kendall tau 0.686, p value 5.18 × 10⁻⁷). Such correlation was not observed on marrons (Spearman rho 0.236, p value 0.235, Kendall tau 0.236, p value 0.084), because, unexpectedly, active/deep-expanding cankers were often associated with hypovirulent fungal isolates. These data indicate that the lack or unequal distribution of naturally occurring hypovirulence were not the cause of substantial marron decay in Lovran. Ecological and age-dependant differences were ruled out because all sampled trees are growing in close proximity and are of similar age. The results imply that the marron genotype is especially vulnerable and its ability to recover is limited even when the hypovirulent strain of the fungus is present in the canker.