Thyreophagus corticalis as a vector of hypovirulence in Cryphonectria parasitica in chestnut stands

The natural spread of hypovirulence in Cryphonectria parasitica (Murr.) Barr. occurs in chestnut (Castanea sativa Mill) stands and orchards in Italy and other European countries, leading to spontaneous recovery of the diseased trees. Little is known about how hypovirulence spreads in chestnut stands but various corticolous mite species frequently detected on chestnut cankers could be one of the many factors playing a role in the spread. Artificial virulent cankers created in inoculation field tests and treated with Thyreophagus corticalis (Acari, Sarcoptiformes, Acaridae) raised on hypovirulent cultures showed similar growth to those treated with mycelia of the hypovirulent strain over 18 months of inoculation. Cultures re-isolated from virulent cankers treated with mites were found to contain hypovirus like those derived from pairings of virulent and hypovirulent strains. Viral dsRNA could be carried externally and/or ingested by mites from the hypovirulent mycelia and then transmitted to the mycelia of virulent strains, causing their conversion. In a laboratory study, all fecal pellets collected from mites reared on hypovirulent and virulent strains grown on semi-selective media gave rise to colonies of C. parasitica with similar morphological characters and virulence to the original cultures. Field inoculation of stump sprouts with the resulting colonies revealed that mite digestive tract passage did not alter the virulence of the studied strains. These results are of interest for the biological control of chestnut blight.