Virus transmission frequencies in the pine root rot pathogen Heterobasidion annosum.

The combined use of Heterobasidion partitiviruses 13 and 15 (HetPV13-an1 and HetPV15-pa1) is considered a promising biocontrol approach against Heterobasidion root and butt rot. In a previous study, the transmission frequency of HetPV15-pa1 was found to be higher from a double partitivirus-infected donor than from a single partitivirus-infected donor. In this study, we included a wider array of recipient isolates to assess whether the phenomenon is widespread across different host strains and conducted transmission experiments on artificial media (in vitro) using a total of 45 different H. annosum donor-recipient pairs. In addition to investigating whether double partitivirus infection improves the transmission of HetPV13-an1 and HetPV15-pa1, we examined for the first time how efficiently co-infecting ssRNA viruses are concomitantly transmitted with the partitiviruses, and whether pre-existing ssRNA viruses in the recipients affect virus transmission. Generally, the transmission rates of HetPV13-an1 and HetPV15-pa1 were high from both single partitivirus-infected and double partitivirus-infected donors to most of the H. annosum recipient strains, with few exceptions. However, in contrast to previous experiments, the transmission frequency was not higher from the double partitivirus-infected donors. Also, ourmiavirus was transmitted between H. annosum strains, but the presence of another ourmiavirus in the recipient might affect the efficacy.