Updated project metadata. Circular agar plugs with fresh mycelium of H. annosum isolates were inoculated onto the center of 9-cm Petri plates containing 2% MEA (malt extract agar) and incubated at 22°C in the dark. Presence of viruses in H. annosum isolates used for the experiment: –1W, isogenic isolates fully cured of viral infections by thermal treatment lasting a one week; –A2, –A4, –A5, –C1, monohyphal cultures created by hyphal tipping; 2052–, isogenic isolates, which have received different combinations of viruses by horizontal transmission from a donor strain denoted by D; –3M, dual cultures maintained for three months.