Tolerance Against Co‐Infection of Two Partitiviruses and Ourmia‐Likevirus Is Common Among <i>Heterobasidion annosum</i> Strains on Artificial Media and in Dead Wood

ABSTRACT Some mycoviruses cause hypovirulence in fungi, but the effects often vary among different host strains. Heterobasidion partitiviruses 13‐an1 and 15‐pa1 (HetPV13‐an1 and HetPV15‐pa1) have been associated with strain‐specific and variable hypovirulence of Heterobasidion annosum , but variation in phenotypic effects of HetPV15‐pa1 or the coinfection of these viruses on different host strains has not been studied previously. In this investigation, the effects of single and double partitivirus infections were first studied using six Finnish H. annosum strains on malt agar plates (MEA). Secondly, the effects of single and double partitivirus infections on the growth rate of four H. annosum strains were tested outdoors using Scots pine billets as a natural substrate. Against our expectations, on MEA plates, the single or double partitivirus infections of HetPV13‐an1 and HetPV15‐pa1 did not have significant effects on three of the fungal strains studied and they slightly accelerated the growth rate of three host strains. In the billet experiment, the double partitivirus‐infected strains were more often assorted to the fastest growing group than virus‐free controls. Based on these results, HetPV13‐an1 and HetPV15‐pa1 do not debilitate the tested H. annosum strains on agar plates or dead wood but may even slightly increase the growth rate of the mycelium on artificial medium and in non‐competitive growth conditions in dead pine wood.