How carton-nest fungi of the ant <i>Lasius fuliginosus</i> interact with each other and with the root-rot fungus <i>Armillaria mellea</i>

<i>Lasius fuliginosus</i>, a fungus-growing ant species distributed across Europe, hosts various fungi inside its carton nests in trees, including the nest fungus SP1 of the order Chaetothyriales, as well as the nest fungi SP5 and SP4 of the order Venturiales. The goal of this study was to gain a better understanding of the fungal interactions inside the <i>L. fuliginosus</i> nests as well as of potential interactions around the nests, including the effects of <i>Armillaria mellea</i>-a root-rot fungus infecting potential host trees. We performed two types of confrontation experiments on Petri dishes between the isolated nest fungi and <i>A. mellea</i>. Firstly, using de Wit experiments, we tested the fungal species in pairwise combinations at three different initial confrontation concentrations. Secondly, a linear confrontation setup focused on differences in directional growth of the fungal species in pairwise combinations as well as on the development of <i>A. mellea</i> rhizomorphs. For the fungi SP1, SP5, and SP4, we found positive influences on each other (SP1 on SP5, SP4 on SP1 and SP5, and SP5 on SP1) alongside no influence (SP1 on SP4, SP5 on SP4). SP1 had a significantly negative impact on the surface growth and directional growth of <i>A. mellea</i>, and SP5 triggered the strongest rhizomorph development of <i>A. mellea</i>, possibly a stress reaction of the root-rot fungus. <i>Armillaria mellea</i> did not negatively impact any of the nest fungi and even promoted the surface growth of SP1. The de Wit setup and the linear setup turned out to be complementary and together facilitated first insights into potential roles of the nest fungi in this association of ants and fungi in trees. Follow-up studies will need to assess how these findings under Petri dish conditions transfer to conditions in natural habitat, in the presence of both the ant and the tree host.