Control of pathogenic PAC strains by non-pathogenic PAC strains in planta does not correlate with higher competitiveness of non-pathogenic PAC strains ex planta

Abstract

Ascomycetes of the Phialocephala fortinii s.l.—Acephala applanata species complex (PAC) are frequent root endophytes of forest trees. Roots are colonized by multiple PAC genotypes that interact, and recent findings indicate that adverse effects on plant performance caused by pathogenic PAC strains are attenuated by non-pathogenic PAC strains. However, it was not known if this “self-control” works only in planta, or also ex planta, i.e., prior to infection during saprotrophic life of the PAC. Interactions between PAC strains were therefore studied in a plant-free system on malt extract agar. The mycelia of two pathogenic (A and T1) and two non-pathogenic (B and C) PAC strains were mixed pairwise 5:1, 1:1 and 1:5 (fresh weight ratios) and incubated at 15 and 25 °C. Mycelial biomass of each strain was measured after 2 and 8 weeks. The combination of strains and the mixture ratio had a significant effect on strain biomass, whereas temperature influenced only the biomass of pathogenic strain T1. Biomass production of strain T1 was inhibited by all other strains, whereas biomass production of the other pathogenic strain A was significantly stimulated by the two non-pathogenic strains. This contrasts strongly with results from a previous experiment in planta using strains A, B and C, because the two non-pathogenic PAC strains successfully inhibited the pathogenic strain, probably by space occupation or the induction of host resistance. Therefore, it is impossible to predict the outcome of PAC-PAC interactions in planta based on the results gained from interactions ex planta.