Diversity and abundance of culturable fungal endophytes in leaves of susceptible and resistant alternate hosts of Cronartium pini and C. ribicola

Cronartium pini and C. ribicola are rust fungi that cause destructive diseases of pines (Pinus spp.). These rusts spread via alternate hosts, among which Melampyrum spp., Veronica spp. and Impatiens spp. are important for C. pini and Ribes spp. for C. ribicola. Congeneric alternate hosts vary in their susceptibility to Cronartium rusts, but the reasons for this variation are not clear. To clarify whether internal, endophytic fungi could explain these differences, we investigated the temporal and spatial variation in fungal endophyte composition of C. pini-resistant M. pratense, V. chamaedrys and I. glandulifera, C. pini-susceptible M. sylvaticum, V. longifolia and I. balsamina, C. ribicola-resistant R. rubrum and C. ribicola-susceptible R. nigrum. In total, 2695 fungal endophytic isolates were obtained and classified into 37 morphotypes, with 1373 cultures isolated in early summer and 1322 in late summer. Fifty-two isolates were identified to species or genus level. The most common morphotypes were identified as Heterophoma sp. Some variation in the abundance of morphotypes occurred between collection sites, but the same morphotypes dominated across the sites and species. The diversity of morphotypes was higher in early September than in late June in all species and the same morphotypes dominated in both early and late season. The diversity of fungal endophytes was higher in resistant Veronica and Ribes than in susceptible congeneric species, but the results suggest that the diversity or abundance of culturable fungal endophytes does not explain the differences in the congeneric species’ susceptibility to rust fungi.