Interactions of Gremmeniella abietina and endophytic fungi in shoots of Scots pine trees treated with simulated acid rain

1. Detached shoots of Scots pine trees with naturally occurring endophytic fungi were inoculated with mycelium of the pathogenic fungus Gremmeniella abietina. The trees had been subjected to simulated acid rain (pH 3, both H 2 SO 4 and HNO 3 ) or control irrigation (pH 6) during the previous five growing seasons. The shoots were incubated in controlled conditions for 6 weeks, after which they were surface sterilized, cut into pieces and plated on agar medium. The frequency of G. abietina and endophytic isolations was measured. 2. The shoots were often colonized by endophytic fungi, two Hormonema species being clearly dominant. Although the frequency of these fungi tended to be low in trees treated with simulated acid rain, the treatment effect was not statistically significant. The isolation frequency of Hormonema 1 increased significantly with height of the host tree. The frequency of Hormonema 2 isolations differed significantly in two separate sub-areas. The frequency of isolations of G. abietina was not affected by simulated acid rain treatment. 3. Gremmeniella abietina was isolated more often from shoots with Hormonema 2 isolations, which suggests that there are common factors determining the success of the endophyte and G. abietina. On the other hand, the frequency of endophytic isolations was lower than expected in those parts of the shoots invaded by G. abietina, implying that the latter was a stronger competitor