Using string trimmer line effectively and efficiently comes with technique and practice.
It sounds like you could use more of both since your concerned about secondary issues due to in proper use of your line trimmer is causing fungus. Although the fungus problem you have mentioned could be related to the use of a line trimmer, another cause
more likely is due to to much moisture. Fungus needs moisture to thrive and grow.
Manual edging or trimming devices may help. All the best.