Do you consider the response to be the presence or absence of a fungal species, and you count the number of plants (out of 25) with the fungus?
It would be best not to use Poisson distribution in a GLM analysis. Use a Binomial model (absent, present) because you know how many of each. That is, y is the number with fungus and 25-y is the number of without.
Your data table should like like this:
Enter the counts in the last column. I saved a table script to complete the action of selecting Analyze > Fit Model. The launch dialog looks like this:
The data table is attached for you.