Is this fungal

I have a little used Tamron 18-270mm f3.5-6.3 Di II VC PZD which I was going to let go but decided to keep as a walkabout. I've had reasonable/good quality images from it but nothing as good as my L-Stock glass would return obviously.
I'd lent it to my girlfriend and she's had it for a few months.
I'm just checking it out on an 80D and its not as sharp as I would expect at 270 checking the leaves on my 'test tree' down our avenue.
In line with what most reviews are saying it pulls into line about f10
Looking into the lens the back optic seems to have a mottled texture, which I've tried to get a picture of unsuccessfully. there are hairlines in there as well.
Is this damaging the lenses performance and is this a case of fungus
I'm particularly concerned because for a fair while she kept it in the kitchen, yeah... a great environment for a lens - not.
What do you guys think

1 - about the lens
2 - about having a girlfriend like this

..seriously though

It could be fungus, without a picture it's impossible to tell. However, at f10 you are well beyond the diffraction limit of your lens especially on the 80D. It almost sounds as if the lens is either back/front focusing a tad and closing the aperture to f11 gets you a DoF great enough to get your test tree in focus.

Try taking a picture at f/5.6 using live view zoomed in at 10x to get a perfect focus and evaluate the lens performance.

As to the girlfriend, take a look at the mom and see your future.

Following another tip that I read, I've viewed the optics under a bright light and it's obviously on the front optic.
Managed to get a shot, not fantastic but not looking good....
Is this bad fungus or average fungus!