Greenhouse A.M.S Grow

hi, i am growing one feminized a.m.s plant from greenhouse, it is growing nice and big, very quickly, hard to keep up!
A couple of the leaves are getting rusty spots and yellowish spots on them. this is mainly happening at the bottom.
Are the avereage spots generally a problem???????!!!!!???????

Also, seing as i dont have much space for the plant, i wanted to maximize the yield by topping the plant. the two grow tips are forming and are just about visible; you cant see the two stems yet, its just a tight bundle of leaves. so would it be ok to start flowering now before the new stems appear from where i topped it?

In my opinion what would happen is; the topped plant would not grow taller stem wise, but would still develop two main stems worth of bud, but just in a more compact form.
If id have left it so the stems had grown longer (to about an inch), the buds would have grown normally, but on seperate stems. possibly not producing as much.
i would prefer to have a topped plant with as shorter stems as possible so to achieve the most amount of compact MJ in what looks like one fat bud!
Thanks for any advice.

hi im growing greenhouse ams . it grows like stink . but streaches a lot when u flower.. u need to switch them over about 12 inches high . or u will end up with 4-5 ft plants .. the rusty spots are usualy a magniesium defiency i flush with plain water every nute change they seem to love that .also try some liquid oxygen ...good luck

just read your other bit m8 they still grow so much up to 3 weeks bloom .. i flowered at 18 inches and i now have 4 ft plants . ams is a fast grower .even after u switch the lights