Bottom leaves yellow and dying on young plants

Info:
Indoor in soil (Organicare Pure Earth)
Lighting: was cfl is now (2) 400 watt hps
temps vary ( 65 deg. at night to 85 deg during the day )
water ph is adjusted from 7.0 well water down to 5.8 - 6.0 with ph down
several different strains: white widow, Power Plant, C99, and a thai x haze hybrid
plants are all young and small, 6 inches or so and vary in age 3 to 8 weeks, except my mother plants. All are infected.
Have not fed with any nutriants, except Epsom salts, as I thought maybe a mag. problem.

History: Had spider mites which I treated with 'Organocide' by Organic Laboratories and no pest strips. No sign of bugs now.

Transplanted several to larger pots thinking maybe a root issue. No positive results from that. All roots lookes healthy in the plants that I transplanted.

Problem:
The lower leaves turn yellow, then die. They show splotches of grey. ( see the pictures)

I'm stumped. Is this a fungas problem maybe? can fungas problems be cured or do I have to scrap this entire grow and start over?

Do you think maybe the Organocide has damaged my plants somehow?

I'm at my wits end trying to figure this out.

Its always hard to tell from pics. It 'might' be phosphorous issue which would give necrotic spots and then shrivel and go malformed? If so you can mix inorganic phosphate fertilizer thoroughly through the soil.

Bat guano is great added to soil and helps with many of these lower problems. If it is feasible you could add some perhaps.

Peace