Could be mites. If you nudge the black dots with your fingernail and they start moving, they're mites.

Also, the older leaves sometimes yellow before they fall off. It's a natural process. How old are those leaves that are yellowing? Are they from winter time or are they new? Once the plant gets really growing, I wouldn't worry about leaves so much-- unless they all fall off at once -- in which case it's probably stress from something -- too cold, too wet, really bad bugs, improperly applied bug repellents, etc..

But I'd treat for mites if they're there. Don't treat in full sun or in heat of the day. Better to treat either early morning or later in evening.

Even a regular hard spray of water will help deter mites...like every day or so.

Hope others will chime in.

Its probably just mites. You can use a water hose to give it a once over then treat again. Be sure to get the undersides of the leaves with the hose. I have been told doing so interrupts the egg laying cycles of the mites.