Hey Dave,

You can try something like immunox. When I see this I usually have very high humidity and conditions that are a bit on the cold side. If you want to get rid of it, you should treat and try to cut-back on humidity a little. Also when was the last time you repotted?

Is this a powdery substance on the leaf? If so, it's probably powdery mildew. This tends to occur when temperatures are low, humidity high, and there isn't enough air circulation. Typically, increasing temps and setting up a fan to get air moving in the room (don't have it blow directly on the plants though) will remedy the situation. Spray with your Physan and then do the above steps and things should improve.

What is your humidity? Get yourself a thermometer/hygrometer with high and low readings - I got one for $9.95 in Walmart.
They can take quite low temperature - so may be you just put them on a windowsill on a natural light and do not crowd them.to increase an airflow.
Would be nice if you post a pic. Usually the streps are more resistant to Powdery Mildew than AVs. The treatment is the same - you need to remove the affected leaves - when the mildew is in the tissue - you cannot kill it - it will show up again and again. You can use Physan or Neem oil or Green Cure ( Potassium bicarbonate) and spray them once a week - or get more serious fungicide - and spray them once a month. Keep them on a dry side - the juicy leaves suffer more than lean.

Good luck

Irina