Oops, I had trouble posting the pictures. Here they are now:

Yellowing leaves:
{{gwi:98036}}

Scaly feature on leaf:
{{gwi:98037}}

Scaly features at base of leaves:
{{gwi:98038}}

Hi...I have not seen your pics yet. I can't here at work..

But I am definately sure that if you are seeing fungas knats around your plant, that is a tell tale sign that your soil is too wet too often for your jade. If you let the soil dry out to the point of fungas knats not able to bread, then your plant would have much happier feet..Just an observation..
Could be starting root rot..:-(
That is what happened to my moms jade when it got root rot, all the leaves turned yellow and fell as you are saying.
Someone else might have alot to say of the pics that I have not seen yet.
I have 4 jades, and I can tell you for sure that they do not like wet soil..
I hope you get the help you need. This is a fun place to come where people really care..Welcome
Goodluck,
Mike

On more thing...

Usually weak plants can become very suseptable to insect invasion and desease. :-(

Mike,

Thank you for your response! I think you are right about the overwatering. Earlier in the summer, I was watering more often--about once every 3 weeks--and once I saw the fungus gnats, I figured it was too often for my plant. Thankfully, I haven't seen any gnats for over a week now.

I'm going to continue letting the soil dry out. In the winter, I would wait until the soil was starting to pull in from the sides of the pot before watering, and it seemed to absolutely love that.

Did your mom's jade survive? I hope so!

I remove any yellowed, diseased, or disfigured leaves.

And, since leaves hold moisture, you can reduce moisture in the plant by reducing leaves (if too much moisture is the problem).

Is this a single-trunk Jade, or is it multiple stems rooted in a pot?

You mention that the soil pulls in from the sides of the pot. Soil should not shrink like that. The cause, most likely, is that the soil has too much peat-moss. Peat-moss holds too much moisture for too long; but, when dry, it is extremely hydrophobic and hard to re-wet. This proves a terrible combination for a Jade.

A change of soil is in order, and the sooner the better.

Josh

Josh, thank you for your response. My jade has two stems in one pot.

I haven't bothered to use a succulent mix for it up until now because it seemed happy enough in regular soil. I guess it's time to make the switch!

Do you have any ideas as to whether or not it is infected?

Thanks!

I don't know if it's infected.
But I'd recommend that you remove any affected leaves or branches.

Then, try one or two drops of non-scented dish soap in a spray bottle (some folks add a bit of alcohol, too). Spray the affected areas, let sit for about ten minutes, then spray with a freshwater rinse. Cover the soil so that it doesn't get wet during this operation.

Repeat this treatment several times over the next few weeks. It won't hurt the plant, but it may help it significantly.

On my small Jades, I've also used a Q-tip with alcohol and water to wipe down areas where I suspected infestation.

Josh