Question
We have had a 4ft kumquat tree in our south window for about 3 years.  Each year it bears fruit and is in a 3-4 gal pot.(original) It has had a fresh growth of leaves since April and May. Since last week a fair amount of the new leaves and some of the full size ones are quickly turning a rusty brown color and falling off. Not the same kind of crispy dry like when you forget to water it.  When the leaves drop they are quite supple. The soil is moist.  I water it about once a week. It looks a little compacted and some of the finer roots are starting to show. Any ideas what's going wrong.

Answer
The leaf drop you describe has many symptoms.  I have been thinking about this and it is hard to say (for me) without seeing the plant.  But not impossible.

Your Kumquat is now 3 years old -- have you repotted it?  This is the best time of year to do that.  A rich, loamy soil, lightened with sand and peatmoss for lowered pH, should be used.

Leaf drop is sometimes a sign that roots that are damaged or constricted.  Especially since you point out that you can see some of the finer roots.  Insufficient light and/or watering and/or nutrition will sometimes cause this problem.

Nutrient deficiencies can also show up about this time.  Please tell me how you have been handling the feeding of this Kumquat.  Coupled with underpotting your Kumquat could be enduring some serious stress.  Please check for these and let me know what you find.