If your clay pot is not fully sealed on the outside, evaporation will cool the soil; and citrus roots do not like to grow into cool soil. Better to use ceramic or plastic or composite, especially in cooler climates.

@johnmerr

Yes, unfortunately pot is sealed (painted with some sort of glaze) on the exterior.

It can get relativelt warm and humid here during Chicago summers, but the past couple of weeks we have been experiencing some unusual cool temps (70s high, 60s low).

Perhaps your soil/pot are not draining well and you have root rot issues, which would cause the leaf color/drop you are having.

Just looks like old leaves to me. Did most of the leaves shown in your picture drop from the lower portion of the tree?

I just checked and I am out of it, but in the past, when I had yellowing on my meyer lemon and other citrus, the local nursery sold me a product that I put around the roots and the yellow went away. Sorry I can't tell you the name, but I think it's a pretty common product.

"pretty common product",, you mean, fertilizer??? Maybe you are talking about epsom salt, which is magnesium sulfate; but your leaves are most definitely not magnesium deficient.

@johnmerr

I did take the tree out the pot to investigate and sure enough a lot of the roots near the bottom of the planter looked rotten =(

I'm not sure if I was supposed to but I trimmed off the rotten roots, threw out the old soil from the bottom of the planter and replaced it with 50% perlite and 50% potting mix for citrus trees. Placed the tree back in the planter and pressed it in.

Now my worries are:

1- can the tree survive with half of its root matter gone?

2- since it is late in the season, will I cause any harm to the tree now that its root system does not fill the entire pot?

@Silica

No, a little bit from everywhere, not just the bottom

The rotten roots you cut were not producing anything, anyway. Your tree will be stronger and healthier and should survive.

Put it in the shade for a week or two to let it recover before you put it back into full sun.

The root rot was caused by cool roots and over watering. Before you water, dig into the top 2 - 3 inches and feel if it is moist. If moist, hold off on watering.

I would also suggest putting it in a better draining (unglazed terracotta) pot to help with drainage and make sure that the pot is not much bigger than the root ball. That's what I did after I had root rot and lost about half of my first Meyer. Now I keep all of my citrus is terracotta pots that are just big enough, with well draining soil. I'm also in NY, so I also have to worry about too much water, as opposed to you lucky folks in the West, South West, and South.

Alanna terracotta pots in the northern hemisphere will create cold feed when moisture evaporates and favorable conditions for root rot. A plastic pot with lots of extra holes drilled in and a good draining soil is a better practice for citrus.

Susanne I just responded in the other thread, before I saw this. So for citrus that is indoor/outdoor, plastic is best? I had no idea that root rot could be caused by cold too! I thought only wet roots were my enemy.

Alanna citrus just don't like cold feet. If you use plastik and make sure the holes are lockated on the sides rather then only at the bottom, it will help to drain faster. In my opinion plastik is best in out climate. Now people in the south with scorching heat could utilize on the cooling effect of the clay.

Right on, Sussanne! That heavy glazed pot with tiny single hole at the bottom again! Those pots are a death sentence for citrus and other potted plants during the cooler fall/ winter months.

Mike is correct. A combination of cold and lack of oxygen due to overly water retentive growth media are what cause root rot. I have used heat mats, of the type used to germinate seeds, to keep my citrus containers warm during winter.