We viewed the photos. This looks like the plant is showing symptoms of a nutrient deficiency.  It is okay to pick off badly affected leaves.  The plant needs adequate light, moisture, and nutrients to grow well.  Citrus are heavy nitrogen feeders. Even though you have fertilized several times this season, we recommend that you fertilize one more time before bringing back indoors for the winter. We do not think this will hurt the plant. Do not fertilize in the winter. See the link below for fertilization. 

You will have to bring the plant in very soon before we get cold temperatures.  Before moving houseplants back indoors check plants for ants, pillbugs, etc. Prior to bringing citrus plants indoors for the winter, you can apply a spray of horticultural oil or insecticidal soap labeled for use on houseplants to manage these pests like scale or spider mites. Here is how to grow citrus indoors https://extension.umd.edu/resource/growing-dwarf-citrus

Marian

On Wednesday, October 6, 2021, 2:06 PM, Ask Extension <<personal data hidden>> wrote:

You are welcome.

Marian

We cannot say for sure why the lemon tree is showing the yellowing symptoms on the foliage. 

Here are some thoughts. Once bringing the plant indoors for the winter, it will most likely lose leaves and any lemons as it adjusts to indoor conditions. Do not overwater and make sure the container has holes for good drainage. Allow the surface of the soil to dry between waterings. If the container is a self watering pot, they are tricky to water properly. If so, the next time it gets repotted, swap it out with a normally drained pot. 

 You did not mention the type of fertilizer you used. Make sure it is a complete fertilizer and it has some micronutrients in it besides N-P-K such as a citrus type fertilizer. They are heavy nitrogen feeders.  If you did not use this type of fertilizer, you can apply one dose of a citrus type fertilizer and see if it helps. Symptoms probably will not clear up in the old foliage only new growth. If the foliage drops, you may not notice anything until next season.

If you have used a citrus type fertilizer or a complete fertilizer, we tend to think that you may be dealing with some root less due to overwatering. 

Repot in the spring sometime in April before you take the plant outdoors.  Look at the roots and do some root pruning. Repot into fresh potting soil and select another container with drainage holes if the container was self watering. If using a saucer to collect excess water, this should be dumped from drainage saucers. The container can absorb excess water contributing to root loss. Next spring see if the plant takes off and produces buds. Here is our page on dwarf citrus and care during the winter.

Marian 

On Wednesday, November 3, 2021, 11:00 AM, Ask Extension <<personal data hidden>> wrote:

We are in the process of consulting with a plant nutrition specialist to see how we can further help you. 

Any other information that you can give us about when it was brought in, how/when you water, how much sun it receives, soil mix used, when it was last repotted, etc. will be helpful.

Christine

Hello Terry,

Thank you for the additional photos. We'll reply again when we receive feedback from the specialist (it might be early next week).

The watering frequency sounds high given the time of year and the plant's pot size. Although how dry your indoor air is (if from an HVAC system, which typically has very low humidity) and how warm the home is will influence how quickly potting soil dries out, we'd estimate that one watering a week - or less - would be a better interval. Citrus like drying out more than most houseplants, and will require even less water as leaves are lost.

The best way to judge when to water is to feel the soil with a finger about two or three inches beneath the surface (for a pot of this size) and water only when it's getting fairly dry at that depth. You could also pick up the pot and feel it's weight if it's made from a lightweight material like plastic; a container needing water and one freshly watered will have noticeably different weights. This method may take more getting used to but can be just as reliable as using the finger test.

When plants shed leaves, they cut off the leaf's supply of moisture and nutrients from inside it's stem, meaning that once that process is set in motion, it won't stop even if conditions improve. (This is why autumn leaves on deciduous trees don't return to green or stay attached longer if we have a mild November or December.) Because of this, it's hard to tell exactly when the plant has been relieved of its stress since these leaf-drop symptoms can be a delayed reaction. It could be weeks between the formation of that tissue barrier inside the leaf's stem and when it actually yellows and falls.

If new growth starts emerging along the stems or branch tips, that will be a good sign that conditions are stabilized enough for the plant to start to recover. However, since daylight is very limited indoors in winter and citrus needs bright light to grow, it's possible it will instead just stagnate and survive the winter semi-dormant, waiting to grow new foliage until spring. A very sunny window like your south-facing one is a good spot, but may not be enough light to stimulate active growth. If desired, you could supplement its lighting with a plant grow light, but with so little foliage at this point that may still shed, we'd suggest waiting to see how it responds before investigating your lighting options. The plant doesn't have to actively grow in winter, so it would be alright to just let it wait out the winter largely leafless and put it back out in spring to recover.

It's possible for plants to display nutrient deficiencies in their foliage when the soil itself has plenty of nutrients. This happens when their roots are unhealthy or have died back some, and the typical cause of that is accidental overwatering. Symptoms don't always appear right away, which can make diagnosis tricky. Your May repotting should be fine as far as the soil itself being fresh enough, but a common method of overwatering involves letting drained water pool in the saucer under a pot after watering. If this tends to happen, just empty the saucer into the sink before putting the pot back on it, or just water the plant in the sink and let it drip-dry there before putting it back onto the saucer. (You may already be doing this. We don't see water in the saucer in the photo, but it's hard to tell if it ever did retain water.)

Aside from water in the saucer itself, the other common method of overwatering is just watering a plant too frequently. Evaporation through the plant's leaves plus evaporation from the soil surface itself is the only way the potting soil loses water. Cooler temperatures slow evaporation, as does slower plant growth. The tiny spaces between soil particles need to have a combination of moisture and air to keep roots healthy, and when plants are overwatered, too little air is available to roots due to the consistent dampness and they essentially suffocate. Plants with dying roots can't absorb the nutrients they need, which is why foliage can become deficient. Plants can grow replacement roots if they get a respite by having the watering frequency better adjusted so the roots have a chance to "breathe," which the finger test is very helpful in determining.

Miri