Wish I could send a longer response but its past bedtime this end of the pond! Look up everything you can on this forum on Winter Leaf Drop (WLD) using the search function at the bottom of citrus forum page. Use google too. Nothing to be alarmed about if this is the case. More info on everything about how you look after these plants is required. Watering, fertilizer, growing medium, environment, size etc

Looks like a soil issue... salt? Too high pH? Are you by any chance watering with water from a water softener? You would be surprised how many plants are killed that way, as water softeners replace minerals with salt. Can you tell us what type soil you have? Looks like it may be planted next to a wall; if so, is your house new? Sometimes around new buildings concrete or lime is left in the soil and that will kill a lemon tree.

Are the lemons of equal quality on all the stems? I see spines on one stem which implies that it is from a seedling and is maybe a rootstock. In that case that stem could outgrow the others. But if the fruit is the same on all stems why change it? It's only a matter of aesthetics.

If you're using a water-retentive medium, deep pots are easier to grow in because they will have a higher % of soil that isn't saturated when a perched water table is in play: Choosing an Appropriate Size Container How large a container ‘can’ or ‘should’ be, depends on the relationship between the mass of the plant material you are working with and your choice of soil. We often concern ourselves with "over-potting" (using a container that is too large), but "over-potting" is a term that arises from a lack of a basic understanding about the relationship we will look at, which logically determines appropriate container size. It's often parroted that you should only move up one container size when "potting-up". The reasoning is, that when potting up to a container more than one size larger, the soil will remain wet too long and cause root rot issues, but it is the size/mass of the plant material you are working with, and the physical properties of the soil you choose that determines both the upper & lower limits of appropriate container size - not a formulaic upward progression of container sizes. In many cases, after root pruning a plant, it may even be appropriate to step down a container size or two, but as you will see, that also depends on the physical properties of the soil you choose. It's not uncommon for me, after a repot/root-pruning to pot in containers as small as 1/5 the size as that which the plant had been growing in prior to the work. Plants grown in ‘slow’ (slow-draining/water-retentive) soils need to be grown in containers with smaller soil volumes so that the plant can use water quickly, allowing air to return to the soil before root issues beyond impaired root function/metabolism become a limiting factor. We know that the anaerobic (airless) conditions that accompany soggy soils quickly kill fine roots and impair root function/metabolism. We also know smaller soil volumes and the root constriction that accompany them cause plants to both extend branches and gain o/a mass much more slowly - a bane if rapid growth is the goal - a boon if growth restriction and a compact plant are what you have your sights set on. Conversely, rampant growth can be had by growing in very large containers and in very fast soils where frequent watering and fertilizing is required - so it's not that plants rebel at being potted into very large containers per se, but rather, they rebel at being potted into very large containers with a soil that is too slow and water-retentive. This is a key point. We know that there is an inverse relationship between soil particle size and the height of the perched water table (PWT) in containers. As particle size increases, the height of the PWT decreases, until at about a particle size of just under 1/8 inch, soils will no longer hold perched water. If there is no perched water, the soil is ALWAYS well aerated, even when the soil is at container capacity (fully saturated). So, if you aim for a soil (like the gritty mix) composed primarily of particles larger than 1/16", there is no upper limit to container size, other than what you can practically manage. The lower size limit will be determined by the soil volume's ability to allow room for roots to ’run’ and to furnish water enough to sustain the plant between irrigations. Bearing heavily on this ability is the ratio of fine roots to coarse roots. It takes a minimum amount of fine rootage to support the canopy under high water demand. If the container is full of large roots, there may not be room for a sufficient volume of the fine roots that do all the water/nutrient delivery work and the coarse roots, too. You can grow a very large plant in a very small container if the roots have been well managed and the lion's share of the rootage is fine. You can also grow very small plants, even seedlings, in very large containers if the soil is fast (free-draining and well-aerated) enough that the soil holds no, or very little perched water. I have just offered clear illustration why the oft repeated advice to ‘resist potting up more than one pot size at a time’, only applies when using heavy, water-retentive soils. Those using well-aerated soils are not bound by the same restrictions. As the ht and volume of the perched water table are reduced, the potential for negative effects associated with over-potting are diminished in a direct relationship with the reduction - up to the point at which the soil holds no (or an insignificant amount) of perched water and over-potting pretty much becomes a non-issue. Appropriate Medium From my perspective, an appropriate medium is a medium that allows us to water to beyond the point of saturation at will, so we're flushing the dissolved solids (salts) that are present in tapwater and fertilizer solutions. These dissolved solids are left behind and accumulate in soils whenever we're forced to water in sips, which is commonly employed as a strategy in order to avoid the sogginess that limits root function and wrecks root health. It's important to realize that a healthy plant is not possible w/o a healthy root system. If you have established goals that include healthy and attractive plants, it's critical that you have a plan to avoid the limitations imposed by over-watering and an accumulation of dissolved solids (salts) in the soil solution. Not every grower fully understands the dilemmic issues associated with inappropriate soils that force the plant to pay a vitality tax resultant of an unhealthy amount of water being retained for extended periods when we water correctly – which is to say, when we flush the soil to limit salt build-up. On one hand, we have the potential for over-watering, and when we act to avoid it by offering dribs and drabs of water here and there, we have high salt levels to deal with. It's easy to see how we all might benefit from use of a soil that allows us to water so we're flushing away excess salts without limiting our plant's vitality via waterlogged soils. Al