Can anyone tell me what is wrong with my lemon tree? It's in a pot, in the sun. Not all the leaves look like this. About half are still normal. Living in Uruguay, South America.

This looks like a calcium (Ca) deficiency that might either be due to an actual deficiency of Ca in the soil/ soil solution or a culturally induced deficiency. There isn't enough information provided to allow anyone to actually make that case, but let me ask some questions.

Does the pot have a drain hole?
Are you using a commercially prepared potting medium or something else, topsoil perhaps?
Are you fertilizing regularly? With what product?
Does the fertilizer contain Ca? Magnesium (Mg)?
Have you used anything that added supplemental Mg?
How do you determine when it's time to water?

Waterlogged media and/or poorly oxygenated media very commonly cause Ca deficiencies, which most conspicuously affects new foliage due to the fact Ca is immobile in the plant. IOW, the plant cannot move Ca from one location to another like it can nitrogen, phosphorous, potassium, and magnesium, so the deficiency shows up in new growth.

Al

Being a Californian from a citrus growing area, I am always very aware of the possibility of HLB (Greening Disease). I found this article from March, 2023:

https://citrusindustry.net/202...

And this from the University of California:

https://ipm.ucanr.edu/agricult...

Thank you for you help. As these are new saplings, they have only been in their pots for 2 months. I have not fed them yet. It is possible with all the rain recently that water logging could be an issue. The pots have drain holes and the soil medium was purchased from a garden center. The soil medium looks to be mostly composted matter. There is no top soil.

Can you confirm that the grow medium you used is actually a potting medium as opposed to garden soil? Reasons: 1) The size of the particles that make up the grow medium is the primary driver of water retention. Garden soil usually has a high % of mineral ingredients, and tends to compact and hold too much water to be used for container growing. Both excess water and compaction limit oxygen supply which decreases Ca uptake. 2) Potting media almost always contains dolomitic lime, which serves to raise pH and supply both Ca and Mg. Media packaged for use in gardens/landscapes would typically not contain dolomitic lime as an additive. FYI, a calcium deficiency can be created by adding magnesium-containing supplements like Epsom salts, even if there is an adequate supply of Ca in the grow medium. These culturally induced deficiencies are called 'antagonistic deficiencies' and occur when an excess of one nutrient limits uptake of one or more other nutrient. Examples: Ca & Mg are antagonists and an excess of one can cause a deficiency of the other. Phosphorous shares antagonistic tendencies with Ca, K, Cu, Fe, Zn.

Since Ca is immobile in the plant, it MUST be in the soil and available for uptake at all times. If the nutrient stream lacks Ca, cells cannot form normally, the most conspicuous symptoms being poorly formed or distorted leaves, holes or tears in leaves, and weak cell walls (and/or areas of tissue that contains the weakened or poorly formed cells) that often produce necrotic areas in/on leaves.

There are a couple of things you can do to reduce water retention during extended periods of drought. Simply tipping the pot to a 45* angle can significantly limit water retention. Compare B to A below:

The pots are full of soil. The shaded area represents (perched) water that defies the force of gravity, refusing to drain from the pot.

Also note how proper use of ballast (D) and a drainage wick inserted through the drain hole (E) work to limit water retention.

Al