wow... need more info... any changes in the soil or weather ? (recent flooding / drought ?)

have you checked under the leaves for fungus or insect damage ?

How fast did this happen... was it healthy looking a few months ago or ?

have you fertilized recently ?

the yellowing looks like mineral deficiency, but if it happened all of a sudden, maybe not...

the 1 thing i know about Lychee, is they have a very close relationship with soil fungi... if the right fungi is in the soil, they do great. if not, they normally just dont grow and start to slowly die.

If it is the soil, you can add fish fertilizer, mycorrhizal fungi, good compost and worm castings, and it should perk up.

you have another plant growing near it...? what is it ? the roots may interfere...

Thanks a lot for your great advise, we will definitely try some of those! The thing is, we now lives abroad most of the year and only comes back for a month or 2 during summer. Besides watering regularly, we didn't do that much for that tree recently...This is what I saw when I came back this year :-( The soil looks good to me, is fungus good or not?

I live in san diego too, be award that these tropical fruits, once the tree is bearing fruit, you should water it less, NOT more, overwater will make your fruit fall...I had that experience my first several years, now I hardly have a fruit fall at all.

Lychee trees need LOTS of water from everything I've read and the problem is you live in CA and I know that you have very strict water restrictions...lychee trees need a lot more water than mango trees even at 5 years old it still needs water at least once a week or every 10 days especially while blooming and fruiting...Call lycheesonline and or Pine Island Nursery for advise. I've done this in the past and they have both been very helpful...

Fertilize them "lightly" in small amounts during the growing season. They do not like a lot of fertilizer or high dosage. After you've picked the all the fruits I would 'tip prune' all the branches to promote more new growths...now is the time to do after you harvest your fruits...

Good luck

" is fungus good or not?"

there are thousands of types. some good, some not, but you need some types in the soil for this tree.

looking again, i think i see grass near the trunk. you can pull it out (dont use chemicals),, or, you can put a few pieces of cardboard down on top to kill it. put some mulch or compost over that.

whats the brown stuff under the fence ? is that the bare soil ?

if its very sandy soil, then you will need to feed this tree a lot of organics, like fish, worm castings, compost etc...

I agree with phamaily, overwatering is not good. The real issue in San Diego is the humidity, or rather lack of it. Lychees are native to tropical humid climates. When you have all the sun but not the humidity, that can really strain the plant. In such a case less sun would be better. I would advise making a cover over your plant to reduce sun exposure, frequently misting the leaves may also help.