I can't see close enough, but it looks like a bad case of aphids or scale, possibly mealybugs. Aphids are soft, round or egg-shaped, green, yellow, or pinkish, pinhead-sized bugs attached to new stems or the underside of leaves. Scale look like spots or blobs of wax or lacquer left on the stems and/or leaves. Mealybugs look like little spots of sticky white cotton where the leaves join the stem, or along the underside midribs of the leaves. All three suck sap from the plant, weakening it and yellowing the leaves. The undigested sap is excreted as a sticky residue called "honeydew".

The plant looks like it might need feeding, too, but I would check for the bugs, first.

Kevin : )

Hey Kevin, thanks for your reply. I've checked very carefully and don't see any bugs. Maybe just need some fertilizer.

Is it truly sap? Have you squeezed one with a thumbnail? If anything squirted out from under it, then it is scale. If not, I wouldn't worry about it. The plant looks fine in the photos. It may be a tad chlorotic but be careful amending that. Is it getting enough sun? It should be receiving full sun. Easy on the ferts.

Have you been on the Gardenweb's Tropical Fruit forum yet?

Hi Ohiojay. The substance on the leaves are very sticky, so I assume thats what you call sap right? For newly airlayered trees, I thought it was best to keep it in a shady spot with very little sun light. Judging by the pics that you saw, do you think its safe for it to receive full sun?
And yes, I have been on the Gardenweb's Tropical Fruit forum yet.

Let's look at this another way. When the airlayer was still on the tree, it was in full sun correct? I would have kept the plant somewhat shaded for a few days or so but that would have been it. When was the layer removed from the tree? These plants need as much sun as they can get. Start getting it more sun...not all at once though. Make is gradual.

Not sure what and where the sap is coming from. Damaged leaf or branch? Something dropped onto it? If there is no sign of insects, at most I would just keep an eye out for more and try to reason out the "why?". A lot of trees release sap when there has been some kind of trauma...branch broken, leaf snapped off, pruning. Trees such as sapodilla, mango, jackfruit, and bananas will do this and it is just a reaction to the trauma.

The thing with scale is that they don't actually look like bugs! They look like a spot of red-brown varnish, or a blob of gray wax, or a yellow fish scale, or a small, dark brown, plastic dome. The "sap" I saw on the leaves was pretty indicative of sucking insects, so you might just want to check again. Mealybugs can be pretty tough to find, too, especially on plants that naturally have a tuft of wool in the leaf axil--I don't know if longans have that.

Good luck!
Kevin : )

Thanks again Kevin.You've been a great help. Happy New Year!