Update October 14th 2014
How to tell if ants are the problem
For this tree there is a suggestion that ants are causing damage because of their relationship with plant sucking insects. How can you tell.
1) They are running up and down. Ants require sugar and protein. When you see ants moving up and down a tree trunk going towards the canopy it is most likely they are collecting sugar and bringing that back to their nest. Ants do form trails to collect protein (usually a dead insect) but it is much rarer and would not last for many days as they would quickly exhaust the protein supply
2) At some points in the canopy, under the leaves, on the fruit or on twigs you will see little bumps. These are the plant feeding insects. You can see different stages on the guava photo; larger ones that are colored and smaller ones that are whiter. You need to approach these very slowly and you will see the ants tending them. The ants are after sugar and these insects secrete this sugar. By moving the leaves, fruit or blowing on them you can cause the ants to leave the insects. But be patient. You will see them
The ants are likely not nesting in the crown of your tree. But try to find out if the trails are going into any holes.
Lets assume they are nesting elsewhere and coming to your tree for sugar. Then you can block them on the trunk using a sticky substance called ant guard, over which they dont pass. You should also spray them off with water and/or soapy water, creating. This is also good for removing the plant feeding insects. It will also help remove that mold. See more details on a previous answer I gave here
I would like to know where the ants are going and if you can see them collected around groups of insects on the underside of leaves. I think there could be a sooty mould problem where the insects feeding on the tree release sugar as honeydew and this promotes the sooty mould (as black growth)
Would like a sharper picture of the fruit too