Edited May 23rd 2019

This looks like a colony of plant feeding insects belong to the group Hemiptera. They stick their mouthparts in the leaf and remove nuntrients. The excrete sugar which can promote the growth of fungus on the leaf. That is the dark discoloration of the leaf you see

You can spary the leaf with soapy water as this kills the insects (they have a waxy covering which the soap breaks down). You can also use botanicals like neem.
Synthetic pesticides of course work but the other two solutions are better in that they are
a) Less expensive
b) less toxic to you
c) not going to kill the good insects


You should consider how big the problem is as you take action. Also, scout for ants. They are partners to these nasty insects and protect them/move them around your tree.

As you spray the plant-feeding insects it should discourage the ants


good luck and let us know how you get on

d








Hi Marianne
Welcome to PlantVillage

This is a hard question. The leaf as three possible problems. See the attached image.

We would need additional images of this leaf and other leaves.

In general, if you have insects feeding on your plant it is very bad. You can use high powered soapy water to remove.