Kmk,
This is what I would do, I would take off every leaf that has that brown on it, and spray it real good with Organicide.
You can find this at any big home store.
It is really concentrated, so buy a spray bottle and mix according to directions.
Spray every inch of it, in the early morning.
Wait 10 days, then spray it again.
Even if you don't see symptoms, spray it.
Remove every brown leaf in the meantime, and if any leaves fall off, remove them from under the plant.
If it's a fungus, or a bug, the Organicide will take care of your problem.
Works great on any plant, will even kill fleas in your yard, and if you have pets, it is safe.
I swear by it. I have pittosporums that get horrible scale, and its the only thing that really works good.
Read the bottle, it's good for what ails your plants.
Have a great Thanksgiving!

Something is eating my bougainvillea also; so this morning I called a nursery who said he had received lots of similar calls from people whose bushes had similar symptoms.

Part of a leaf gets papery thin and loses color. Another leaf has jagged holes in the leaf and sometimes a semi-circle eaten-from-the-edge hole. Sometimes the whole branch shrivels up and dies. And sometimes the leaves fall off, but the branch puts out new growth, which is later eaten. This cycle has repeated itself for 2 years.

The nursery guy says it's caterpillars who feed at night and he gave me the name of several insecticides. 1. Monterey BT 2. Spinosad 3. Malathion

Close inspection shows a small piece of web, and perhaps an egg. I'll be out tonight with a flashlight looking for caterpillars, meanwhile a friend is coming over with a sprayer and Malathion in a few days. Stay tuned.

from PoorMimi

Just be aware, malathion is an organophosphate insecticide, and by definition, a broad-spectrum nerve poison. The Bt product will be far safer albeit toxic to any and all Lepidoptera (butterflies and moths). Finally, the spinosad is also toxic to moths/butterflies but the larvae of some such insect is probably responsible for this damage. Spinosad also kills bees, but your timing-doing the spraying after dusk-will render it safe to those guys. The dried material on the leaves is not toxic to bees, only the fresh spray.

Good luck!

+oM