Wonder what this disease is on the Guava tree see picture. Looks like white spots?

You might have whiteflies. I can't actually see any whitefly nymphs or adults in the picture, but the white flocking is evidence. Whitefly nymphs secrete a white wax that get all over. http://tinyurl.com/8vrg8cf

Appreciated so much

Not a disease, an infestation. Looks like cottony-cushiony scale (anyway, some kind of mealy bug or unarmored scale). Almost any insecticide will work if applied to underside of leaves. Organic could be Neem Oil extract. Least toxic non-organic is probably Bayer Total Insect Control (a synthetic pyrethrin). Most (including those mentioned) are non-systemic, meaning they only kill what they touch, or what crawls across them. Any non-movers (what you see) will be killed if you hit them with spray. Babies that haven't crawled away while insecticide is active will not. You will have to repeat a few times, at perhaps 2 week intervals. to knock infestation down to tolerable level.

If you don't want to spray with chemicals, you might use a strong water stream at bottom of leaves, frequently (but has little chance of actual control).

Thanks. Have used pure Castile Soap water

I agree that is the typical look for whitefles. Insecticidal soap or horticultural oils will work, but need to be reapplied frequently. If the tree is small and the infestations aren't that bad you can blast them off with a jet of water. Purple ladybugs eat whiteflies, but they have a predator-prey cycle so whiteflies are usually bad every two years.

Wonderful information