I can't determine precisely what all the problems are; it's not reliable to diagnose definitively from just an image. However, the white, and possibly the black (I can't quite see them in the image clearly) are, insects: a good dose of aphids). These can be knocked off with a firm spray of water or simply wiping the backs of the leaves. You'll need to do that every week or few days as more generations will continue to emerge until predators take over and deal with them. There is no need for insecticide; indeed that will make the aphid problem worse in the long run.
As for the other leaf degradations (yellowing and black splotching): the yellowing could simply be senescence (older leaves that are dying naturally). The black irregular splotches, however, could be signs of more serious fungal infection. Although there is no treatment to stop an infection that has started, regular spraying with a "bordo" (sulphur)-type fungicide, giving the plants lots of room, and regular pruning to improve air circulation can help slow it down.