We do not see typical disease symptoms, but the damage (which appears relatively minor) does look more consistent with insect feeding. It's possible one or more culprits are responsible - leafhoppers, spider mites, thrips, or planthoppers. The white residue you've been finding are probably not mealybug, as they are rarely problematic on outdoor plants, but instead planthopper nymphs. If so, no treatment for them is needed.

Mites and thrips can be treated with either horticultural oil or insecticidal soap, but their presence should be verified first by checking leaf undersides, especially near the veins and where yellowing appears. A magnifying glass may help, though they are visible to the naked eye. A strong blast of water to each leaf underside may knock off enough that pesticide sprays are not needed. Either way, if numbers are high, repeat sprays (of any sort) will be needed. The leaf damage here doesn't look extensive enough to suggest that pest populations are out of control, though. Beneficial insects may also arrive to knock-down their numbers. Existing leaf damage won't heal, but if you don't see new damage on younger growth, the populations can be considered reduced to a tolerable level.

Miri