Purely a guess - but I've read about powdery mildew that is supposedly white and blotchy like that. Try googling Powdery Mildew on Oxalis and see if that helps. Good luck!

Idk about pests, thankfully I haven't had much experience! But the leaves /stems only live for a couple months and then die back. When this happens the leaves wilt and become discolored and sickly looking. I carefully pluck the dying stems whenever I see them, usually not more than 2 or 3 at a time. Each stem grows from a little "bulb" in the soil and I'm careful not to pull that out because a new leaf will grow. I do this year round and it does get a little sparse looking in the fall /winter, but it's never died all the way back or gone completely dormant. It can also be pretty dramatic when thirsty. Id give it a good drink and pluck whatever leaves/stems don't perk up. If powdery mildew is the problem, removing the dying bits should help open it up and increase air flow around the plant in general.