Answer: Your oxalis grows from a bulb and, as you have discovered, has a natural dormancy cycle. First the foliage dies and then the bulb rests. After sufficient resting, the bulb sends out new foliage. Since the plant has been growing indoors all these years the dormancy cycle may or may not be the same as for plants growing outdoors. If the leaves become spotted just before they wither and die, it could be because they are not as healthy and vigorous as when they first emerged. It could be natural, or it could be that older, stressed leaves are more susceptible to disease or insect infestations. Once the leaves disappear, so do the pests. Without seeing the problem first-hand, it's difficult to diagnose as a real pest, or as a natural withering of the leaves. Oxalis doesn't require regular fertilizer applications, and the plant is hardy in zones 6-8, so it might be borderline outdoors in your zone 5/6. You might want to take your plant to your local Extension office and have the spots positively diagnosed. (Phone number 502/425-4482)
Hope this answers your questions!
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