Black spot, a common fungal problem especially noticeable during warm, wet weather, causes black spots followed by yellowing and leaf drop. Remove these affected leaves. Abundant sunlight, good air circulation and a healthy soil help discourage this disease, but you may want to consider a fungicide. Ten years ago a friend gave my wife a pretty purple flower growing out of a three-leaf clover. There are four genera in the Oxalidaceae, or wood sorrel, family.
All oxalis have foliage that closes at night or on cloudy days.
It does resemble that of clover. Oxalis crassipes is a pretty pink perennial wood sorrel used in Southern cottage gardens for many years. This oxalis flowers for long periods but often disappears in hot weather. But O. violacea , or violet wood sorrel, is a wildflower that can become too happy in the home garden. It springs up in lawns and flower beds.