Downy mildew, uncontrolled leaf spot, or the presence of nematodes on the plant all cause yellowing of impatiens leaves. The specific problem can be pinpointed by observation of other common symptoms that occur in conjunction with the yellow leaves.
Downy mildew begins with wilted flowers, pale leaves and a white, purple or beige film on the underside of the leaf. As the disease progresses, the leaves turn yellow and fall off. Leaf spot disease first appears as brown spots on the leaves. Left untreated, more spots appear, and the leaves turn yellow and shed. Wilted plants that have partially decayed root systems along with yellow leaves are most likely infested with nematodes, which are unsegmented roundworms that feed on impatiens.