Are there white specks or webs on the back? Then it is spider mite. Are there aphid on the leaves or tips above? Then perhaps it is honey dew from the aphid. Has the weather been warm and wet? A leaf spot fungus could be the culprit. You may need to send more images or bring the leaves to a Master Gardener clinic or local nursery. Also let me know what type of plant it is. Camellias often get aphid, fuchsias get mites, leaf spot happens to many plants. Looking forward to making a more accurate diagnosis. Keep growing, Marianne Binetti