Leaf Blight

Not positive, but it looks like a leaf blight on your plant. This is often a windborne disease, more likely to be a problem in damp, warm, humid weather. Avoid disturbing wet soil and spray the plants with a fungicide labelled for use on Cercis. Always read and follow label instructions when using garden products. And water in the morning (not later in the day – early morning allows the moisture to dry off a bit before evening) at the base of the plant, not overhead to prevent the spread of the disease. Unfortunately if the disease is extensive, it might be best to dispose of the plant in the trash and select a more fungal resistant variety. Here is a link with more info: http://www.forestryimages.org/series/viewseries.cfm?ser=197