Certified GKH Gardening Expert
Without seeing the tree, it is hard to say for certain. My best guess will be a fungal infection. This can happen when the tree is planted in heat, and gets watered just a little too much in compensation.
Many times, the winter should take care of it. Freezing temperatures can help drastically. I would also treat with a fungicide to be sure. This article will give you more information: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will help you to care for a weeping cherry tree: https://www.gardeningknowhow.com/ornamental/trees/weeping-cherry/weeping-cherry-growing-tips.htm