This can happen for a few reasons, but it appears to be fungal or bacterial. If this was planted in the heat of summer, then this is likely what started the issue. Overwatering in compensation for the heat seals the deal.
The heat, and constantly wet soils are a perfect combination for disease. The best way to get around this is to only plant in early Spring in zone 7. That way, you won't have to water, unless it remains dry enough to sap all moisture out of the root ball. It has the best chance of taking to the soil around it at this time.
If this has been established for a while, then sometimes constant rains for too long will bring about infections.
The best course of action from here will be to use a fungicide for the upper portions of the tree. This article will help you with fungicide use: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
I would also recommend treating the soil with DOLOMITIC LIME and WETTABLE SULFUR around the tree. This will help condition the soil, as well as treat what is left in the soil. It will also give the tree much needed calcium, magnesium, sulfur, and carbon.
This article will help you to care for weeping cherry trees: https://www.gardeningknowhow.com/ornamental/trees/weeping-cherry/weeping-cherry-growing-tips.htm