Our established Black Walnuts and and Burr Oaks were budding as they should in late April. At the beginning of May there were two mornings where the temperature dropped into the low 20's. A few days ago I noticed that probably 80-90% of the buds that were in the process of leafing out on the black walnut and just about every bud on the lower quarter of the canopy of the burr oaks shrivelled up and died. A few ashes also have similar problems. Is the frost the cause or might something else be going on? I'll post pictures if needed.