I have had episodes of canker sore since I was a child. After suffering for about 35 years I figured out that my canker sores were triggered by certain tree nuts. I am not otherwise allergic to tree nuts. If I eat walnuts or hazelnuts, or products containing them or their oil, canker sores will develop in 24-48 hours. I also think that seasonal pollen allergies may trigger them, and found that a prescription antihistamine (I know, not a folk remedy) completely prevents them in pollen season. As for "cures" I have never found one, but am intrigued by the alum testimonials and will try that.