It was probably degrading during the crushing of the leaves or if the sample was frozen in storage. On cell lysis dna degrading enzymes are released and there is some cutting of dna until the enzymes are degraded. When you run the gel it just shows you the amount of degradation as a smear. This is not usually a problem if you just want to do pcr when degraded dna amplifies perfectly well. If you are sure that it is the electrophoresis that is causing the degradation then this can happen when the buffer in the gel tank is re used many times and gets warm and bugs grow in the buffer and release enzymes that get into the gel and can degrade dna and rna

Make sure that you have EDTA in your regents to chelate divalent ions needed by dnases when they degrade dna