It looks cool but I wonder if it might be some type of disease or fungus?

Oh no. I certainly hope not. :c(

No disease or fungus but probably just a chance mutation - and it doesn't look as if it will recur since it's just the one leaf. Now if every leaf on a stem did that,you could propagate and it would be worth a bundle. There are some roses with variegated leaves - mostly due to virus and pretty miserable.