A photo would be helpful. I assume you are asking about "trails" on the underside of the leaf. I can only guess that what you have may be the slime trail of slugs! If so, you may try to use a bait to eliminate the slug which is nocturnal.
Regards,
Kent

{{gwi:535209}}

It looks like this.

Sorry, but you have me. I do not believe this is a disease. WhatÂs the chance of this being eggs deposited by a moth or another insect?
How many leaves or plants are affected?
Kent

I noticed it first on a plant. I cut the leaves but it showed up on others a day later. There have been a lot of butterflies flying around. I'll observe a bit more. Thanks anyway.

I would suggest you take some of your leaves into the County Extension Office. They can offer suggestions and should be able to send it off to be examined if necessary. I would also like to have an answer also..
Regards,
Kent

That's bizarre!