Garden Pests and Diseases: What is this blackish brown stuff on my Colocasia?, 1 by SavvyDaze

SavvyDaze wrote:
A new leaf grew on my Colocasia Esculenta about 4 days ago. It looked healthy until today when I looked at the underside of the leaf. I noticed a blackish brown substance (doesn't wipe off) on the leaf vein. And on the leaf between these veins are 1/4 to 1/2 inch thin lines of the same blackish brown color. (Reminds me the color of car oil). (Pics 1 and 2) I noticed the same thing happening to a relatively new leaf next to that one. Except it is on the stem. (Pic 3 and 4 - a little blurry, but you can see it.)

Does anyone know what caused this? Insect? Disease? Fungus? No chemicals have been sprayed, so that isn't an issue. I am clueless. Hope someone here can help.

Thanks!