I am growing sweet corn in pots. The plants are about 2' tall. The leaves keep getting hard black deposits on the them. Not a leaf disease, just black nodules on the leaves, especially deep into the plant. I sprayed them off with water but they return. They are not tightly attached. I have a picture with some untouched and some crushed. They actually look like brown dirt when I crush them. I water with a gentle shower so there shouldn't be dirt splashing up onto the plants. The only insects I see on the corn are small flies with green eyes and a light brown body. I've also recently fertilized with Milorganite but I didn't throw it on the leaves, I spread it into the dirt. Can you identify what is on this corn?