Half of my tomato plants have developed a few dark brown, almost black areas, mostly along the stem. At first, I thought it was dirt and gently scraped at it, taking off some of the epidermis. So far, the affected areas don't feel mushy, but it looks like so kind of rot. The surrounding stems are fine. There is a little bit of yellowing on the lower foilage on just a few of the plants, but doesn't have me worried, as it is normal this time of year. I've ruled out cold damage as that affects the new, tender growth. Are these the early symptoms of late blight? If not, then what is this?