I noticed one of my tomato plants had some leaves wilting. When I looked at the stem there was a length of stem about 6 inches long and hallway up the stem that looked darker than the rest. Then I noticed some other stems with wilted leaves that had an area of stem that was totally dried out. I opened all of these stems at the site of the disease only to find they were hollow. The entire inside of the stem was simply gone. It looked like something had eaten the inner portion of the stem. There were small "globs" about 2 inches apart in each of these hollow stems that looked like "droppings." I never found any worms or other living thing inside these hollow stems. What did this?