Symptom: Stem plant growing apparently well, and healthily, good color, etc. Then a part of the stem just goes necrotic and turns black. No apparent physical damage that I can see to cause it. I have had this occur on various Ludwigias, and Proserpinaca palustris, no other plants that I have tried. The areas that go necrotic are not shaded. It doesn't seem to occur down at the base of the plant but either in the middle or at the growing stem. With Ludwigias in particular, it's really bugging me. Any and all suggestions are welcome! Thanks.