A succulent's stem turning black is not necessarily black rot, it could also be lignification
In succulent care, misunderstandings are inevitable. Some friends think a blackened stem means black rot. Many growers have encountered this: suddenly finding that the part of their succulent's stem in the soil has turned black, and a purplish black at that. Isn't that black rot? But why is the succulent fine? Because that is not black rot, that is normal lignification of the stem, and it only looks that frightening black-rot color because it is in the soil.
How do you tell them apart? Look at the plant's condition. If the plant is healthy and plump and the stem is hard, it is fine, completely healthy. If the stem is slightly soft and the leaves fall off at a touch, cut it off quickly, that is black rot. Succulent black rot is caused by the fungus Fusarium oxysporum (the pathogenic fungus of black rot is not necessarily the same for different plants). Fusarium oxysporum is a facultative parasitic fungus that can both infect plants and survive in soil. Like other plant pathogens, it has infraspecific differentiation and can infect many plant hosts.
Once you have experienced black rot even one time, you know that the black of black rot and the black of an old stem are worlds apart. Although an old succulent stem will lignify, its color basically does not turn black, but rather gray or brown and other dark colors, whereas the black of black rot is a black that reaches to the marrow, truly black. Black rot is accompanied by leaves turning black as well, falling off without being touched, falling off at a touch, while a lignified succulent is still in a relatively healthy form. In addition, black rot spreads very quickly, with a big difference in just one day.
The symptoms of succulent black rot start from the middle of the succulent's stem. Some people take the black spots and watery rot that appear on succulent leaves as black rot, and think that black rot can be treated, which is unscientific. This is usually not black rot but black spot disease. In fact, even without medication, if you stop watering and let its growing environment become dry, black spot disease can also stop spreading. The symptoms of black rot damaging leaves should be as described below: the leaves fall off at a touch, and blackening and rot start from the leaf's growth point (the place where leaf cuttings sprout) and spread to the leaf.
Why does black rot mostly occur in summer? Why is it more common after overwatering or after rain, and yet many succulent growers still think their succulents died of heat? Actually, understanding this question is simple. Fusarium oxysporum is a fungus, and its brothers and sisters are mushrooms. Why do mushrooms tend to grow after summer rain? Because it is warm and humid, and the conditions for reproduction are met. In the literature on controlling cotton wilt, it is described that the most suitable growth temperature for Fusarium oxysporum is 27 to 32 degrees, so in summer please pay close attention.
Finally, to judge whether a succulent's stem has a problem, you can pinch it with your hand. If it is still hard, then it is fine. A black-rotted stem will feel soft when pinched and will also ooze juice. If the outside of the stem looks fine but pinching reveals that the inside has already dried out and shriveled, then behead it early and replant it to root.
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