Question: Can I save my cactus?
My old cactus (around 25) got sick. I state never to have paid any particular attention to him and that, nevertheless, he became healthy until some time ago (I think, having moved him from the sunny balcony to the landing was a bad idea!). Now it has the central (and main) stem dried from the center to the tip, of a brownish color, with some dark spots and, at the point where the disease begins, there is a white line, similar to a kind of foam. I would like to know if, also given the age, the plant is now "dead", or if I can do something to save it. Thanks.
Sick Cactus: Answer: a sick cactus
cacti are long-lived plants, which survive many decades in nature, the twenty-five years of your specimen do not make it an old man, on the contrary, they could represent its salvation, as aging may have become a more robust and vigorous plant. If your plant lived well on the terrace in the sun, it is because the climatic conditions were the ideal ones: a good insolation, continuous air exchange, shelter from bad weather. This life is not so dissimilar from what a cactus can find in the Mexican highlands, indeed, including the winter cold. I believe that moving to the landing, in very different conditions, has been the main cause of problems for your cactus: in bad cultivation conditions succulent plants tend to get sick quickly. What may have attacked the plant is rot, but I do not exclude that it could be some bacterial or viral infection, perhaps starting from the bite of some insect, such as cochineal.
The only way you can try to save your cactus is to remove all the diseased part, hoping that the stem, deprived of so much photosynthetic tissue, will develop lateral shoots, to replace the removed tissues.
Since the plant is so large, I suggest you use a clean and sharp kitchen knife for cutting; you have to remove all the diseased tissues, going down until you find a completely healthy part, even inside the stem, and not sorry or soggy. After each single cut, clean the knife and disinfect it with alcohol.
If it is root rot, it is probable that the rottenness starts from the base of the plant, and continues inside the stem; in this case, it becomes very likely that your plant is doomed.