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Question: Cactus with dark spots
I bought a nice Cereus that a short time later showed clear spots and some speckles the size of a lentil always on the top, probably necrotic.
I read the answer to the reader about Echinocactus but I would like to know, in addition to putting it in the shade, what can I do about it.
Answer: Cactus with dark spots
the spots on the green parts of a plant can have the most varied causes, to better understand what it is, it is advisable to check the evolution of these spots; but since some spots may be caused by diseases that have then progressed inauspiciously for the plant, or by insects that sometimes tend to develop very quickly, perhaps we should try to treat our plant as soon as we see the first symptoms of a problem. Often the succulent plants are subject to problems related to the presence of fungi or molds: these parasites penetrate inside the stem, and in a short time small light spots can grow very large, or creep deep into the pulp that forms the stem of the cacti, to then become soft and rotten, or dark and necrotic, or even with a rigid and colorless appearance. In all these cases these are fungal diseases that can also kill the plant, because starting from the epidermis, they penetrate inside the stem, until causing its complete decay. Against these plants we can use fungicides, to spray on the stem of the plant and on the ground, so that it also kills any mushrooms that have not yet attacked the plant. If the stains have already penetrated deep, it is advisable to remove them using a well sharpened cutter, to be disinfected after each cut; subsequently the wounds will also be treated with the fungicide. I understand that this treatment will make your cactus unwatchable, but the choice is between this treatment and the death of the plant. But the yellowish spots can also be caused by insects, or rather by their punctures, which pierce the external cuticle that covers the stem, to suck its lymph; also in this case it is good to promptly intervene, treating the plant with an insecticide, and cleaning up the tiny holes with a cotton bud soaked in disinfectant, to prevent the penetration of fungal spores or bacteria through the small ovens; if it is insects, you should see some specimens on the plant, then look carefully between the thorns and the crevices of the stem. Small spots may also be due to accidental "bruises", for example from hail, or from pebbles that have fallen on the plant; in this case it will not be necessary to treat the plant. In any case, pests of any kind tend to attack plants that are not entirely healthy, or in any case: a plant cultivated in the best way, tends to know how to defend itself from parasites. So in addition to checking what problem you can deal with, remember that fungi and insects attack plants that are grown with good lighting less, water them properly (in the case of succulent plants, watering is provided from May to September, every 3 -4 days, abundantly watering the soil, only when it is perfectly dry), fertilized once a month, with fertilizer rich in potassium and poor in nitrogen, and placed in a well-ventilated place, with excellent ventilation.