Response : 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 advisable to control the evolution of these spots, but since some spots can be caused by diseases that have then unfortunate course for the plant, or by insects that sometimes tend to develop very quickly, perhaps it is worth trying to treat our plant as soon as we see the first symptoms of a problem.
Often succulent plants are subject to problems related to the presence of fungi or moulds: these parasites creep inside the stem, and in a short time small light spots can become very large, or creep deep into the pulp that forms the stem of the cacti, and then become soft and rotten, or dark and necrotic, or even rigid and colorless looking.
In all these cases, they are fungal diseases which can also kill the plant, because starting from the epidermis, they penetrate inside the stem, up to cause its complete rottenness. Against these plants we can use some fungicides, to be sprayed on the stem of the plant and on the ground, so that it kills also possible fungi which have not yet attacked the plant.
If the stains have already penetrated deeply, it is advisable to remove them using a well-sharpened cutter, to be disinfected after each cut; afterwards, the wounds must also be treated with the fungicide. I understand that this treatment will make your cactus unsightly, but the choice is between this treatment and the death of the plant.
But yellowish stains can also be caused by insects, or rather, by their stings, which pierce the outer cuticle that covers the stem, to suck the sap; also in this case, it is good to intervene immediately, treating the plant with an insecticide, and cleaning the tiny holes with a Cotton fioc soaked in disinfectant, in order to avoid that, through the small ovens, fungal spores or bacteria penetrate the plant; if these are insects, however, you should see some specimens on the plant, then observe carefully between the thorns and the recesses of the stem.
Small spots can also be due to accidental “contusions”, 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, parasites of any kind tend to attack plants that are not completely healthy, or at least: a plant cultivated in the best way tends to be able to defend itself from parasites.
So, in addition to checking what problem can be treated, remember that fungi and insects attack less plants that are grown with good lighting, watered properly (in the case of succulent plants, watering is provided from May to September, every 3-4 days, watering abundantly 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.
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