how can I cure the rust of my succulents with natural methods?
the rust of succulents is caused by a fungus, commonly called septoria; for this reason it would be better to call the disease septoriosis of succulent plants; these mushrooms creep into superficial areas of the cactus, where they find wounds or fractures of the cuticle that covers the stems and leaves; once entered, they tend to develop both superficially and penetrate deeper, causing even very serious damage. If the fungus has recently penetrated the plant, it is possible to remove it by removing with a scalpel (or with a sharp and clean knife) all the yellowed part. Clear that this method leaves many scars on the plant, which should be repaired with pruning mastic. Generally, a systemic fungicide is used against septoria, which means that it has the characteristic of penetrating inside the plants, through the cuticle that covers them; these fungicides must be mixed in water and then vaporized on the areas affected by the diseases: it will not be possible to make all the spots disappear, but at least you will stop their growth, and kill the fungi that caused them.
Unfortunately for this type of use you will have to buy a synthetic fungicide, such as propeconazole, or similar products; to prevent the development of other fungi, and the spread of those already present, you can treat your plants with products based on copper, which does not appear to be as toxic as synthetic fungicides. It would also be advisable to ensure your plants are more aerated, and prevent them from getting wet, either because of the weather, or by watering them from below and not sprinkling them from above, as if they were going to shower; in fact many fungal diseases develop due to poor ventilation and high humidity.