There are 2 possible causes for your Mammillaria elongata succulent dying: Overwatering and pest infestation. 1. Overwatering   Overwatering Ladyfinger Cactus will lead to its roots rotting and possibly infecting the entire plant with fungal infection.   Mammillaria elongata’s roots can be preserved simply by giving the plant water when the soil is completely dry and free of moisture. As a cactus, Mammillaria elongate has the ability to store water. It will not require as much watering as other succulents.   2. Pest Infestation   Mammillaria elongata is particularly attractive to many types of pests. The list of insects that may infest Ladyfinger Cactus include mealybugs and red spiders.   The best chemical remedy to remove mealybugs from Mammillaria elongata is to spray the succulent with a systemic insecticide such as imidacloprid once every few months. You can also wipe the insects with a cotton ball that is dabbed in denatured alcohol.   Red spiders can be removed by overhead watering.