Cleistocactus strausii
Silver Torch cactus can thrive in low-nitrogen soil without facing the consequences of low nitrogen. Too much water will make the plant weak and lead to root rot. 
Least Concern. This cactus is locally abundant in its native land and has few local threats to its survival.
This cactus can be propagated by cuttings or seed.
Pitaya
Overwatering or excessive rainfall can cause the flowers to drop and fruit to rot. Also, extended over-watering can cause maturing fruit to split on the branch. Birds can be a nuisance. The bacterium "Xanthomonas campestris" causes the stems to rot. "Dothiorella" fungi can cause brown spots on the fruit, but this is not common. Other fungi known to infect pitaya include "Botryosphaeria dothidea", "Colletotrichum gloesporioides" and "Bipolaris cactivora".