Mammillaria elongata · Succulent
Mealybugs: Isolate the plant and dab infested areas with a cotton bud soaked in methylated spirits.
Root rot: Ensure well-draining compost and allow soil to dry completely between waterings; repot if necessary.
Scale insects: Remove manually with a soft brush or treat with an appropriate horticultural oil spray.
Sunburn: Acclimate plants gradually to brighter conditions and avoid intense midday sun in summer.