Whitening cacti are often caused by a deficiency of light or a change in temperature. To properly diagnose why your cactus is changing color, you must first identify the cause.
Crossed sunlight can cause spotting or discoloration on some types of cacti and should be avoided. Excessively bright conditions can lead to cactus leaves turning white from sunburn.
A sudden drop in temperatures may have an adverse effect and cause whitening, especially when combined with poor air circulation. To prevent this, ensure there’s good ventilation around your plant, and don’t expose it to open drafts or cold rooms for extended periods of time.
Fungal diseases, like powdery mildew, can also cause cacti to appear pale and mottled. If this is the issue, isolate affected plants to reduce further spread and apply fungicide if necessary.
Lack of nutrition can also result in yellowing or bleaching out leaves – soil lacking essential nutrients results in stunted growth. Determine whether existing soil is suitable for your type of plant and supplement as needed with organic fertilizer or compost tea.
If all else fails, isolated pruning may help preserve their health by removing the source of infection or damage from the rest of the plant body – but beware of fully replacing each stem as restarting growth is labor intensive for many species!