Yellowing leaves
Yellow yucca leaves are an issue most commonly attributed to overwatering. Most yucca varieties require little water and are generally happy with a generous weekly watering in summer and a decent monthly watering in the cooler months. Yellow yucca leaves can also be caused by a lack of direct sunlight.
Gray or brown spots on yucca leaves
Patchy or spotted gray areas found on the leaves of the yucca plant are most often a result of fungal disease. When these blemishes are found, it is best to remove any affected leaves and treat the plant with copper fungicide which can be purchased from most plant nurseries.
Leaves dying off
The drying out of leaves on a yucca is commonly a sign of overwatering or underwatering, although this could also be a sign of a dying plant due to other underlying causes such as root rot or overfertilization.
Dried out leaf tips
When yucca leaf tips dry out it is most likely that the yucca has not been watered properly. See overwatering below.
Plant is dying / Trunk rotting away
Unfortunately, this problem can have a range of causes and can sometimes be difficult to diagnose the cause. When a trunk begins to dry out or rot, it may take several months to be noticeable and thus it is often too late to repair the damage done. Causes for a drying or rotting yucca trunks include root rot, over-watering, under-watering and over fertilization.
Dark spots on top of leaves
This may be attributed to scale or fungal disease. If the spots are removable by scratching, then the problem is indeed scale and will require a treatment of insecticide followed by rinsing with water to remove. However if the spots are not scale, then they are probably a fungal disease and infected leaves will need to be removed to stop further infection.
Rotting roots
Root rot is caused by overwatering or poor soil that does not dry out quickly enough. Large pots or highly compact soil may cause moisture to be retained in a pot for extended periods and contribute to root rot. If poor soil is the cause of root rot, removing the plant from its pot, removing excess soil from the root ball and adding fresh well draining soil will usually combat this problem.
Overwatering
Overwatering a yucca is without a doubt the most common cause of unhealthy yuccas. As mentioned above, a yucca should be watered weekly in summer and monthly in cooler months. Soil should have a chance to dry out between watering to avoid root rot and other issues.
Underwatering
The most common sign of lacking water in yuccas are the drying out of leaves and leaf tips as well as trunks drying out and dying off. See overwatering (above) to ensure that plants are watered appropriately.
