Q. I have a five-foot
Yucca tree that I cannot get to be disease free after two years of trying. I have tried several insecticides and
fungicides. I hope you can identify the problem from the picture.
A. The Yucca leaves are
scorching or turning brownish white in the middle, at the bend where the leaf
is curving down. This is not a problem you can correct by treating with
insecticides or fungicides.
This is an ornamental Yucca called Spanish dagger with
flat leaves coming from the trunk and curving slightly downward. You would
think that yuccas would be a plant not requiring much care in the Mojave
Desert. Well, this one does. It is native to coastal Carolinas, Georgia,
Florida and Alabama.
This is a cultural management problem. It should be
addressed by transplanting it to a new, protected location and adding
amendments the soil at the time of planting. Water it more like a palm tree
than a cactus.
Although classified as a disease, it is not caused by
anything biological so pesticides won’t do any good. This problem is more
correctly classified as “leaf scorch”. It is a soil/planting location/water
management problem.
Don’t plant this Yucca like you would yuccas that grow
easily in desert environments. This particular Yucca prefers protection from the
hot, late afternoon sun and grown in richer soils, soils amended with compost
at the time of planting, and not surrounded by rock mulch.
What to do now? Move it to a new location which gives it
protection from direct, late afternoon sun. If it is surrounded by rock mulch,
get rid of it. With a posthole digger or soil auger make several vertical
chimneys in the soil about two feet deep and 18 inches from the trunk. Pack
these vertical chimneys with compost and then soak them with water.
Water this plant like a
normal landscape plant with good drainage and make sure it is watered on four
different sides with enough water each time to percolate 18 inches deep. 
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