I was adding root stimulator to my orange tree today and noticed this spot on the stem. Can anyone tell me what it is?
Looks like an old mower wound. If you are trying to get your tree to produce more support roots, a thicker trunk, I would get rid of the staking stuff. When a tree is able to move in the slightest bit of wind that causes the best support root growth and definitely thickens the trunk. Is there a reason you are trying to support this tree?
Think of a broken arm or leg. What does that limb look like when the cast is removed? Thin, weak, atrophied. I only stake newbie bare root trees and older trees that have been blown over. I walk through parking lots and clip the staking stuff on trees. Makes a huge difference in growth and sturdiness.
I am also a bit worried your tree was planted too deeply. Could you pull the soil and bark away from the trunk of your tree to make sure only the roots are covered? Any mulch, soil against the bark of the trunk will enhance bacteria that will easily girdle your tree and kill it.