I have a grapefruit tree that is about 30 feet high. I did not know anything about growing one so it got really high and it is difficult to get the fruit at the top. I had about 3 years of bumper crops, about 600 each year. The problem is that it's leaves are very mildewy and black on something white on the underside. This year not much fruit. I have sprayed with soap as instructed from a nursery but has not gotten any better. Please help.
Harris County Texas
Based on the symptoms you described, my best educated guess is that you are dealing with the sooty mold fungus which explains why the nursery personnel recommended that you spray with soap. While that should be effective against the mold fungus, it does nothing to kill the sucking insects that are the main cause of the problem. Going by your description of whitish mass under the leaves affected by the mold, the insects involved could be mealybugs, scale insect or even whiteflies. What happens is as these insects feed by sucking the sap from your leaves, they excrete excess sugars which gets deposited on your leaves. The deposited sugars are what the sooty mold fungus thrives on hence, you will continue to see expanded mildew on your leaves unless you deal with the root cause of the problem which are the sucking insects.
My recommendation is that in addition to the soap treatment, you should immediately treat your tree with an insecticide capable of killing the sucking insects. I prefer you use a systemic insecticide that could be absolved by your tree roots and taken up through the vascular system especially given the architecture of your tree. A good one will be neonicotinoids and one brand name that comes to mind is Bayer Advance which you could get at your nearest Home Depot or gardening center. The added benefit of a systemic insecticide is that you just need to apply the treatment to the drip line of your tree and not have to worry about coverage like you would with a contact alternative. Alternatively, you could tree with neem oil but you might need to do repeated treatments to kill future emerging nymphs of the insect.
As per the problem with the height of your tree, you might consider pruning the tree (topping and hedging) once you finish harvesting the seasons crop. Doing so will not only solve your tree height problem, it will also rejuvenate your tree.
Hope these helps.