Answer: Several things could be wrong. When young leaves (those near the end of shoots) turn yellow, we ususally consider an iron deficiency to be the cause. High pH, high phosphorous and of course low soil iron levels all can result in iron deficiency symptoms. If older leaves are yellowing, nitrogen may be deficient. However, with iron or nitrogen deficiency, the leaves usually do not fall from the plant.
Root problems are a more likely cause. Root rot infection, physical damage to roots, drought and overwatering (soggy, waterlogged soil) can all cause leaves to turn yellow and fall.
Try to determine which of the cultural problems listed above may be the cause and take steps to alleviate it. If a root rot disease is present, there may be little that you can do at this time other than to avoid overwatering which tends to make things worse.
Another possible culprit is Citrus Mesophyll Collapse. Grapefruits are particularly susceptible but any citrus may be affected. It is not uncommon for grapefruit trees to be affected and other nearby citrus to be fine. Leaves and fruit of citrus trees dry up and drop. Stems and branches remain green and living. Rarely is the whole tree affected. Mesophyll collapse occurs when there are unseasonable weather changes. For example if it gets abnormally cold in October for several days and then heats back up above average temperature in November. This fluctuation stresses the trees as they kick into and out of and then back into their winter slow down. It is especially stressful after a dry summer.
Remove fallen fruit to prevent attracting fungal and bacterial diseases. Continue to water the tree on its normal cycle and it will leaf-out again in the spring. Do not prune out the defoliated branches until spring, and then only after the tree has completely leafed out.
Good luck with your citrus tree!
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