What is wrong with my grapefruit?
I believe you have a disease called Greasy spot or Greasy spot rind blotch (GSRP). Greasy spot is caused by a fungus and commonly found on grapefruit, Hamlin oranges, and tangelos. Greasy spot rind blotch (GSRB) is particularly problematic for fresh fruit grapefruit and this occurs on the rind of the grapefruit as the spots join to form larger areas called blotches. The spores from the fungi produce in the decaying leaf material from the tree. The spores are taken up by the wind and attach to the underside of the grapefruit leaves forming black spots with yellow halos. When the weather conditions are perfect (moist and warm) the spores multiply causing damage to the leaves and ultimately the fruit rind. The highest levels of spore production usually occur during the months of April through June causing infection to occur from June through September. Best management practices for controlling the disease is to be sure to clean up leaf litter from under the tree and apply horticulture oil and/or copper once in mid-May to June with the second application in late July. Although the rinds of the fruit look poor, the fruit is still edible.