We live in Central California with hot, hot summers and winters down to
about 30F. We bought an older home last year with four citrus trees; two
orange and two lemon. Crop of oranges was very good this winter but now it
is Fall and unripened lemons and oranges are falling off trees. I have
noticed a round brown spot spreading out from the center stem on each fallen
lemon, almost like a scald mark or sunburn. The oranges that have fallen
(and some that are on the tree) appear to have split skins, almost like the
fruit got too big, too fast for the rind. How must water should citrus trees
have in the summer? I would be most appreciative if we could get an answer
on this although I think it may be too late to do anything this year. If we
did something wrong, I want to avoid it next year. Thank you for your help!
We are not too sure about this problem. The fruit falling prematurely and the brown, round mark point to Centre Rot disease (caused by the fungus Alternaria citri). Remove any infected fruit as future source of infection. If it is in fact Centre Rot then cutting through the fruit should reveal a dark/black internal core.
Another possibility is Copper deficiency. Symptoms - on branches, causes stained terminals, small gum pockets, black excrescences, multiple buds, and distorted S-shaped growth, and dying-back. On leaves, symptoms are large, dark-green coloured leaves, changing to small mottled leaves in late stages. Fruit is stained with brownish, sun-soaked areas, and frequently splits and drops. Gum may extrude from fruit. First do a tissue test to confirm a deficiency. Be careful adding Copper because it is very toxic!
A photo and more info on Alternaria citri.