Question
dear sir, i live on the greek island of corfu. we have about 10 lemon trees and three orange trees that are about 25yrs old. they have never been any trouble until this past year. the leaves have turned blackish and look as if they re covered in a sticky sooty substance, including the fruit. what can we do as they are quite high? obviously we will try anything suggested but not harmful to our grandchildren who play in the garden. sorry i cant attach photo as not too good with computers,thanking you.

Answer
Hi Emily:
Great to hear from such a far away place from North Carolina!  I hope your weather is cooler there than here.

Your description sounds like an insect issue.  When certain insects feed on the leaves, they secrete a high sugary content of "poop" called honeydew.  This stuff drips/falls down and gets deposited on the leaves/twigs below.  This substance supports the growth of several fungal organisms called sooty mold.  Sooty mold is not a disease situation, but an insect problem.  The black sooty mold can buildup on the leaves and can actually be peeled off when it gets real thick.  Sooty mold can block the light being absorbed into the leaves. This can lead to a drop in photosynthesis.

Sooty mold can be removed from a plant by spraying a diluted soapy solution followed by a strong water rinse.  It will come back IF the insects remain on the tree.  The solution is an appropriate insecticide to control the insects.  Check with your local garden center/nursery for such materials as Malathion or other approved insecticide for such insects as aphids and/or scale insects on your tree.

Regards
Steve