My guess would be that is has a pest of some kind, likely aphids or a similar sap-sucking insect. They generally tend to secrete honeydew, a sticky substance which may be present on the leaves. This honeydew is what attracts the flies. I would recommend treating the tree with neem oil, which should take care of the pest problem and in turn take care of the black flies. It is also safe to use and an effective fungicide as well should any other issues pop up due to the stress from the pests. It is often sold as fruit tree spray at stores. Check the label. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm
Answered on June 29, 2015
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