Answer: It sounds to me as though your tree is stressed and to add insult to injury, insect pests have moved in. I suspect aphids or whiteflies because they suck the fluids from the tissues and leave a sticky excrement called honeydew. The curled, darkened, sticky leaves are a dead giveaway. Since your tree is large, you may not be able to effectively spray to control these pests; you may have to wait for natural predators to arrive (namely lady bugs) to help balance out the population.
Best wishes with your tree!
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