Answer: I think you're on the right track! Based on your description I'll bet your plants are being attacked by black bean aphids. These critters are a charcoal gray as opposed to the green aphids we're used to seeing, but their feeding habits are the same - they suck the fluids out of plant tissues! Beans and nasturtiums are their favorite plants, but it sounds like your trees have fallen victim to their voracious appetites, as well.
You might try hosing the insects off until beneficial ladybugs find your stash of black bean aphids. If all else fails, you can control the pests with insecticidal soap sprays.
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