Answer: Annuals are the types of plants that flower, set seeds and then die. If your annuals have stopped producing flowers and are setting seeds, they have finished their lifecycles. Yellowing leaves are the first sign that the plants are dying. You can collect and save the seeds for sowing next year.
Black leaves, on the other hand, indicate a fungal infection. But, without knowing the names of the plants involved, I can't make a recommendation. You might try using a general garden fungicide on your plants. Be sure to read the label thoroughly to make sure your plants are listed as safe to be sprayed.
Best wishes with your annuals.
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