My hibiscus is planted in big pot with good drainage. I have had it for 3 years and it has always been beautiful. I take it in during the winter. This year it had black spots on the leaves and not very many leaves. I ask my local HD about this and the recomended a spray for the black spots. I have used it once but my plant looks so bad I am afraid to do it again. It starts new growth but then it dies off. Right now it has about 20 leaves and half of them are yellow. What should I do to save it?