Answer: You probably didn't do anything wrong and I'm not sure you can prevent it from happening again. The foliage turned brown because the flowers are blocking sunlight and it turned yucky because water that would normally evaporate if the leaves were open to fresh air and sunshine just stayed and rotted the leaves. If the plant were smaller so air and sun could reach the center, the leaves would be green and healthy. So, to prevent this problem you'd have to keep your aster plant smaller than it is now. I think it's a trade-off because a plant that's full of blooms looks so attractive.
Q&A Library Searching Tips