OK so we have had 3 hibiscus trees in our garden in France for the past 10 years at least. They are in the same stretch of garden and have always done really well with lots of flowers and just the occasional need for insect spray. However, this year the middle hibiscus has a problem. The leaves have all turned yellow and now brown, are shrivelling up and dying. It was/is in mid flower and the flowers are still there, but it is a shadow of its former self. We have had a really hot, dry spell but the other 2 hibiscus are fine. It has also been raining now for 3 days straight and it looks no better. Any ideas?