quince trees dying

jimblob79 starts with ...
l have 4 quince trees 15 years old, up until now they have been healthy, but this years they are dying the leaves have turned brown and black spots and only a few fruit on them, theres apple trees next to them and they are all healthy. In october there was a big flood in my area but l dont know if that has anything to do with it. is there anything l can do to save them such as spraying the leaves incase its a disease