No disease. Your blue berry bush is being crowded out by the other two. They are using all the neutriants from the soil and it is not getting any. Your best bet is to crush some egg shells into a powder, add 2 -3 teaspoons of vinegar and 1/2 to 1 teaspoon of sugar mixed with a water slurry. Then put a rusty nail or two into the slurry (for the iron) let sit for 24 or so hours then (after removing the nails) pour the slurry at the base of the sick blue berry bush. You should see a remarkable difference in about 7 - 10 days. -Birdy