Its a type of fungus. I had to spray all my rabbiteyes this season because of how wet it has been in my area. I would spray late in the evening, night or early in the morning. Some fungus sprays require a second application 2 weeks after the first one.

Is this a fungus that spreads from plant to plant quickly?

Will it kill a plant?

I dont believe it will kill a plant but it will make the plant defoliate early and that will limit the amount of fruit buds that develope in the fall. What I did was after I picked the last fruits I thinned out the branches and canes to limit the amount of spraying I had to do and to remove alot of the infected leafs. Its also good for the plant because it opens up the plant for air circulation and sunlight which will keep alot of fungus at bay with out even spraying. Usually thats the problem to begin with. I dont think most people prune blueberries hard enough so if the plants are too thick with branches and leafs it doesnt allow the plants to dry out quickly enough and causes fungus issues. If you go ahead and prune it hard now and spray you will have plenty of time left this growing season to put on some growth and fruit buds for next spring.

If you do prune be sure to sterilize your clippers with a bleach rag between every cut so you dont spread disease. Also be sure to remove any fallen leafs around your plants. Good house keeping will help keep fungus and disease at a minimum. Also try not water the leafs if you can. Only water the soil under the plant.