Fire blight

Fire blight is a contagious disease of rosaceous plants
 The pathogen is "Erwinia amylovora", a Gram-negative bacterium in the family Enterobacteriaceae. If the conditions are right, it can destroy an entire orchard in a single growing season.
Infections get into open blossoms and tender new shoots and leaves in the spring when blossoms are open. There is no known cure; prevention is the key.
Spraying plants with streptomycin or injecting plants with oxytetracycline can prevent new infections. That leads to antibiotic resistance. Spraying beneficial bacteria or yeast may also prevent fire blight from infecting new trees. The only effective treatment for plants already infected is to cut off the affected branches and dispose of them.