The bacterium, Erwinia amylovora, overwinters in trunk and branch cankers. In the spring, the bacteria resume multiplication when temperatures are above 65 F.
Shoot blight occurs when infections begin at shoot tips, moving rapidly down the shoots and then to limbs and trunk. Apple and crabapple leaves turn brown, pear leaves turn black. Frequently, the tip of the blighted shoot bends over and resembles a shepherd’s crook.
A combination of pruning, reduced fertilization and chemicals can help control fire blight. Prune and remove all stems showing symptoms as they first appear. Cut back into the healthy portion of both stems and limbs.
Over fertilization will cause rapid new growth which is most susceptable to the blight. Moderate fertilizer to reduce rapid tree or shrub growth. Over pruning can have the same effect so you should refrain from heavy pruning.
The antibiotic streptomycin is the most effective spray material for controlling fire blight. It will prevent but not control infections. Use streptomycin in Spring during bloom.
Resistant varieties to fire blight are apple, pear, crabapple, ornamental pear and pyracantha. You need to stick with these varieties when planting. Check your planting stock for fire blight resistance.