The bacteria is spread during wet Springtimes. It spreads from blossom to blossom. The bacteria can be carried by water droplets, pruners and pollinating bees, visiting blooms carry it from plant to plant.
It is often described as a Shepherd's crook or a cane. The Bactria move down from the blooms into the branches. The tip of the branch typically crooks or cruels into a cane shape. The wood darkens and becomes sunken in slightly. The leaves die on the infected branch and in the next winter the dead branch holds the dead leaves. The healthy part of the plant goes dormant for the winter and drops leaves.
There is no easy cure for Fireblight. Fireblight is serous enough that it will cause a steady decline in the plants health to the point that the plant dies. "Blight" after all does refer to death.
A very good control for Fireblight is prevention. Dormant pruning done in or before March to thin the plant and promote air movement through the tree. The Fireblight is also dormant at this time. First, remove all dead, dieing or diseased parts of the tree. If you are removing Fireblight cut back as far as reasonably possible from the diseased wood. Then, remove the small inside branches, remove branches that are crossing or rubbing on each other. Without stripping the tree to nothing the idea is to just open it up so that air can move through and dry the moister that is on tree in the Springtime.
Cut Fireblight out when you see it during the growing season. Try to cut well below the visibly diseased wood. It is very important that you sterilize your pruners with something like Lysol (kills Bactria) between each and every cut you make. Chlorine Bleach and water will also work. The Bactria can be spread by the pruners from branch to branch and to other trees.
When you start your sprinkler system up in the Spring make sure it is adjusted so that it is not pounding young short Crabapple trees with water.
Chemical control of this disease is hard this makes cultural control and prevention all the more important! It may not work well, it is expensive and may even make the problem worse in the long run. Because Fireblight is a Bactria it can become resistant to the chemical control. Controls include Copper Sulfate and Streptomycin. Copper Sulfate is used as a fungicide and has some effect on Fireblight. Streptomycin is an antibiotic drug. Spraying it on trees may control Fireblight but I fear that used too much it will cause the Bactria to become resistant. What if using antibiotics in the environment on a tree could cause other Bactria to become resistant to antibiotic drugs? When the discussion becomes Bactria that cause human and animal disease it is far out of my area of expertise. It is something I think about and make certain assumptions on but do not know as fact, if I am correct. I strongly promote prevention.