Wet, rainy springs with temperatures of 65 and above are the best environment for fire blight. This bacteria, Erwinia amylovora, infects certain apple trees, quince, crabapple and mountain ash. Chanticleer pear and Bradford pear are ornamental trees for the landscape. Both are not resistant to fireblight. Other plants also not resistant to fire blight are hawthorn trees.
Apple trees that have some resistance are Early McIntosh, Grimes, Golden Delicious, Missouri Pippin, Sharon, Red Delicious, Winesap, Duchess, Turley and Haralson. According to University of Idaho, the commercial varieties of Enterprise and Liberty do not live up to their claim of better resistance.
Pear trees that have some resistance to fireblight are Douglas, Golden Kieffer, Seckell, Anjou, Magness, Moonglow, Brandywine and Centurion.
Crabapples that have some resistance to fireblight are Radiant, Kelsey, Red Splendor, Royalty, Snow Cloud, Vanguard and Dolgo.
The signs of fire blight begin with the flower petals. The blossoms turn brown, mushy and wilt. Then, the bacteria moves down into the branches and creates cankers. Next, the leaves darken, usually along the veins and through the rest of the leaf. Eventually, the leaves will be entirely darkened or appear to have a scorched look to them. Lastly, the newest growth at the ends of the branches will blacken and curl creating a "shepherd's crook" symptom.
If fire blight is left untreated the disease will progress to the point where the cankers will girdle the tree. Do not feed the tree with nitrogen fertilizer and avoid any overhead irrigation.
The proper treatment starts with pruning the infected areas. Each cut needs to be 6 to 12 inches from the infected area and in healthy tissue. When pruning a 10% household bleach is recommended to be used to clean the pruners before the next cut. The bacteria is carried on the pruners and is taken to another location.
There are certain chemicals that are useful in controlling the spread in healthy areas of the tree. These chemicals provide good coverage over the healthy tissue. What is already infected, the chemicals cannot bring the infected tissue back into a healthy state. Lime sulfur and/or streptomycin (Agristrep) are recommended. Bordeaux mix, a copper compound is recommended to be applied when the blossoms are at 50% open. Follow directions when using any chemical. Copper compounds can kill healthy tissue, if they are overused during the season.
Just remember bacteria are opportunistic. When the right set of circumstances occurs, they begin to spread. So be proactive if you have trees that lack a higher resistance to fire blight.