UNITED STATES—Bunches of blackened leaves hanging from blackened stems in otherwise healthy pear trees really are as serious as they look. They probably appeared as new growth was developing in spring, and are still as dead now as they were then. As surrounding foliage colors and falls, the blackened foliage will remain until it gets knocked out by rain and wind in winter, or until it gets pruned out.
Because these dead bits seem to have been scorched by fire, the bacterial infection that causes them is known as ‘fireblight.’ Pear, flowering pear and evergreen pear are most susceptible to it. Apple, flowering crabapple, quince, flowering quince, hawthorn, loquat, cotoneaster, pyracantha and photinia are also very susceptible. Some cultivars are more resistant to fireblight than others. Few other members of the ‘rose family’ are rarely infected.
At the base of each dead bit (or toward the supporting limb from dead bits that do not stand upright) is a lesion where the bacteria that cause fireblight infected the stem. These infections not only obstruct vascular activity and kill the distal (outward) portion of the infected stems, but they also spread proximally (inward) to more important limbs, branch unions and even main trunks and roots.
Fireblight is transmitted mostly by bees, and also by other insects, birds, rain and wind, while trees are blooming in spring. It most often infects through flowers, and can also infect where infected debris, particularly falling flower parts, get caught in branch unions. If it infects root suckers, it can infect the roots right away, and kill an entire tree. Root suckers should be pruned away anyway.
There is no remedy for fireblight. Because it is very contagious, all infection should be pruned out. Because infection extends inward from obvious lesions, infected stems should be pruned back at least a foot and a half below (inward from) obvious infection. Sadly, this often disfigures infected trees, sometimes severely. Pruning scraps should be removed from around susceptible trees.
Some say that fireblight should be pruned out in summer. Others say that winter is best. Really though, the only bad time is spring, while weather is warm but still damp, and the trees that are so susceptible to fireblight are active and blooming. The reason for pruning it out in winter is that the disease is inactive while cold. However, it does not get cold enough here to slow it down much.
Highlight: evergreen pear
If it were not so seriously susceptible to fireblight, the evergreen pear, Pyrus kawakamii, would be a practical evergreen shade tree for small garden spaces. Mature trees do not often get much taller or wider than 25 feet. Aggressively pruned trees that do not bloom much are less susceptible to fireblight. Regularly groomed trees can live with fireblight for many years or decades.
The nicely furrowed bark and irregular branch structure give evergreen pear trees the distinction of larger trees. Lower limbs will probably sag low enough to need pruning for clearance. Clusters of small white flowers, like those of other pears, bloom in spring, but are partly obscured by the evergreen foliage. Distressed trees bloom more profusely and are not so densely foliated in bloom.