Fire blight kills fruit-bearing spurs, branches, and entire trees. It can be distinguished from other disorders by droplets of red-brown, sticky, liquid, bacterial ooze that seeps from the surfaces of infected tissue. The disease gains entry to the tree through two main points--blossoms and new shoots--and often appears first in the spring as blossom, fruit spur, and new shoot blight. Infected blossoms wilt rapidly and turn light to dark brown. Bacteria might move through the pedicel to the fruit spur and out into the leaves, where they follow the midrib and main veins, which soon darken. The leaves wilt, turning brown on apple and quince, and dark brown to black on pear. The blighted leaves remain attached for much, if not all, of the growing season. Some remain even after normal leaf fall.
Fire blight's two main symptoms are shoot blight and cankers on limbs. Shoot blight begins with the infection of the young, succulent growing tip, which turns brown to black. This infection might occur at any time during the season while the shoots are still growing and when environmental conditions are most favorable for the disease. The leaves wilt rapidly, turn dark, and remain attached, as in the case of spur blight. A characteristic symptom of shoot blight is the bending of terminal growth into the shape of a shepherd's crook. Pearly or amber-colored droplets of bacterial ooze are often present on diseased blossoms, fruit, and leaf stems, on succulent shoot stems, and on the exterior of infected fruits. Inside these droplets are millions of bacteria, which can cause new infections.
Fire blight bacteria can move from blighted spurs and shoots into larger limbs and tree trunks. Infected branches can be girdled, resulting in loss of the entire branch. Suckers at the base of trees often are invaded and can blight back to the trunk or rootstock, causing the loss of the entire tree in one season. This is true of susceptible pears, especially Bartlett, Bosc, and Clapp's Favorite, and certain clonal apple rootstocks, especially M.26 and M.9.
Cankers, slightly sunken areas of various sizes surrounded by irregular cracks, occur on small to large limbs, trunks, and even roots. They often begin at the bases of blighted spurs, shoots, and suckers. Active blight cankers are characterized by an amber or brown exudate on their surfaces or on the bark below.
The bacteria also can invade fruit, which becomes water soaked. Droplets of bacterial ooze appear on the surface. Later the fruit becomes leathery, turns brown on apples and black on pears and quinces, shrivels, and usually remains attached to the fruit spur.