This disease is known to affect several fruit - trees and a few ornamentals belonging to the apple family. The discussion presented here concerns only the pear.
All varieties of the pear are more or less susceptible to Fire Blight. But it is generally believed that the Bartlett, Flemish and Clapp Favorite are more frequently and more severely attacked than the Kieffer, Angouleme and Seckel. However, these apparent variations in resistance may not be traced to an inherent difference among kinds of pears, but to other factors and conditions frequently overlooked. The presence of the disseminating agents (insects), the source of the inoculum, and the period of growth - activity of the host rule the situation more than real immunity.
The disease is very generally known as Fire Blight. It is also referred to as pear blight, blossom blight, twig blight, fruit blight and blight canker. Various combinations of these names are used. The term blight signifies a sudden killing of a part or all of the plant, and in case of Fire Blight the blossoms, leaves and limbs are so affected.
The disease is of American origin, having first been known and described in 1794 from the Hudson River Highlands in New York State. Since its discovery Fire Blight has appeared in the South and West, and about 1880 it was the chief topic of discussion in the horticultural meetings in the Middle West. For several years it was unknown west of the Rocky Mountains, but about 1900 it was described from California. The disease now has a very general range throughout the United States and southern Canada, but does not occur elsewhere.
Fire Blight is the most universally destructive of all poma-ceous fruit diseases. The range and perennial nature of the host and the nature of the disease account for this condition. Being an epiphytotic disease, it may appear suddenly in a locality and rapidly cause severe injury or complete destruction to the pear industry of that section. Nursery-stock suffers severely, and often thousands of affected trees are ruined. In some cases entire blocks of pears are destroyed. Orchard trees may be killed in one season. In some years the attack subsides, but with the recurrence of favorable conditions the ravages are renewed. The disease is said to be generally least troublesome in the United States and in portions of Canada bordering the Great Lakes. But in New York and Michigan the outlook is sometimes discouraging. In the region south of these two states Bartlett pear-growing has been largely abandoned. In the southern states the Kieffer, generally regarded as resistant to the disease, was once widely grown, but its culture has been discontinued, owing to the destructiveness of Fire Blight. In California two - thirds of the Bartlett trees had been destroyed by 1908.
The nature of the injury makes it difficult to obtain accurate figures representing the annual loss from this disease, either in a given locality or in the country as a whole. The blossoms are affected, which means a loss of the current year's fruit-crop, and often that of the next on account of the death of the spur. Twigs may blight in great numbers, thus destroying the twig - growth of the current year; this may later result in death of the larger limbs bearing such twigs. Cankers which may eventually kill the tree are produced on the limbs. Finally, the fruit may be attacked directly; in such cases it is rendered unmarketable.
Attacks on the blossoms result in blossom blight (Fig. 85). This phase of the disease is evidenced by a sudden wilting and darkening of the young fruits after the petals fall, followed by similar changes in the spur. This is a very common form of the disease in New York State. The most striking symptom to be recognized by the grower is that of twig blight (Fig. 86). In the Middle West twig blight, is the most common form of blight. The affected organ is blackened and bears darkened, drooping leaves, the whole appearing as if burned by fire. In no other diseased condition of fruit trees does the foliage cling so tenaciously to the dead branches. Cankers occur in the bark of the body or of large branches (Figs. 87 and 88). In the winter the spot is dark, smooth and sunken, the margin definite and usually marked by a crevice (Figs. 87 and 88). In the spring the canker has a soaked appearance, the advancing margin being indefinite or raised. Sometimes milky or reddish brown drops ooze from the lenticels (Fig. 89); the presence of this ooze, however, is not a constant character. Not infrequently Fire Blight and winter injury are confused. In contrast to a sudden and local dying of the affected organs in the case of Fire Blight, winter - killing manifests itself by a general wilting and a uniform browning of all the foliage simultaneously. Cankers most frequently surround the base of a spur (Fig. 85), watersprout, or small limb (Figs. 87 and 88).
Fig. 85. - Fire Blight; the bacteria have spread from the blossom into a twig, forming a canker.
Fig. 86. - Twig blight. Tip of a shoot blighted by the Fire Blight bacteria.
Fig. 87. - Hold - over canker. Note that it surrounds a stub of a twig.
Fire Blight is caused by the bacterial pathogene Bacillus amylowrus. This pathogene passes its entire life-history within the tissues of the living host, except, of course, during dissemination from one place to another. It cannot survive long, even in the dead parts of the plant attacked. The organism passes the winter in an inactive condition in the tissues along the margin of the blight cankers both on larger limbs and on twigs; such lesions are sometimes called hold-over cankers (Figs. 87 and 88). In the spring the bacteria become active, multiply rapidly, and spread into adjoining healthy tissues. Great numbers of them ooze forth in sticky masses from the lenticels (Fig. 89). This ooze is the source of trouble for the season; it is visited by wasps, bees, flies, beetles, bugs, aphids, curculios and leaf-hoppers, any or all of which insects may carry the bacteria to the opening blossoms, to the tender growing-tips of twigs, or to wounds in the bark. Pruning tools are also agents in transmitting the bacteria. In these various infection-courts the bacteria multiply rapidly and within a few days infection results. The first evidence of the disease in the spring is blossom blight, the tender tissues of the blossom and of the embryo fruit being killed suddenly. The bacteria in a blighting blossom are carried from it to other blossoms by insects, especially bees, and so the pathogene spreads through the orchard, and from one orchard to another.
Fig. 88. - Hold - over canker.