PEARS AND 
By JOHN C. DUNEGAN, principal pathologist, Horticul- 
ae al Cr ops Resear ch Br anch, Agricultural Research 
ervice* 
WHAT IS IT? 
The common and frequently destructive dis- 
ease called pear blight, fire blight, or simply 
blight, is caused by a bacterial organism. Pears 
and quinces are extremely susceptible, but the 
organism also attacks apples and at times dam- 
ages ornamental plantings of hawthorn, spirea, 
flowering almond, and mountain-ash. It attacks 
(1) the blossoms and often the young fruits, 
causing them to blacken and die; (2) the tender 
tips of twigs and water sprouts, often killing 
the twigs back for a foot or more and causing 
the leaves to turn black but not fall (fig. 1) ; 
and (38) spurs, large branches, trunks, collars 
(bases of trees), and even roots, chiefly by 
extension of the disease from infected blossoms, 
twigs, and water sprouts. Invasion of large 
branches, trunks, collars, and roots may result 
in the death of a part or all of the tree in one 
season. 
The disease is so destructive to Bartlett, 
Clapp Favorite, Bosc, Flemish Beauty, and 
other high-quality pears that these varieties 
can be grown commercially in only a few fa- 
vored localities. The blight organism some- 
times destroys many of the blossom clusters 
and young shoots of susceptible apple varie- 
ties such as Jonathan, Yellow Transparent, 
* The original edition was prepared by John W. Rob- 
erts, pr incipal pathologist. 
Wealthy, and Transcendent Crab. Although 
destructive in some seasons to apples, the blight 
does not prevent the commercial production of 
even the susceptible varieties. 
HOW IT OVERWINTERS 
Blight appears during the blossoming season 
and is generally active until rapid growth of the 
tree ceases, usually about a month thereafter. 
The bacterium causing blight winters over, 
chiefly in the areas of thick, fleshy bark on large 
branches and trunks, and even roots, which it 
has invaded during the previous year. Occa- 
sionally it may also live over winter in twigs or 
small branches if conditions are such that the 
infected places do not dry up during the sum- 
mer. From these so-called holdover cankers 
the bacteria are carried to the open blossoms, 
and from blossom to blossom and from tree to 
tree by insects visiting the blossoms for nectar 
and pollen. From the diseased blossoms the 
bacteria are carried to the twigs. and water 
sprouts by insects and probably to some extent 
by rain. Usually with the coming of summer, 
the hardening of tissues and the occurrence 
of hot, dry weather prevent new infections, and 
the old ones die out, except those so favorably 
situated that drying out is prevented. These 
are chiefly deep-seated infections in the thick 
bark of the larger branches, trunks, and collars 
and in the roots; they are the holdover cankers 
mentioned above. aeons 
