NECROTIC DISEASES 
FIRE BLIGHT 
_ This is the most common and best known bacterial disease of plants 
occurring in this country. It affects apples, pears, quinces and occasion- 
ally plums, apricots and a few ornamental and wild plants related to the 
ease SYMPTOMS 
The symptoms of this disease will be studied in the order in which 
they manifest themselves during the season on different parts of the tree, 
beginning with the appearance of the cankers in the spring. 
Hold-over cankers. These are the sources of inoculum for the first 
infection of the blossoms in the spring. Study the typical cankers on 
the limbs of apple and pear trees provided and OBSERVE :-— 
1. The smooth, more or less sunken area in the bark,—the canker; 
its margin sharply defined by a definite crack. In cankers in which 
the pathogene is active this margin is not sharply defined. (See illustra- 
tion specimens if available; photograph 1; Cornell Bul. 272, fig. 16, or 
329, fig. 114.) 
2. The margin. Note that it is irregular, the crack being 
formed by the drying away of the diseased tissue from the healthy when 
the active progress of the pathogene is suddenly checked. Dry or cold 
weather may thus check the enlargement of the canker. These specimens 
were collected in the autumn or winter. 
3. The surface of the canker. Note that it is smooth, seldom 
roughened or wrinkled. It is often checked at the margin by drying. 
Compare with the healthy bark in this respect. Locate the lenticles. 
What is their structure and function? 
Make a V-shaped cut across the margin of the canker and DETERMINE :— 
+. How deeply the disease penetrates. What tissues are 
affected? 
Make a DRAWING of the canker studied. Label fully. These cankers 
are formed during the summer and early autumn and in many of 
them the bacteria pass the winter dormant, or only slightly active 
in the partially living tissues along the margin. With the increased 
temperature and the beginning of growth- activities in the spring, these 
bacteria become active, work rapidly into the adjoining healthy tissue, 
increase the area of the canker and ooze out through the lenticles to the 
surface in sticky, milky drops. Study photograph 1; Cornell Bul. 27 
fig. 16, or Re fig. 114. OBSERVE :— 
That the advancing margin of the canker is not distinctly 
evident hire. The diseased area covers nearly all the surface shown in the 
illustration, except on the extreme left. 
; 6. The large viscid milky drops ooz'ng out and running down the 
limb; above to the left, two small globules just oozing out from the lenti- 
cles. Read Cornell Bul. 272:40-41, also Ontario Bul. 176:15-23. 
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