86 MANUAL OF GENERAL AGKICULTURE. 



2. The margin. Note that it is irregular, the crack 

 being formed by ^he drying away of the diseased tissue 

 from the healthy when the active progress of the disease 

 was suddenly checked. Dry or cold weather may thus 

 check the spread of the canker. These specimens were 

 taken in the autumn or winter. 



3. The surface of the canker. Note that it is smooth, 

 seldom roughened or wrinkled. It is often checked in 

 from the margin by drying. Compare with the healthy 

 bark in this respect. Locate the lenticles. 



4. Make a careful drawing of the canker you have 

 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 tem- 

 perature and rise of sap in the spring, these bacteria be- 

 come active, spread rapidly into the adjoining healthy 

 tissue, increasing the area of the canker and oozing out 

 through the lenticles to the surface in sticky, milky drops. 

 If active cankers are available make a careful drawing, 

 showing large viscid, milky drops that have oozed out. 

 (See Fig. 16 N. Y. Cornell Bulletin No. 272.) 



5. Blossom Blight. Bees and flies visit these active 

 cankers in the spring to feed on exuding sap and then 

 visit the opening blossoms, where they leave behind them 

 some of the blight bacteria with which they are smeared. 

 Here in the nectar and in the injuries made by the insects' 

 claws in the tender tissue of the fiower, the bacteria mul- 

 tiply rapidly, killing the blossom. Study the specimen 

 provided or Fig. 6, Cornell Bui. 272. Observe : 



6. The dead and blackened flowers. The leaves of 

 the spur are also dead and brown. The bacteria have 

 spread down the pedicles in the spur. The dead and 

 blackened blossom spurs are usually the first striking evi- 

 dence of the disease in the spring. The oozing cankers 



