SOME BACTERIAL DISEASES OF PLANTS. 125 



by flies or other insects into accidental wounds or bruises. The 

 pruning knife is often an efficient agent in the spread of the bacteria 

 through an orchard. Infection at a pruned surface usually results 

 in a collar-like canker about the pruned stub. Cankers near the 

 base of the tree may also result from infection through wounds 

 made by borers. 



As I have already pointed out it is in these cankers on body 

 and limbs that the bacteria live through the winter. In by far the 

 larger number of these cankers the bacteria die shortly after the 

 canker ceases to spread. In a few of them, however, especially 

 those that arejormed late in the fall, the bacteria remain alive, the 

 line between the diseased and healthy tissue never becoming 

 sharply marked by a crack or depression. These are the so-called 

 "hold-over" cankers of Waite. During the very cold days of winter 

 the bacteria in these hold-over cankers are more or less dormant 

 but on warm days, especially those of March and early April, the bac- 

 teria resume activity, multiplying and working slowly down the 

 limb in the cambium layer. 



With the warm days of April and May the tree becomes gorged 

 with sap and active and vigorous growth in the buds begins. The 

 bacteria in the cankers also become very active, multiplying and 

 spreading into adjoining healthy tissues. On damp cloudy days 

 they ooze out on the surface through the lenticels in the bark in 

 great quantities, often running down the limb as a thick milky viscid 

 fluid. It is with this oozing material that the bees become contam- 

 inated and carry the bacteria away to the blossoms causing them to 

 turn brown and die with the "blossom blight" as we have already 

 seen. 



This brings us to the point in the annual history of the disease 

 from which we started. We have studied the disease in all of its 

 different forms and phases. We have seen that the blight bacteria 

 live continuously in the tissues of the host plant throughout the year. 

 They are never carried by the wind and do not exist in the soil. 

 These two facts are of much importance in simplifying the problem 

 of controlling the disease, as we shall now see. 



It is evident from what we have learned of the disease that spray- 

 ing with a poison of any kind as a means of controlling Fire Blight 

 will be of little value. The bacteria are not scattered by the wind 



