PROCEEDINGS OF THE ANNUAL MEETING 187 



blight may kill a large limb or tree in a single night. In this case the 

 disease may have been in the bark several weeks, and merely spread into 

 the water-conducting tissues at a favorable time and cut off the water 

 supply. 



The pear-blight microbe belongs to the class of bacteria, which, as you 

 doubtless know, is the smallest of all living things. These germs are of 

 vegetable nature, composed of simple cells which multiply by splitting in 

 two. They have minute flagella over their surfaces, and can live only 

 in liquids or moist substances. They swarm in immense numbers in the 

 sap of the tree, and force their way into the tree and into the sap of the 

 cell, killing all the tissues which they reach. The disease causes a greater 

 part of its damage in the month or so following the blooming period, or 

 in young orchards and in the nursery it may be severe all through the 

 summer. In the majority of cases, however, the disease has entirely died 

 out by midsummer. At that time the line of separation betw^een the live 

 and the dead wood is clearly marked, and probably not one case in one 

 hundred would be found where the diseased wood blends off into the 

 healthv. In this case, where there is no distinction between the healthy 

 and blighted wood, the disease is still progressing. In all other cases 

 where the disease has stopped, the microbes have all died or disappeared. 



This microbe is a very delicate organism, and forms no spores. It is 

 readily killed by drying, and in the blighted twigs it dies out in one or 

 two weeks when exposed to ordinary summer weather. It is easily killed 

 by heat. Even hot dry summer weather very closely approaches its 

 death point if not in some cases reaching it. It stands any amount of 

 cold. If frozen below zero, on thawing out it grows as readily as if 

 nothing had happened. It is very easily killed by poisons and fungicides. 

 However, all attempts to prevent the disease by spraying with such mix- 

 tures have proven failures on account of the insidious w^ay in which this 

 parasite grows inside the tissues of the host. The disease dies out so 

 thoroughly in the blighted branches that it was a question for some 

 time just how the microbes live over winter and start the disease the fol- 

 lowing spring. It was found by examining a great many cases of blight 

 that in certain branches the disease may sometimes keep on growing 

 through summer, advancing very slowly into the new bark. Whenever 

 a case of blight succeeds in pushing onward during the summer time, 

 no matter how slowly, and does not dry out and form a definite line 

 between the diseased and healthy wood, and the disease is continued up to 

 the time the leaves fall, such cases result in the blight holding over 

 winter. The cases of hold-over blight make the vulnerable point in the 

 life history of the pear blight germ. The disease can not hold over in the 

 dead bark. 



Extermination of the pear-blight microbe is the only satisfactory treat- 

 ment of the disease. These cases of hold-over blight start the disease 

 the following spring. The source of supply of the pear-blight virus, as it 

 may be called, is spots of the gummy exudation pushed forth from the 

 branches when new growth starts the following spring. This is in fact 

 the dangerous source of infection. We know that no matter how favorable 

 conditions may be for pear blight, if we have not the supply of the 

 germs at hand there will be no blight. These cases of hold-over blight 

 are the key-note of the question. From them the germs will come for a 

 season's destruction. Everv one should be carefullv cut out and burned. 



