PEAR DISEASES 



1623 



degrees centigrade for ten minutes. They 

 are wholly uninjured, on the other hand, 

 by any degree of cold. Temperatures of 

 40 degrees Fahrenheit below zero have 

 no effect whatever. They may be found 

 to be frozen at this temperature, but 

 thaw out immediately when plunged into 

 warm water and go on with their activ- 

 ities uninjured. Cold retards their de- 

 velopment, but it also prolongs their life. 

 Like other vegetable organisms, cold 

 storage has the effect of prolonging life 

 over a long period of time. In the lab- 

 oratory the organism lives for a rela- 

 tively short time at room temperatures, 

 while if the cultures are put into the 

 icebox the germs may live for months, 

 providing the culture medium does not 

 dry out. 



.Tlie Life Cycle 

 In the life cycle of this germ, blossom 

 blight may be considered as the first 

 step, at least this is the first step in its 

 life cycle that is noticed by the casual 

 observer. For a long time it was a great 

 puzzle where the germs came from that 

 produced the first blossom blight in the 

 spring of the year. This one link in the 

 chain, where the germs remained during 

 the dormant season, was missing. No 

 one knew how the first blossoms became 

 infected. Given a number of blossoms 

 infected, it was comparatively easy to 

 discover the methods by which the germs 

 were carried about. Not only in natural 

 infections, but in those artificially pro- 

 duced with pure cultures, insects were 

 found visiting the blighted blossoms. The 

 germ multiplies in the nectaries of the 

 blossoms as readily as it does in a labo- 

 ratory culture medium, since the nectar 

 glands exude a sugary solution which fur- 

 nishes the organism the necessary food 

 supply. The enzyme or ferment given off 

 as a by-product by the germ dissolves the 

 delicate cells beneath, permitting the 

 germ to pass downward with the greatest 

 of ease. Ordinarily, the entire pear tree 

 is sealed up with an air-tight and water- 

 tight cuticle composed of a thin layer of 

 the same composition as cork. Even the 

 breathing pores are plugged up during 

 the dormant season of the tree so as to 



prevent evaporation from the tissues. 

 This cuticle keeps out the pear-blight 

 germ unless it is injured or broken. The 

 nectary is not covered by cuticle and is, 

 therefore, an easy place for the germs to 

 enter. The gummy exudate pushes out 

 of the infected blossoms, adding to, or 

 even taking the place of the nectar; and 

 honey bees, wild bees, wasps, flies, and 

 perhaps fifty other species of insects vis- 

 iting the pear blossoms or apple blossoms 

 carry the germ-infected material. When 

 once an insect's mouth parts and feet are 

 infected, blossoms which it visits there- 

 after become in turn infected as the in- 

 sect drops off a few germs into the nee 

 taries. The blight virus being also a 

 sticky material and usually requiring a 

 considerable mass, speaking from the 

 microscopic standpoint, to produce infec- 

 tion, is not blown by the wind. Of course, 

 the negative of such a proposition is hard 

 to prove conclusively, but experiments 

 have been made to decide this matter by 

 covering blossoms with mosquito netting 

 along the side of artificially infected 

 flowers, and it was readily found that the 

 uncovered flower visited by insects con- 

 tracted the blight, while those covered by 

 bags, mosquito netting and other mate- 

 rial which kept out insects remained free 

 from the disease. Occasionally, a hum- 

 ming bird visits the infected blossoms. 

 This has been observed in a number of 

 instances. Doubtless, birds get the 

 gummy material on their feet and carry 

 the blight long distances. However, we 

 look upon insect distribution as by far 

 the most immediate means of infection, 

 especially in blossoms, in carrying the 

 blight from flower to flower, tree to tree 

 and orchard to orchard, although doubt- 

 less occasional long-distance distribution 

 is accomplished by birds or some other 

 agencies, including man himself. After 

 the blossoming period is over, or evea 

 before it is entirely finished, blight may 

 be found attacking the tender twigs. Our 

 common insects have been found to be 

 active agencies in not only the distribu- 

 tion of the disease, but in puncturing the 

 tissues and thereby introducing the 

 germs into them. While it is easy to 



