24 NEBRASKA STATE HORTICULTURAL SOCIETY. 



and sometimes the larger limbs, and even the trunk of the tree is 

 killed. The same disease attacks the pear, and is known as the "pear 

 blight. 7 ' It is much more severe and destructive on the pear than on 

 the apple. It has a wide distribution in North America and is the 

 cause of great loss to apple and pear growers. 



Cause. The disease is caused by a certain minute single-celled 

 plant, one of the bacteria named Micrococcus amylovorus by Professor 

 Burrill, but since called Bacillus amylovorus (B.) De Toni. Professor 

 Burrill* first discovered that this bacterium was the cause of twig- 

 blight. He inoculated healthy branches with the juices of diseased 

 ones, thus being able to produce the "blight." Dr. J. C. Arthur, f by 

 a series of very carefully conducted experiments, 

 proved beyond doubt that the disease is caused by 

 this particular bacterium and by no other. Ba- 

 G ^ us amyhvorus (B.) De T. is very small, one to 

 ^ > o cncD < o one and one-fourth thousandths of a millimeter 



FIG. 5.-Bacteria which lon g> b - V half as broad - Thev are Oval aud co]or ' 

 produce blight, mag- less, either single or attached in twos or threes. 

 nified 1,00 times. 



young succulent parts of twigs or into the flowers, increase in num- 

 bers and destroy the young growing cells of the twigs or flowers, thus 

 causing their death. 



Infectious. The disease is considered to be extremely infectious, the 

 bacteria being carried in the form of dust from tree to tree bv the 

 wind. Mr. M. B. Waite, of the Division of Vegetable Pathology in 

 the United States Department of Agriculture, has pointed out the 

 fact that insects play a very important part in distributing these germs. 

 He doubts if any other very important means of distribution exists. 



Species of trees affected. The disease is most troublesome in the 

 pear, apple, and quince, but affects many other near relatives of these. 

 The different varieties of apples are affected in different degrees. 



Conditions It is generally held by investigators and those ac- 

 quainted with the disease, that watery, succulent growth offers the best 

 conditions for the development of the disease. 



Remedy Removal and destruction by fire of the diseased parts is, 

 so far as yet known, the best remedy. Anything that will cause hardy 



*Proc. Amer. Assoc. Adv. Sci., vol. XXIX, 1880, p. 583. 

 fProc. Amer. Assoc. Adv. Sci., vol. XXXIV, 1885, p. 295. 



