DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 537 



the blight may extend only a short distance, which results in tip prun- 

 ing. The bark of the tree indicates the progress of the disease, for 

 the soft bark assumes a water-soaked appearance followed by a blacken- 

 ing and shriveling. When the organism ceases to spread rapidly in 

 the tissues, there appears a sharp Hne of separation between the dead 

 and the healthy tissues. The bark is broken and through the bark 

 cracks appear gummy, or gelatinous, drops which vary in color from 

 white to brown, or black. 



Bacilhis amylovorous was described first by Burrill in 1877, a dis- 

 covery full of significance to plant pathology, because it established 

 the first bona fide case of a plant disease due to bacteria. It has been 

 established, that infection takes place through the visits of insects, 

 especially bees, to the pear flowers. From the floral nectary, the 

 bacillus spreads to the softer tissues of bark and cambium, where 

 it is very largely confined, and where it winters over, spreading to 

 other blossoms the next spring. Bacillus amylovorus is an oval 

 microorganism 1.5^ to 2^1 long, growing singly, or several attached 

 end to end, and is motile in fresh cultures. On agar, the cloudy 

 and white surface colonies appear the second day, and attain a di- 

 ameter of 2 to 3 mm. by the fourth or fifth day. Cloudiness appears in 

 bouillon after twenty-four hours, and in milk, thickening of the medium 

 begins at the third or fourth day, which increases until the fifth, or 

 sixth day, when the product is finally partially gelatinous with a clear 

 acid liquid above, changing to slightly alkaline. 



The work of Waite has shown that pear blight can be controlled 

 by pruning out the blight during winter, so as to eliminate the source 

 of infection during the next year, and if this pruning is done thoroughly, 

 the disease can be kept in check. The stubs should be disinfected 

 with corrosive sublimate (i-ioo). 



Pine (Pi litis spp.) 



Blister-rust (Cronartium ribicoliim, Fisch & Waldh. = Pcrl- 

 dermium strobi, Klebahn).^ — This disease, as it appears on white pine, 



^ Spaulding, Perley: The White Pine Blister Rust Situation, American 

 Forestry 22, pp. 137-138, March, 1916; The BHster Rust of White Pine, Bull. 206, 

 U. S. Bureau Plant Industry, 191 1; also consult American Forestry. Feb., Mch., Dec, 

 1916. In the December, 1916, number a map showing the distribution of the 

 disease is given. A conference was held at Washington in January, 1917, to 

 consider the establishment of stricter quarantine regulations of the methods of 

 checking the spread into the western states. 



